Seminars

29 September:  Uddipan Banik (Yale) - hybrid

Pushing the frontiers of gravitational encounters and collisionless dynamics
The long range nature of gravity complicates the dynamics of self-gravitating many-body systems such as galaxies and dark matter (DM) halos. Relaxation/equilibration of perturbed galaxies and cold dark matter halos is typically a collective, collisionless process, and depends on the perturbation timescale (impulsive/fast, adiabatic/slow or resonant). First, I shall briefly discuss a non-perturbative treatment of impulsive encounters between galaxies or halos. Next, I shall present a linear perturbative formalism to compute the response of disk galaxies to external perturbations such as satellite impacts. I shall elucidate how phase-mixing of the disk response gives rise to phase-space spirals akin to those observed by Gaia in the Milky Way disk, and how these features can be used to constrain the dynamical history and DM distribution of our galaxy. Finally, I shall discuss the secular evolution of a perturber due to the back reaction of the host galaxy/halo response. In this context I shall present two novel techniques to model the secular torque (dynamical friction) experienced by a massive perturber on a circular orbit in a spherical host due to resonant interactions with the field particles: 1. a self-consistent, time-dependent, perturbative treatment and 2. a non-perturbative orbit-based framework. These two approaches explain the origin of certain secular phenomena observed in N-body simulations of cored galaxies but unexplained in the standard Chandrasekhar and LBK theories of dynamical friction, namely core-stalling and dynamical buoyancy. I shall briefly discuss some astrophysical implications of these phenomena: potential choking of supermassive black hole mergers in cored galaxies, and the possibility of constraining the inner density profile (core vs cusp) of DM dominated dwarf galaxies and therefore the DM particle nature.

20 October:  Ricarda Beckmann (Cambridge) - hybrid

Intermediate mass black holes in dwarf galaxies and black hole spin evolution
While it is well established that massive galaxies coevolve with, and are shaped by, their central supermassive black holes, it is as of yet less clear whether dwarf galaxies experience similar coevolution with their central (potential) intermediate mass black holes (IMBH). Observational case studies hint at the intriguing possibility that IMBH might play an important role in the evolution of dwarfs galaxies, but both observational and simulated samples have so far remain too small to make general statements about the population of IMBH. This is changing with recent technical advancements in simulations that allow cosmological simulations to push resolution sufficiently high to effectively study the dwarf galaxy regime. In this talk I will present two key results from the recent NewHorizon simulation: First I will discuss intermediate mass black hole population statistics to constrain IMBH populations in dwarf galaxies and make predictions for their impact on the evolution of their host galaxy. Then I will share insights into the spin evolution of massive black holes as their host galaxies form and evolve.

27 October:  Catherine Fielder (Arizona) - virtual

The First UV to IR SED of the Milky Way
Improving our knowledge of global Milky Way (MW) properties is critical for connecting the detailed measurements only possible from within our Galaxy to our understanding of the broader galaxy population. I train Gaussian Process Regression (GPR) models on SDSS galaxies to map from galaxy properties (stellar mass, apparent axis ratio, star formation rate, bulge-to-total ratio, disk scale length, and bar vote fraction) to UV (GALEX FUV/NUV), optical (SDSS ugriz) and IR (2MASS JHKs and WISE W1/W2/W3/W4) fluxes and uncertainties. With these models I estimate the photometric properties of the MW, resulting in a full UV-to-IR spectral energy distribution (SED) as it would be measured externally, viewed face-on. I confirm that the Milky Way lies in the green valley in optical diagnostic diagrams, but show for the first time that the MW is in the star-forming region in standard UV and IR diagnostics---characteristic of the population of red spiral galaxies. Although this GPR method  predicts one band at a time, the resulting MW UV--IR SED is consistent with SEDs of local spirals with characteristics broadly similar to the MW, suggesting that these independent predictions can be combined reliably. The UV--IR SED will be invaluable for reconstructing the MW's star formation history using the same tools employed for external galaxies, allowing comparisons of results from in situ measurements to those from the methods used for extra-galactic objects.

10 November:  Holly Preece (MPA Garching) - hybrid

Forming hot subdwarf B stars from hierarchical triples
Alongside the usual binary interactions, triple systems may also experience short-term dynamical instabilities triggered by mass-loss and eccentricity excitation via von Zeipel-Lidov-Kozai (ZLK) oscillations. Dynamical instabilities can cause ejections, collisions and exchanges. ZLK oscillations create high eccentricities in the inner orbit, potentially triggering mass transfer or collisions. Hot subdwarf B stars are the exposed cores of red giant branch (RGB) stars which have gone on to ignite Helium. Proposed formation scenarios involve binary interactions to strip the RGB star of its envelope, either by Roche lobe overflow or common envelope evolution. This talk presents novel population synthesis calculations of sdB stars formed from hierarchical triples. We examine formation channels, orbital parameters of the sdB systems and initial conditions required to form an sdB from a hierarchical triple. We find we are able to create sdBs as singles, binaries and triple systems.

16 November:  Emma Willett (Birmingham) - hybrid

Asteroseismic inferences on the chemical enrichment of the Milky Way
I will describe the two main projects I have worked on during my PhD, which both involve using asteroseismic constraints in the context of stellar ages and Galactic evolution.  First, I will discuss the radial metallicity distribution of the Milky Way which provides information about the chemical enrichment of the disk and dynamical processes, particularly radial migration. We investigate the metallicity gradient with guiding radius in a sample of red giants, with global  asteroseismic parameters from K2 and chemical abundances from APOGEE. We use asteroseismic ages and apply different modelling techniques to examine the evolution of the characteristics of the metallicity gradient. We find that the gradient flattens towards older ages, while at younger ages the gradient is steeper and its behaviour as a function of age changes. This changing relationship may be a signature of dynamical processes occurring on different timescales and motivates further examination of models of these effects. Second, I will talk about the helium-to-metal enrichment ratio, DY/DZ. The helium abundance of low-mass stars is usually estimated using this parameter, but it is not well constrained by current methods, which introduces significant uncertainties into stellar models. We use the luminosity of red clump (low-mass, core helium-burning) stars as a proxy for Y, and so investigate the helium enrichment history. The approach combines asteroseismic results from Kepler with spectroscopy from APOGEE and astrometry from Gaia to allow red clump stars to be used in this way for the first time.

24 November:  Amandine Dolinksy (Strasbourg Observatory) - hybrid

Detection limits and global properties of Andromeda's dwarf galaxy system
Faint dwarf galaxies are powerful cosmological probes as their properties (numbers, size, luminosity, spatial distribution) can be used to test the cosmological model and, in particular, constrain the dark matter particle mass. But, to use dwarf galaxies as such, it is absolutely crucial to accurately determine the dwarf galaxy detection limits so they can be accurately modeled into the dwarf galaxy system models. I will present the first such effort to characterize fully the dwarf galaxy system of the Andromeda galaxy, based on the PAndAS photometric mapping. As expected, the detection limits are a strong function of the size, luminosity and the location of a dwarf galaxy in the survey. I use these limits to parameterize the satellite system of M31 by forward modeling the combined luminosity function, distribution function, the size-luminosity relation and its total number of dwarf galaxies. I infer that, within a radius of 300 kpc, there should be 133 +/- 40 dwarf galaxies with Mv<-4.5 I will compare the resulting constraints with those from other satellite systems, highlight similarities and differences between the dwarf galaxies orbiting different hosts, and discuss how these impact the use of dwarf galaxies as cosmological probes.

8 December:  Ondrej Pecha (Charles University) - hybrid (postponed to Spring semester)

15 December: Sergio Martin-Alvarez (Stanford University) - hybrid

Pandora's dwarf: combining magnetism, radiation and cosmic rays in galaxy formation
Dwarf galaxies are at the centre of multiple unanswered cosmological and galaxy formation open questions. These galaxies are also a persistent challenge to galaxy formation simulations, which often fail to produce simulated dwarf galaxies matching the properties of their observed counterparts. While approaches such as supernova feedback calibration have allowed some simulations to attain the expected range of stellar masses, various other observable properties (e.g., morphology, kinematics, density profiles, or metal enrichment) are often inconsistent with observations. A promising avenue to overcome this incompatibility is by including additional important physical processes missing in simulations. To this end, I present here our first radiative transfer, cosmic rays, magnetohydrodynamical (RTCRMHD) simulations of a dwarf galaxy evolving in a cosmological context. Incorporating each of these physical processes one at a time into a fiducial star formation + SN feedback model, I will compare my simulations with the observable properties indicated above, and more. Our results provide encouraging prospects for RTCRMHD physics to overcome challenges in galaxy formation, and alleviate the necessity for stellar feedback calibration. I will finally introduce our upcoming large-zoom cosmological simulation of galaxy formation featuring RTCRMHD and its various combinations.

16 February:  Ondrej Pecha (Charles University) - hybrid

Violent interactions of binary stars and the associated outbursts
One of the formation channels of compact object binaries, including sources of gravitational waves, critically depends on violent binary interactions accompanied by the loss of mass, angular momentum, and energy ("common envelope" evolution - CEE). For decades, CEE has been a major unsolved problem in binary star astrophysics with most observations probing CEE only indirectly by looking at progenitor or remnant stellar populations. Recently, the dynamical phase of the CEE has been associated with a class of transient brightenings called Luminous Red Novae (LRNe), which exhibit slow expansion velocities and copious formation of dust and molecules. LRNe offer us a new way to directly probe CEE. In this talk, I will provide a basic review of the CEE and show some surprising results from LRN observations. 

28 February: Elisa Bortolas (University of Milano Bicocca) - hybrid

The path to coalescence of massive black hole binaries across time and space
Coalescing massive black hole binaries are key targets for several ongoing and planned gravitational wave facilities, such as Pulsar Timing Arrays and the LISA mission. These observatories promise to probe the properties of merging massive black holes across the evolution of our Universe, unveiling the clustering and growth of massive black holes and their host galaxies all the way up to the cosmic dawn. In this seminar, I will describe the new advances and challenges in the modelling of the path to coalescence of massive black hole binaries. I will especially focus on recent results that challenge the traditional paradigm of binary evolution, from the large-scale galaxy merger down to the small separations at which the evolution is dominated by stellar hardening, interactions with a gaseous disk and, finally, gravitational-wave emission. I will conclude by  discussing the critical importance of constructing realistic models for the galaxy hosts of massive binaries to properly assess the detection prospects of upcoming gravitational-wave detectors.

23 March: Patrick Gualme (Max Planck Insitute for Solar System Research) - virtual

Understanding the magnetic activity of low and intermediate-mass red giants
According to dynamo theory, stars with convective envelopes efficiently generate surface magnetic fields, which manifest as starspots, faculae, or flares, when their rotational period is shorter than their convective turnover time. Most red giants, having undergone significant spin down while expanding, have slow rotation and no spots. However, based on a sample of 4500 red giants observed by the NASA Kepler mission, Gaulme et al. (2020, A&A 639) showed that 8% display spots, including about 15% that belong to close binary systems. We then focused on a puzzling fact: for rotational periods less than 80 days, a red giant that belongs to a close binary system displays a photometric modulation about an order of magnitude larger than that of a single red giant with similar period and physical properties. We investigated whether binarity leads to larger magnetic fields when tides lock systems, or if a different spot distribution on single versus binary stars can explain this fact. For this, we measured the chromospheric emission in the CaII H & K lines of over 3000 stars studied by Gaulme et al. (2020) thanks to the LAMOST survey. It appears that red giants in a close-binary configuration with spin-orbit resonance display significantly larger chromospheric emission than single stars, suggesting that tidal locking leads to larger magnetic fields at a fixed rotational period (Gehan et al. 2022, A&A 668). In this presentation I will review this project dedicated to a better understanding of red-giant surface magnetism, which combines asteroseismology and high-resolution spectroscopy, and expose future investigation plans in the domain.

30 March: Jonathan Gair (MPI Potsdam) - virtual

Gravitational wave standard sirens as probes of cosmology
Observations of gravitational wave sources provide direct measurements of the luminosity distance of the sources without any need to calibrate to local distance measurements. If these can be combined with measurements of the redshift of the source, these observations can be used to constrain the expansion of the Universe and measure cosmological parameters. Redshifts can be obtained directly from counterparts to events, as in the case of the binary neutron star merger GW170817. They can be obtained statistically by using galaxy catalogues that cover the gravitational wave localisation volume. They can also be obtained by exploiting features in the mass distribution of the sources that can be used to convert the precise gravitational wave measurements of redshifted masses into redshifts. In this talk I will discuss these three approaches, summarise the current status at the end of the O3 observing run and discuss prospects for the future.

20 April: Gwendolyn Eadie (Toronto) - virtual

A Hurdle Model for Globular Cluster Populations and their Host Galaxies
Almost all galaxies in the universe appear to have globular cluster (GC) populations. It is widely accepted that the stellar mass in GC population correlates (log) linearly with the stellar mass of the host galaxy. However, there is a range of galaxy host mass for which this empirical relationship actually breaks down --- when it comes to galaxies at lower mass, some don't have GCs at all, and these data points are often ignored in the application of a log-linear model. In my work, we have introduced a generalized linear model from statistics called a hurdle model, which allows us to include the galaxies with zero GC mass in the analysis. With this type of model, we can not only describe the relationship between GC populations and their host galaxy mass, but also estimate the probability that a particular galaxy will or will not have a GC population given its mass. In this talk, I will describe how this model works and our results when we apply this model to real data. I will also discuss the scientific implications and questions that arise out of our findings, and future applications of our work.

18 May:  Marla Geha (Yale University) - virtual

Our Galaxy in Context:   Exploring Satellite Galaxies Around out Milky Way and Analogs Systems
The Milky Way's satellite galaxies provide critical clues to how low mass galaxies form and the nature of dark matter.  Yet the Milky Way itself is a single realization of a Milky Way-mass galaxy halo.  I will review recent studies of the Milky Way's satellite population and work to put the Milky Way's satellite galaxies in context, highlighting the SAGA Survey.  The SAGA Survey has completed its goal to  measure the distribution of satellite galaxies around 100 systems analogous to the Milky Way down to the luminosity of the Leo I dwarf galaxy.  I will discuss recent results on the quenched fraction of satellites, luminosity functions, radial distributions and more

19 May:  Lucio Mayer (University of Zurich) - hybrid

Direct formation of supermassive black holes from relativistic collapse in high-z galaxy mergers
Multi-scale simulations of gas-rich major mergers between massive high-z disk galaxies have shown that gas inflows can be triggered that damp mass in the cores at more than a 1000 Mo/yr , forming nuclear supermassive disks (SMDs). We report on the first cosmological simulations that follow a merger between two early forming massive disk galaxies, comparable to the present-day Milky Way, occurring at redshift just below 8, which  combine the zoom-in technique and particle splitting to reach pc scale resolution. An  SMD only 4 pc in size forms which has super-solar metallicity due to the highly biased environment, yet it hardly fragments due to the high turbulence of the ISM. The SMD is self-gravitating and bar-unstable. We use an analytical model to study the later evolution of the SMD due to internal transport of angular momentum resulting from the bar instability, showing that it reaches the conditions for the general relativistic radial instability in less than a million years, forming a supermassive black hole with mass up to 100 million solar masses. This mechanism skips the stage of BH seed formation, and provides a natural explanation for the bright high-z QSOs.

25 May: David Nidever (Montana State University) - hybrid

Using Big Data to Understand Galaxy Formation in Our Neighborhood
How galaxies form and evolve remains one of the cornerstone questions in our understanding of the universe on grand scales. While much progress has been made by studying galaxy populations out to high redshift, there is a tremendous amount that can only be learned from near-field cosmology — that is, investigating nearby galaxies in detail using observations of individual stars.  In this talk, I will discuss some recent results in this field that used large surveys to improve our understanding of the formation of our Milky Way galaxy and its largest satellite galaxies, the Magellanic Clouds. In particular, the APOGEE and SMASH surveys have enhanced our understanding of the structure, chemical evolution, and interaction history of the Magellanic Clouds.

1 June:  Francesca Fragkoudi (Durham University) - hybrid (postponed to autumn)

8 June:  Jarle Brinchmann & Daniel Vaz (University of Porto) - hybrid

MUSE-Faint: Dissecting the faintest galaxies with MUSE
Ultra-faint dwarfs are the lowest mass and most dark-matter dominated galaxies known. The shallow potential wells make them susceptible to feedback from star formation and their low baryonic content allows us to use their stars as test-particles in the dark matter potential. Here we will give an overview of the MUSE-Faint survey, a MUSE GTO survey of 10 ultra-faint dwarfs. After introducing the survey, we will outline how the high density of stellar spectra obtainable with MUSE can be used to constrain the dark matter content and density profiles of the galaxies and discuss how the data can be used to constrain dark matter models. After this introduction we will present a detailed study of Leo T, a transition dwarf for which MUSE and the literature provides us with 75 member stars. Armed with these stars we will discuss the stellar content of the dwarf and show that the young stars in Leo T have a distinctly different velocity dispersion from the older stars, while having much the same metallicity.

15 June:  Duncan Forbes (Swinburne University) - hybrid

Ultra Diffuse Galaxies: Galaxies at the Extreme
Ultra Diffuse Galaxies (UDGs) were first identified using the Dragonfly Telescope Array in 2015. Their extreme properties (of low surface brightness, large size, and in some cases rich globular cluster systems) continue to present challenges for standard cosmological simulations. UDGs may represent galaxies with a range of properties: from puffed-up dwarfs to failed galaxies, from those with overly massive dark matter halos to some that are dark matter free. In my talk I will present new observations, and contrast these with the latest simulations, summarising our current understanding of this extreme class of galaxy.

7 July:  Ting Li (University of Toronto) - hybrid

The Power of Milky Way's Stellar Streams Enabled by Multi-Object Spectroscopic Surveys
We are entering an extremely data-rich era in the next decade, with full 6D+chemistry information on dozens of stellar streams, to shape our understanding on the chemo-dynamical evolution of the Milky Way, as well as the nature of the dark matter. In this talk, I will discuss two ongoing spectroscopic programs to study the stellar streams in our Milky Way and highlight a few latest scientific results from these two programs. The Southern Stellar Stream Spectroscopic Survey (S5), started in 2018, is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere. The science results from S5 include a homogeneous study of the kinematic and chemical properties of dozen streams in our Milky Way, the finding of a stream at ~30 kpc possibly perturbed by the dark matter subhalo, the constraints on the mass of the Milky Way and the Large Magellanic Cloud with stellar streams, and the discovery of the fastest hyper velocity stars ejected from Galactic center that can be used to study the shape of the Milky Way halo. The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI), on the other hand, is a recently started 5-yr spectroscopic program in the Northern Hemisphere. With just the first year of data collected in 2021-2022, 4 million unique stars have been observed by DESI, including many stars in the streams of the northern sky (e.g. GD-1) and showing some interesting features as well as the streams beyond the Milky Way.

10 July: David Martinez-Delgado (Institute of Astrophysics of Andalusia) - hybrid

Stellar Tidal Streams beyond the Local Group
Within the hierarchical framework for galaxy formation, merging and tidal interactions are expected to shape large galaxies up to the present day. While major mergers are quite rare at present, minor mergers and satellite disruptions - that result in stellar streams - should be common, and are indeed seen in the stellar halos of the Milky Way and the Andromeda galaxy. In the last years, the Stellar Stream Legacy Survey (PI. Martinez-Delgado) has exploited available deep imaging of some nearby spiral galaxies with the ultimate aim of estimating the frequency, morphology and stellar luminosity/mass distribution of these structures in the local Universe. In this talk, I will present the current results of our systematic survey of stellar streams together with some recent follow-up observations (e.g. Megara, Subaru) and N-body modelling of the most striking streams. Finally, I will discuss what we can learn about galaxy formation from the results of this survey, including the comparison with the available L-CDM cosmological simulations, and our future plans to extend this stream survey at lower surface brightness regime with the ARRAKHIS mission, using small telescopes on-board a micro-satellite in Low Earth Orbit (LEO).

12 July:  Geraint Lewis (University of Sydney) - hybrid

On the Cosmological Time Dilation of High Redshift Quasars
One of the fundamental predictions of modern cosmology is that we should see the distant universe run slowly, with time dilated by the expansion of space. Searches for this cosmic time dilation in the light curves of distant quasars, some of the most luminous objects in the universe, have so far failed to find the expected signal, with claims this challenges the standard cosmological model. I will present an analysis of a new sample of quasars monitored over two decades, testing competing hypotheses within a Bayesian framework to search for the influence of cosmology. The key result is that modern cosmology is safe, quasars are truly luminous objects at immense distances,, and I will explore how previous studies were limited by available data and chosen approaches.

14 October:  Anna Nierenberg (UC Merced) - virtual

Revealing dark matter with strongly lensed quasars

The shapes and abundance of dark matter halos are directly tied to the fundamental physics of dark matter particles such as its free streaming-length and self-interaction cross section, and have thus proven to be essential probes of this otherwise elusive form of matter. Traditional measurements of the shapes and abundance of dark matter halos rely on observations of stars and galaxies which form within them. Such measurements become difficult at low halo masses, as galaxy formation becomes increasingly suppressed. Thus tests of dark matter at low halo-mass scales require a tracer which does not rely on the presence of stars. We have shown that strong gravitational narrow-line lensing provides a powerful probe of the presence of low mass structure in a much larger sample of systems than was previously possible, and enables the detection of dark matter halos well into the regime where the majority of halos are expected to be dark. I will present our results placing limits on a turnover in the halo mass function and measuring the low-mass end of the halo-mass concentration relation. I will also describe our upcoming JWST program to measure cold-torus flux ratios, and conclude by looking to the future for the next generation of instruments, telescopes, and surveys which will enable us to expand the narrow-line method to an order of magnitude larger sample.

11 November:  Rohan Naidu (Harvard) - virtual

Unraveling The Milky Way’s Stellar Halo

The Milky Way has assimilated smaller, immigrant galaxies throughout its history. Its outer reaches (the stellar halo) are a melting pot for stars that were born elsewhere, but now call the MW their home. Despite being scattered across the Galaxy, accreted stars retain memory of their common origin that may be accessed via their shared chemistry and dynamics. In this talk I will present results from the H3 Survey -- the first dedicated halo survey in the Gaia era (150+ nights on the 6.5m MMT, 150k+ spectra of distant stars). I will present a comprehensive anatomy of our Galaxy out to 50 kpc, charting debris from various known and newly discovered mergers. I will present results from two exciting lines of inquiry that this inventory of immigrant galaxies has opened up: (i) they provide convenient, star-by-star access to the stellar chemistry of ``high-z” galaxies at a resolution and scale inaccessible even to JWST -- I will discuss multiple case-studies, including new constraints on the birth-sites of r-process elements; (ii) detailed models of the dark matter that accompanied each immigrant galaxy are upending our picture of the Galaxy -- as a case study I will argue that the last major merger at z~2 induced a ~30 degree tilt in the Milky Way’s dark matter halo with respect to the disk that remarkably persists to this day, requiring us to revisit several inferences about our Galaxy that assume axisymmetry.

15 November:  Pavel E. Mancera Piña (Groningen) - hybrid

Extreme galaxies in fundamental scaling relations

I will discuss two of the most important scaling laws for disc galaxies: the baryonic Tully-Fisher relation (BTFR) and the specific angular momentum-mass relation. Specifically, I will focus on the location of rather extreme galaxy populations in such scaling laws. First, I will present recent results on the dynamics of gas-rich, extended low surface brightness galaxies, sometimes called ultra-diffuse galaxies (UDGs). Using a robust kinematic modelling fitting technique, we find that gas-rich UDGs are outliers of the BTFR and have baryon fractions as high as the cosmological average. Moreover, I will show that mass models of a gas-rich UDG cannot be reconciled with a standard cold dark matter halo, posing a strong challenge to cold dark matter theories. In the second part of the talk, I will present new measurements of the stellar, gas, and baryonic specific angular momenta of disc galaxies, to then derive the relations between the specific angular momenta (j) and mass (M). I will report the discovery of empirical correlations between j, M, and the gas fraction. These relations are very tight and are followed very well not only by normal galaxies but also by a set of extreme galaxies that are outliers of other scaling relations. Moreover, our scaling laws provide a direct benchmark to test the outcome and predictions of simulations and models.

25 November:  Peter Coles (Maynooth) - cancelled

27 January:  Wilma Trick (MPA Garching) - virtual

Gaia & the fingerprints of the Galactic bar - A dynamical Milky Way mystery

The Milky Way's disk around the Sun is interspersed by 7 streams of stars, amongst them the famous "Hercules", "Sirius", and "Hyades" moving groups. These streaming motions are caused by some powerful perturber. Suspects are the spiral arms, satellites, and/or the Galactic bar. So far, the true culprit has been notoriously elusive.  This talk shows how stars whose orbits are in resonance with the Galactic bar exhibit distinctive patterns in action-angle space. Based on these fingerprints, the exquisite Gaia data narrows down the search for the bar's Outer Lindblad Resonance (OLR) to two likely candidates: the "Hat" and the "Sirius" moving groups. Finally pinning down the true OLR (and therefore the pattern speed of the Galactic bar) will be a milestone in uncovering the Milky Way's structure and evolution.

10 February:  Massimo Dotti (Milan Bicocca) - virtual

Dynamics of LISA massive black hole pairs in galaxy (minor) mergers

I will describe the ongoing development of a novel semi-analytical code modeling the pairing of unequal mass MBH, from ~100 kpc scales down to the (possible) formation of a bound binary. The physical prescriptions implemented so far have been validated against high-resolution N-body simulations, include the effect of strong deviations from axisymmetry (e.g. stellar bars), and will soon include a cosmologically motivated evolution of the main galaxy during the pairing. I will discuss some future perspectives and alternative applications of the algorithm

24 February:  Anna Lisa Varri (Edinburgh) - hybrid (postponed)

10 March:  Sarah Pearson (New York University) - virtual (postponed)

24 March:  Hans-Thomas Janka (MPA Garching) - virtual

Core-Collapse Supernovae: From Neutrino-driven Explosion Models to Observations

Supernova explosions terminate the lives of massive stars, produce and disseminate a major fraction of the heavy elements, play an important role as neutrino and particle laboratory, and give birth to neutron stars and stellar-mass black holes, which have recently become sources of measured gravitational waves. After more than 50 years of progressively improved computational modeling, first-principle three-dimensional hydrodynamical simulations with detailed neutrino physics have meanwhile achieved to demonstrate the viability of the neutrino-driven mechanism. The consequences of this mechanism can now be confronted with observations. In particular, 3D simulations of such explosions provide new insights into the geometrical and chemical structure of young supernova remnants, possible explosion-progenitor connections, and the natal properties (masses, kicks, spins) of the compact objects formed in stellar core collapse events.

7 April:  Bob Nichol (Surrey) - hybrid

Time-Domain Extragalactic Survey (TiDES) for LSST Rubin

Type Ia supernovae have revolutionised our view of the Cosmos leading to a Nobel Prize in Physics in 2011 for the discovery of the accelerating Universe. Cosmologists believe this acceleration in the expansion rate of the Universe is driven by a new substance called “dark energy” that dominates the energy density today. In this seminar, I will review the use of Type Ia supernovae in making such measurements and how present surveys of such supernovae have improved our understanding of our Universe. I will also review the future of transient and supernova cosmology with new experiments like the LSST on the Rubin Observatory and the Time-Domain Extragalactic Survey (TiDES) using the 4MOST instrument on the VISTA telescope.  I also review my career for the interest of early-career researchers.

21 April:  Easter break

5 May:  Axel Widmark (University of Copenhagen) - virtual

Weighing the Galactic disk using phase-space spirals

We have developed a new method for inferring the gravitational potential of the Milky Way disk, using the time-varying structure of the phase-space spiral in the plane of vertical position and vertical velocity. Our method of inference extracts information from the shape of the spiral and disregards the bulk density distribution that is usually used to perform dynamical mass measurements. In this manner, it is complementary to traditional methods that are based on the assumption of a steady state. Furthermore, this method is especially robust with respect to severe and unknown selection effects, which allows us to apply it in distant regions of the Galactic disk where dust extinction and stellar crowding would otherwise be detrimental. We have demonstrated the accuracy of our method on simulations. We have also applied our method to Gaia data, from which we inferred the local halo dark matter density and placed the most stringent constraints to the surface density of a thin dark disk.

12 May:  Kelly Hambleton (Villanova University) - virtual

Pulsations and Precision: Why Binary Stars Form the Building Blocks of Stellar Astrophysics

Thanks to precise photometry from satellites such as TESS, Kepler and CoRoT, the field of asteroseismology is advancing at an astounding rate. For many stars, it is now possible to obtain fundamental stellar masses and radii based primarily on empirical asteroseismic formulæ. In complement, the field of binary star physics has long been viewed as the cornerstone of astrophysics, providing direct measurements of fundamental stellar parameters and giving insight into the evolution of stars. In this talk, I will examine each field individually, following which I will look at what happens when these two diverse fields are combined. Specifically, I will focus on the use of pulsating stars in binary systems to validate the results of asteroseismology and tidally driven pulsations: pulsations driven by gravitational interactions between the two stars in a binary system.

30 May:  Eric Andersson (Lund University) - hybrid

Resolved stars in galaxy simulations

Hydrodynamical simulations have been hugely successful in explaining many of the details governing the formation and evolution of galaxies. The most recent advances in this fields has in large part been facilitated by parsec-resolution simulations, now reaching the scales of star forming clouds. This has provided an incredibly detailed view of how stellar feedback and chemical enrichment operate on galactic scales. With parsec resolution, galaxy simulations can now be done star-by-star, and in fact should be done this way to avoid the many restrictions imposed by the traditional IMF-averaged approach. In this seminar, I will go through the most recent advances in this field, and present my own model incorporating individual stars in galaxy scale simulations. In this context, I will focus on stellar feedback and galactic wind driving mechanics, explaining how these are crucial aspects for successfully explaining galaxy evolution. Furthermore, I will shed light on the controversy regarding massive, fast-moving runaway stars and how they can affect these aspects. Finally, I will show preliminary results, where I applied my star-by-star model to ultra-faint dwarf galaxies simulated in a cosmological context. This enables us to study the relationships between properties of specific stars and the global properties of the dwarf galaxy. This allows for a very detailed comparison to data collected in spectroscopic surveys, which are limited to giant stars due to extremely low surface brightness.

9 June:  Annika Peter (Ohio State University) - virtual

Count your halos

One of the strongest predictions of the standard cold dark matter paradigm is the hierarchy of structure down to Earth-mass scales.  However, individual self-bound clumps of dark matter--"halos"--are difficult to detect directly.  Instead, we use galaxies as lampposts for halos.  By counting galaxies, we can measure the underlying population of dark matter halos.  In this talk, I describe results that seem completely at odds with each other in measuring the population of small halos.  I argue that the resolution to the problem is a better mapping between galaxies and halos.  I will show what my group is doing so far to address the problem, and what opportunities lie ahead in the wide-field surveys of the 2020's.

16 June:  Duncan Forbes (Swinburne University) - hybrid

Ultra Diffuse Galaxies: Galaxies at the Extreme

Ultra Diffuse Galaxies (UDGs) were first identified using the Dragonfly Telescope Array in 2015. Their extreme properties (of low surface brightness, large size, and in some cases rich globular cluster systems) continue to present challenges for standard cosmological simulations. UDGs may represent galaxies with a range of properties: from puffed-up dwarfs to failed galaxies, from those with overly massive dark matter halos to some that are dark matter free. In my talk I will present new observations, and contrast these with the latest simulations, summarising our current understanding of this extreme class of galaxy.

11 July: Philip Wiseman (Southampton University) - hybrid

4 August:  Mattia Sormani (Heidelberg University) - hybrid

Gas dynamics, inflow and star formation in the innermost 3 kpc of the Milky Way

I will give an introduction to gas dynamics and star formation in the central region (R<3kpc) of the Milky Way, which is dominated by the strongly non-axisymmetric gravitational potential of the Galactic bar. After reviewing the basic theoretical tools, I will discuss several topics including (i) how we can interpret the large-scale spectral line observations of CO, HI and other interstellar gas tracers; (ii) how we can use the gas dynamics to constrain the properties of the Galactic bar; (iii) how the gas is driven inwards from the Galactic disc (R~3kpc) down to the Central Molecular Zone (CMZ, R~120pc) and then to the central black hole SgrA*; (iv) the spatial and temporal distribution of star formation in the Galactic centre; (v) the structure and dynamics of the Nuclear Stellar Disc, the flattened stellar structure that dominates the gravitational potential at Galactocentric radii 30pc<R<300pc. Finally, I will highlight some open questions and directions of future research.

15 October:  Morgan Fraser (University College Dublin)

Strange Transients in Strange Times

Much of the impetus for wide-field time domain surveys has come from supernova cosmology. But along with finding ever larger samples of Type Ia supernovae at high redshift, these surveys have revealed a dizzying array of other classes of transients. I will highlight some of the most interesting and peculiar examples of these, ranging from stellar mergers, kilonovae and the extra-galactic analogs of Eta Car.

22 October:  Marcin Semczuk (University of Leicester)

Tidally induced morphology of late-type galaxies

Roughly half of the nearby spiral galaxies have warped gaseous discs. All three of the massive spirals of the Local Group have warps with M33 having the most pronounced one. The morphology of M33 was hypothesized in the literature to be shaped by a past possible interaction with M31. During the first part of my talk I will present the results of the simulations of such potential past interaction and discuss how our model corresponds to the observations.  While for M33’s warp the tidal origin seems plausible, it is unclear how often such scenario occurs in other galaxies. In the second part of my talk I will present the results of the analysis of warped spiral galaxies from the cosmological simulation IllustrisTNG. We found that in this simulation warps are common features of the discs and interactions contribute significantly to their formation. 

29 October:  Simona Pirani (University of Copenhagen)

Disentangling the early history of the solar system

Giant planet formation models have radically changed from the standard in situ growth via planetesimal accretion. Pebble accretion, indeed, allows cores of giant planets to grow rapidly enough also at large radii. Hydrodynamical simulations, on the other hand, show that a protoplanet embedded in a gas disc interacts with it and starts to migrate, typically inwards. Exoplanets in resonances or resonant chains also hint that migration is a common process in planetary systems. We investigated the possibility that also the solar system has been affected by an early inward migration of our giant planets. This kind of large-scale event could have left traces in the population of small bodies of our solar system, especially in the Jupiter trojan asteroids that share the same orbit as Jupiter. 

12 November:  Sebastian Trujillo-Gomez (Heidelberg University)

Gas, stars, and globular clusters in the local Universe as tracers of galaxy formation and evolution, and the nature of dark matter

In this talk I will discuss how understanding the stellar, globular cluster (GC), and gas components of galaxies allows us to trace the assembly of galaxies and their dark matter (DM) haloes, and how these provide constraints on the complex physics of galaxy formation and the nature of DM. To illustrate this, I will use examples from three studies. In the first, I will discuss the phase-space distribution of the MW GCs in the E-MOSAICS simulations provides a detailed picture of the formation of the Galaxy. Using Gaia data and a statistical method based on 25 MW-mass simulations, we obtain 18 quantitative predictions, including the MW's stellar and DM assembly timescales, merger demographics, and balance of in-situ and accreted star formation. In the second example, I will show how the unusual GC populations in galaxies like Fornax and NGC1052-DF2 (the galaxy 'lacking' DM) can be used to rewind the clock and obtain a snapshot of their galactic progenitors at cosmic noon. Using a model for the feedback-regulated hierarchical formation of clusters to predict the environmental conditions, indicates that these galaxies must have undergone strong structural evolution. These early galactic snapshots have important implications for the effect of clustered star formation on galaxy evolution. Lastly, I will describe how detailed knowledge of the HI kinematics of field dwarf galaxies can be used to probe the abundance of low mass DM haloes using the velocity function. Once the systematics in the recovery of the circular velocity are fully understood, upcoming interferometric 21-cm surveys will provide unprecedented constraints on the physics of the DM particle as well as on the physics of galaxy formation through the relation between stellar mass and halo mass.  

19 November:  Gijs Neleman (Radboud University)

How can observations teach us about binary evolution?

Binary stars are very common and produce lots of interesting phenomena, ranging from types of supernovae, gamma-ray bursts, and kilonovae to Gravitational Wave sources and important sources of chemical evolution. The evolution of binary systems, through stages of mass transfer, supernovae etc is complicated and often not well understood. In principle, observations of different (populations) of binary systems at different phases in their lives can be used to determine their evolution and thus constrain the uncertain physics of their interactions. However, in practice this is not easy, since many uncertainties are combined and observations are strongly biased. Yet, there are a number of very positive developments: large surveys of stars in the Galaxy like SDSS and many others have produced much more homogeneous samples of systems; Gaia is rapidly providing more information about populations and their absolute numbers and the new field of Gravitational Wave astronomy is providing new and sometimes orthogonal information. I will describe a number of efforts to utilise these new developments and an outlook into the future of observationally constraining binary evolution.

26 November:  Paul Jofre (Universidad Diego Portales)

Stellar Phylogenies: Using Tools from Evolutionary Biology to Reconstruct the History of the Milky Way

The evolution of galaxies can be studied using the abundance ratios of long lived stars, which serve as fossil records of the chemical composition from the gas clouds they formed. It is standard in our field to use the these fossil records to study how the chemical information is inherited from one stellar generation to the next using the principle of descent with modification, and therefore underpin evolution. What is novel is that this very principle allows us to further depict the patterns of descent among stars as an evolutionary tree. Today, in all branches of biology, evolutionary trees (more generally known as phylogenies) are a major tool for analysing the pattern and process of evolutionary history. In this talk, I will show how we can adapt these tools and use it to study the history of the Milky Way.

10 December:  Yumi Choi (Space Telescope Science Institute)

Resolved stars in nearby dwarf galaxies: Probes of dust, stellar structures and kinematics, and the epoch of reionization

Nearby galaxies are important laboratories for studying fundamental astrophysical processes in (sub)-pc scales. The detailed knowledge acquired from rich dataset available for nearby galaxies can be used not only to understand their current status and evolution over time, but also to infer the role of similar galaxies in driving the cosmic evolution. Analyses of resolved stellar populations in nearby galaxies provide a wealth of information about spatial distributions of dust and stars, stellar motions, and star formation histories and their links to the multiphase interstellar medium. Furthermore, as excellent analogs of high-redshift UV-faint small galaxies, low-metallicity (lower than the SMC) dwarfs in the Local Volume provide useful insights into unveiling the galaxy formation and evolution in the early universe. In this talk, I will present power of resolved stars to understand (1) dust reddening in the Magellanic Clouds, (2) stellar structures and kinematics in the Magellanic Clouds, and (3) the role of UV-faint dwarf galaxies in the epoch of reionization.

21 January:  Johnny Greco (Ohio State)

Hunting Diffuse Galaxies with Next Generation Imaging Surveys

We are entering a new era for deep wide-field imaging surveys, which promises to extend our census of the galaxy population to lower surface brightnesses than has ever been possible over large areas of the sky. These surveys will uncover low surface brightness (LSB) galaxies across halo environments, which will provide important tests for theoretical predictions of galaxy and star formation, stellar feedback processes, and the distribution and nature of dark matter. I will present results from our ongoing efforts to discover and characterize diffuse galaxies with the Hyper Suprime-Cam Survey, a new generation imaging survey that is serving as a stepping stone into the LSST era. We are uncovering a diverse population of objects, spanning dwarf ellipticals in nearby galaxy groups to ultra-diffuse galaxies in the field to giant LSB spirals. Importantly, reliable distances are required to study the numbers and physical properties of these objects as a function of environment, and we are taking a number of approaches to constrain the distance distribution of our sample. I will focus in particular on our efforts to use the globular cluster luminosity function and ground-based surface brightness fluctuations as distance indicators. Pushing such studies to lower surface brightnesses will be necessary to form a more complete census of the galaxy population, which will ultimately provide one of the strongest tests of the standard LCDM framework.

11 February:  Michael Petersen (University of Edinburgh)

Modelling Milky Way dark matter from the largest to the smallest scales

With most evidence pointing towards the Milky Way hosting a massive cold dark matter halo, dynamicists are now on the hunt for unambiguous signatures of the structure and evolution of the unseen component. Some models for the Milky Way dark matter halo have begun to include a new ingredient: time dependent structure. I will present one family of models. Starting at the largest scales, I will review recent results parameterising the halo from information gleaned through modelling the infall of the Large Magellanic Cloud. I will then present ongoing efforts to move to smaller scales, including dark matter-mediated processes driving stellar disc evolution.

4 March:  Ling Zhu (Shanghai Astronomical Observatory)

The discovery of an ancient massive merger event in the Fornax cluster galaxy NGC 1380

Driven by gravity, galaxies are expected to continuously grow through the merging of smaller systems. To derive their past merger history is challenging, as the accreted stars disperse quickly, yet, it is a needed step to test the theory of hierarchical evolution. The merger his- tories of the most massive Local Group spirals, the Milky Way and M31, have been recently uncovered by using the motion and chemistry of their individual stars. On the other hand, the details of the merger history of galaxies at further distance have so far remained hidden. Here we report the discovery of an ancient, massive merger event in a lenticular galaxy NGC 1380 in the Fornax cluster. By applying a recently developed population-orbital superposition model1 to NGC 1380’s surface brightness as well as stellar kinematic, age, and metallicity maps from VLT/MUSE IFU data, we obtain the stellar orbits, age and metallicity distribu- tions of this galaxy. The highly radial orbits which make up an inner stellar halo are ∼ 13 Gyr old with metallicity Z/Z⊙ ∼ 1.2 and comprise a stellar mass of M∗,halo(r<2Re) ∼ 3.4×10^10 M⊙. By comparing to analogues from the cosmological galaxy simulation TNG50, we find that the formation of the inner stellar halo of NGC 1380 requires a merger with a massive satellite galaxy with stellar mass of ∼ 3 × 10^10 M⊙and occurring roughly ∼ 10 Gyr ago. Moreover, we infer the total accreted stellar mass of NGC 1380 to be ∼ 6 × 10^10 M⊙. The massive merger in NGC 1380 is the first major merger event found in a nearby galaxy beyond the Local Volume, and it is the oldest and most massive one identified in nearby galaxies so far. Our chemo-dynamical method, when applied to extended deep IFU data and in combination with cosmological galaxy simulations, can quantitatively unravel the merger history of a large number of nearby galaxies.

18 March:  Kathryn Johnston (Columbia University)

Snails Across Scales or Fun with Phase-Mixing or Gaia, the Galaxy and Galactic Dynamics

Results from ESA’s Gaia mission paint a picture of our Galaxy in 6-D - revealing abundant signatures of departures from purely random samples drawn from equilibrium distributions. This talk outlines some fun connections back to basic dynamical concepts, and forward to interpretation and implications.

25 March:  Cristina Chiappini (Leibniz Institute for Astrophysics Potsdam)

Galactic Archaeology with precise ages, chemistry and kinematics: what have we learned?

Because most stars carry in their outer envelopes the chemical composition inhered at birth, it should be possible to map the star formation history in different parts of the Milky Way by measuring, for stars of different ages, a large array of chemical elements covering different nucleosynthetic sites. This goal seems to be still reachable even in the presence of mergers and radial stellar mixing, i.e., the fact that stars can move away from their birth places, losing most of their kinematical memory.  In particular we will highlight recent results obtained from the combination of asteroseismology, Gaia, photometric and spectroscopic surveys. The new data can take us out from the very local volume, enabling finally a more holistic view of our Galaxy. We close by discussing where more data is needed and the future plans of 4MOST.

1 April:  Easter Break

8 April:  Easter Break

15 April:  Jamie Tayar (University of Hawai'i)

How much should you trust stellar models?

Stellar models are used for a wide range of purposes from characterizing extrasolar planets to untangling the history of our own and other galaxies. Unfortunately, there are still significant uncertainties in the physics of these models, and inconsistencies with recent observations. I will discuss how we model stellar interiors, and what that means for our ability to accurately and precisely estimate stellar parameters. I will also show that large photometric and spectroscopic surveys have significantly improved our understanding of stellar convection, rotation, and mixing in the past few years. Finally, I will discuss how this improved physical understanding impacts the utility of stellar models, including the estimation of stellar ages, and the prospects for future improvements.

22 April:  Monica Colpi - postponed

29 April:  Lewis Whitehouse - postponed

6 May:  Tom Hands (University of Zurich)

Super stellar abundances of alkali metals suggest significant migration for Hot Jupiters, and how our own Jupiter might capture interstellar objects

I investigate the origin of the observed over-abundance of alkali metals and a lack of water in hot gas giants relative to their host stars. I show that formation exterior to the snow line followed by inward disc-driven migration results in excess accretion of oxygen-poor, refractory-rich material from within the snow-line. This naturally leads to enrichment of alkali metals in the planetary atmosphere relative to the bulk composition of its host disc but relative abundances of water that are similar to the host disc. These relative abundances cannot be explained by in situ formation which places the refractory elements in the planetary deep interior rather than the atmosphere. I therefore suggest that this is clear evidence of significant migration for at least a subset of hot gas giants. In the second part of the talk, I will focus on another project which aims to understand how easily Interstellar Objects such as 1I/Oumuamua and 2I/Borisov might be captured into our Solar System, and appear as long period comets. Current estimates for orbital lifetimes and space densities then imply steady-state captured populations of ∼10^2 comets and ∼10^5 `Oumuamua-like rocks, of which 0.033% are within 6au at any time.

13 May:  Paolo Bianchini (Strasbourg)

A new perspective on the dynamics of Globular Clusters

The traditional picture of globular clusters (GCs) as simple stellar systems is being radically revolutionized by state-of-the-art observations, in particular thanks to precision astrometry led by the Gaia mission. We now know that present-day Milky Way GCs display rich properties (e.g. internal rotation, tidal tails, complex stellar populations, stellar-mass black holes) and a fraction of them is expected to be the remnant of accreted dwarf galaxies, possibly still containing leftover dark matter. In this talk, I will show how this revitalized perspective offers the exciting possibility to unlock the mystery of GCs origin in the high-redshift Universe, but, at the same time, demands for a renewed and much more refined theoretical framework. First, I will highlight my effort in exploiting astrometric data to unveil the kinematic complexity in today's GCs, both in their inner regions and in their outskirts. Subsequently, I will focus on the role of dynamical evolution driven by repeated star-by-star interactions (collisionality) which strongly reshapes GCs and leaves strong imprints in their properties, such as their mass-to-light ratios. Finally, I will introduce a new theoretical approach –the algorithm π-DOC– based on the combination of neural networks and N-body simulations, which aims at dramatically improving the measurement of GCs properties, explicitly taking into account their complex >10 Gyr dynamical evolution. Ultimately, this novel strategy, combined with state-of-the art data, will allow us to exploit GCs as probes of the high-redshift Universe.

20 May:  Sarah Casewell (Leicester)

Irradiated brown dwarfs: Providing insights into exoplanet atmospheres

Brown dwarfs are often described as failed stars, however the flip side of this description is that they can also be described as over-ambitious planets. With masses between 13-70 Jupiter masses they have cool atmospheres dominated by cloud features, molecules and show features due to weather.  These atmospheres have a lot of similarities with atmospheres we see in planets in our solar system, and also directly imaged exoplanets.  The question then is: How like hot Jupiters are irradiated brown dwarfs? In this seminar I will describe the known irradiated brown dwarfs and how they evolve into post-common envelope systems containing a white dwarf. These rare binaries have very short periods (~hrs) and the brown dwarf is irradiated by the white dwarf companion, often with large amounts of UV radiation. I will discuss the atmospheres of these highly irradiated brown dwarfs and their similarities with irradiated exoplanets.

3 June:  Monica Colpi (University of Milan Bicocca)

LISA, the Gravitational Universe and the Origins of Massive Black Holes

The Laser Interferometer Space Antenna LISA, the third ESA’s large mission with foreseen launch in the mid thirties, will shed light on how, when and where the supermassive black holes, powering quasars and lurking at the centre of the bright galaxies today, formed, grew and assembled  in concordance with the evolution of cosmic structures. Their origin is still a mystery which LISA aims at unraveling. Numerous signals of coalescing massive black hole binaries are expected, occurring across the epoch of cosmic reionization and during the culmination of the star formation rate in the Universe. I will describe how, in synergy with third generation ground based interferometers as Einstein Telescope we might be able to establish the plausible link between stellar  and supermassive black holes to learn about the black hole “seeds” upon which the supermassive ones have grown. I will highlight unsolved problems related to the formation of “binary” black holes on various mass scales. 

10 June:  David Hernandez (Harvard University)

Are long term N-body simulations reliable?

N-body integrations are used to model a wide range of astrophysical dynamics, such as those in galaxies or planets, but they suffer from errors which make their orbits diverge exponentially in time from the correct orbits. Over long time-scales, their reliability needs to be established. In this talk, I describe analytic and numerical techniques we used for establishing the reliability of long-term N-body simulations.  In our results, the criteria for reliable N-body simulations lay in resolving orbits in time such that stepsize resonance chaos is avoided.  I will use the the last portion of my time to discuss how we applied related concepts and tools to solve the transit timing variation problem.  Specifically, we deduced the masses and densities of the TRAPPIST-1 exoplanetary system through the variations in the transit timings of its planets.

The intermediate neutron capture process

Speaker: Richard Stancliffe (University of Hull)

Date: 24 September 2020

The seminal work of the late Margaret Burbidge and her collaborators in 1957 laid down the foundations of most of stellar nucleosynthesis. From a consideration of the Solar abundance pattern and a knowledge of nuclear structure, the possible processes for producing all the elements could be outline. Two neutron capture processes -- dubbed the slow and rapid processes -- were shown to be key to producing the heavy elements. However, recent evidence from metal poor stars suggests a third neutron capture process may be need.  In this talk, I will lay out the evidence pointing to the operation of an intermediate neutron capture process in certain astrophysical sites. I will discuss the reactions that lead to the production of neutrons, and the physical conditions that can give rise to them. I will describe some of the possible sites where the intermediate (or i) process may take place, and discuss some of the implications for the chemical evolution of the Galaxy.

Galactic Archaeology with Gaia and Large Spectroscopic Surveys

Speaker: Keith Hawkins (University of Texas, Austin)

Date: 10 September 2020

One of the key objectives of modern astrophysics is to understand the formation and evolution galaxies. In this regard, the Milky Way is a critical testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical  chemical, and age distributions of its stellar populations.  Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have and can been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.

Cosmic Concordance or Tension: Cosmology from the Kilo-Degree Survey

Speaker: Catherine Heymans (University of Edinburgh)

Date: 3 September 2020

Surviving and destroyed dwarf galaxies: the build-up of the Milky Way's stellar halo

Speaker: Azadeh Fattahi (Durham University)

Date: 20 August 2020

The hierarchical nature of galaxy formation in the standard model of cosmology predicts a diffuse stellar halo component around Milky Way-mass galaxies, formed from the accretion and disruption of dwarf galaxies. Using cosmological hydrodynamical simulations from the Auriga project, I will compare the properties of these destroyed dwarfs which built up the stellar halo of Milky Way-like galaxies with those of existing satellites. I discuss their abundance, infall times and metallicities. In the second half of the paper, I connect these results with recent findings from the Gaia mission related to the major event in the formation history of the Milky Way (Gaia-sausage or Gaia-Enceladus).   

Dynamical Chaos near Corotation: Limits on cold radial migration

Speaker: Kate Daniel (Bryn Mawr College)

Date: 23 July 2020

After an initial epoch of assembly, spiral galaxies like the Milky Way evolve primarily under the influence of slow, internal processes.  This secular evolution rearranges the orbital angular momentum and energy of the disk, thus altering its kinematics, morphology and chemical distribution.  Dynamical resonances with spiral arms cause stars to migrate large radial distances from their birth radii.  Most radial migration is associated with kinematic heating.  However, transient spiral arms drive a particularly important process, called cold torquing, that can change the orbital sizes of a substantial fraction of disk stars over the lifetime of the disk without kinematically heating it.  The relative importance of cold or heating radial migration significantly impacts how disks evolve.  High resolution simulations of spiral galaxies, as well as large, high precision observational surveys of the Milky Way disk, stress a critical need to further develop a theoretical framework for interpretation of these results.  In this talk, I will quickly demystify the physics that governs radial migration and cold torquing and present scaling relations that constrain its efficiency.  I will then argue that in some limits cold torquing can, in fact, kinematically heat the disk.  First steps have been taken, but there is an ongoing need to better understand the nature of transient spiral structure and the role cold torquing has played in the evolution of the Milky Way.

Signatures of planet formation and orbital evolution in the cold dust emission of protoplanetary discs

Speaker: Clement Baruteau (CNRS, University of Toulouse)

Date: 14 May 2020

The classical picture of protoplanetary discs being smooth, continuous structures of gas and dust has been challenged by the growing number of spatially resolved observations. These observations tell us that rings, gaps, and large-scale asymmetries like spirals are common features of the emission of protoplanetary discs, which are often interpreted as signatures of the presence of (unseen) planetary companions. During this seminar, I will report on our recent and ongoing work on how the formation and orbital evolution of planets impact the dust emission in protoplanetary discs, mainly at radio wavelengths. Through gas and dust hydrodynamical simulations post-processed with dust radiative transfer calculations, I will show that recent ALMA observations strongly suggest the presence of several planets in the discs around two young stars, MWC 758 and HD 169142.

The IKEA model of self-regulated galaxy formation

Speaker: Tom Theuns (Durham University)

Date: 30 Apr 2020

I will present an analytical model of galaxy formation called IKEA, in which a galaxy's star formation rate balances the increase in its energy due to feedback from massive stars with energy losses associated with cosmological accretion onto its host halo. Such balancing of energies is self-regulating provided the star formation law is such that the star formation rate increases with gas pressure (such a the Kennicutt-Schmidt law, for example), but does not dependent on the details of the star-formation law.  The model has four parameters - hence the name - two of which depend on cosmology while the remaining two depend on stars and their interaction with the interstellar medium. Keeping all four parameters constant, IKEA reproduces very accurately the star formation rate as a function of halo mass and redshift in the Eagle cosmological simulation. Other observables, for example the shape and evolution of the galaxy stellar mass function, are also reproduced reasonably well. The onset of AGN feedback is shown to be a consequence of the failure of stars to regulate cosmological accretion once the potential well of the halo becomes too deep. The characteristic transition scale is set by how much energy is injected in the ISM per solar mass of star formed. The relative success of the model demonstrates that star-forming galaxies are shaped by the balance between stellar feedback and cosmological accretion onto their host halos, with accurately accounting for radiative losses associated with supernova feedback the most crucial ingredient of any model.

Nuclear stellar clusters and supermassive black holes

Speaker: Melvyn Davies (Lund Observatory)

Date: 12 Mar 2020

Supermassive black holes are found in most galactic nuclei. A large fraction of these nuclei also contain a nuclear stellar cluster surrounding the black hole. Here we consider the idea that the nuclear stellar cluster formed first and that the supermassive black hole grew later. In particular we consider the merger of  stellar clusters to form a nuclear stellar cluster, where some of these clusters contain a single intermediate-mass black hole (IMBH). In the cases where multiple clusters contain IMBHs, we discuss whether the black holes are likely to merge and whether such mergers are likely to result in the ejection of the merged black hole from the nuclear stellar cluster. In some cases, no supermassive black hole will form as any merger product is not retained. This is a natural pathway to explain those galactic nuclei that contain a nuclear stellar cluster but apparently lack a supermassive black hole; M33 being a nearby example. Alternatively, if an IMBH merger product is retained within the nuclear stellar cluster, it may subsequently grow, e.g. via the tidal disruption of stars, to form a supermassive black hole.

General relativistic effects around the Galactic Centre black hole

Speaker: Stefan Gillessen (MPIE)

30 Jan 2020

The Galactic Center offers the unique possibility to quantitatively test general relativity in the so-far unexplored regime close to a massive black hole. Here we present the results from the last two years of GRAVITY observations, in particular (1) the detection of the gravitational redshift in the orbit of the star S2, and (2) the orbital motion of matter close to the last stable orbit during a flare. The GRAVITY instrument, which we have developed specifically for the observations of the Galactic Center black hole and its orbiting stars, is now routinely achieving ~3 milli-arcsec imaging interferometry and with a sensitivity several hundred times better than previous instruments. Its astrometric precision of few ten micro-arcseconds corresponds to only few Schwarzschild radii of Galactic Center massive black hole, which opens up the possibility to test the fundamentals of gravity, all the way from the underlying equivalence principles, to considerations on new physics and their characteristic scales and strengths. The Galactic Center is and will remain the Rosetta-stone for deciphering strong gravity around massive black holes.

The discovery of a nearby 1700 km/s hyper-velocity star ejected from the Galactic Centre

Speaker: Sergey Koposov (Carnegie Mellon University)

Date: 24 Jan 2020

In this talk I will present a recent discovery of a new hyper-velocity star in the S5 survey. While the S5 survey is devoted to the study of the Milky Way stellar streams, the efficiency of target selection enabled us to target a large numbers of other potentially interesting targets. The search among those revealed a star with the radial velocity of ~ 1000 km/s. After combining the radial velocity measurement with the Gaia proper motion and spectrophotometric distance we found out that the star is more than twice as fast as other known hyper-velocity stars and its orbit can be for the first time confidently traced back to the Galactic Centre. The star was found to be ejected from Sgr A* ~ 5 Myr ago and can be possibly connected to the disk of young stars around the Galactic center.

Revealing the complex evolution of dwarf galaxies through spatially resolved spectroscopy

Speaker: Samantha Penny (University of Portsmouth)

Date: 16 Jan 2020

Dwarf galaxies are the dominant galaxy population by number in the nearby Universe. Despite their ubiquity, the processes that govern their star formation activity are not well understood. In particular, what causes star formation to cease/quench in these low-mass galaxies? Addressing this question requires spatial information for dwarf galaxies in a range of local environments. Using data from the SDSS-IV MaNGA IFU survey, we identify a sample of quenched dwarf galaxies fainter than Mr = -19, selected independently of morphology and environment.  I will show the majority of quenched dwarfs exhibit coherent rotation in their stellar kinematics, and several host disc or spiral features, inconsistent with a primordial origin. I will also show that a number of bright dwarf galaxies with signatures of gas accretion host active galactic nuclei, which are likely maintaining their quiescence. While they are the "simplest" galaxies in our current models of galaxy formation, quenched dwarf galaxies are a diverse population, with further surveys and modelling needed to understand their origin.

A geometric probe of cosmology: Gravitational lensing time delays and quasar reverberation mapping

Speaker: Angela Ng (University of Sydney)

Date: 12 Dec 2019

I present a novel, purely geometric test of cosmology based on measurements of differential time delays between images of strongly lensed quasars due to finite source effects. This approach is solely dependent on cosmology via a ratio of angular diameter distances, the image separation, and the source size. It thereby entirely avoids the challenges of lens modelling that conventionally limit time delay cosmography, and instead entails the lensed reverberation mapping of the quasar Broad Line Region. I demonstrate that differential time delays are measurable with short cadence spectroscopic monitoring of lensed quasars, through the timing of kinematically identified features within the broad emission lines. This provides a geometric determination of an angular diameter distance ratio complementary to standard probes, and as a result is a powerful new method of constraining cosmology.

Runaways from young star-forming regions in simulations and observations

Speaker: Christina Schoettler (University of Sheffield)

Date: 21 Nov 2019

Theory predicts that we should find fast ejected (runaway) stars of all masses around dense, young star-forming regions. N-body simulations show that the number and distribution of these ejected stars could be used to constrain the initial spatial and kinematic substructure of these regions, which in turn places constraints on the star formation process. Most current observations of these runaway stars are high-mass stars (O or B stars). Until now, observational limitations have made it difficult to find lower-mass runaways, but this has changed with the advent of Gaia. In this seminar, I will begin with an introduction into the dynamical evolution of young star-forming regions and how N-body simulations can help us to constrain some of their initial conditions. I will then turn to stars that have become unbound or ejected as runaways and what they might be able to tell us about the initial conditions in their birth regions. Using the Orion Nebula Cluster as an example, I will show what simulations predict about runaways from this region and how we can use Gaia to search for these runaways. Finally, I will present results from a Gaia DR2 search in the vicinity (∼100 pc) of the ONC, finding a number of runaway (and also walkaway star candidates at lower velocities) that could have been ejected from the ONC during its past dynamical evolution. I conclude by commenting on the likely initial conditions of the ONC, as constrained by these ejected stars.

Spin alignment of stars in old open clusters

Speaker: Enrico Corsaro (University of Catania)

Date: 7 Nov 2019

Stars originate by the gravitational collapse of a turbulent molecular cloud of a diffuse medium, and are often observed to form clusters. Stellar clusters therefore play an important role in our understanding of star formation and of the dynamical processes at play. However, investigating the cluster formation is difficult because the density of the molecular cloud undergoes a change of many orders of magnitude. Hierarchical-step approaches to decompose the problem into different stages are therefore required, as well as reliable assumptions on the initial conditions in the clouds. In this seminar I report on the use of the full potential of NASA Kepler asteroseismic observations coupled with 3D numerical simulations, to put for the first time strong constraints on the early formation stages of open clusters. Thanks to a Bayesian asteroseismic analysis of about 60 red giant members of NGC 6791 and NGC 6819, the two most populated open clusters observed in the nominal Kepler mission, a complete set of detailed oscillation mode properties for each star, with thousands of oscillation modes characterized, is obtained. I therefore show how these asteroseismic properties lead to a discovery about the rotation history of stellar clusters. Finally, the observational findings are compared with hydrodynamical simulations for stellar cluster formation to constrain the physical processes of turbulence, rotation, and magnetic fields that are in action during the collapse of the progenitor cloud into a proto-cluster. Some brief mentioning about other more recent works presented in the literature will also be given.

Stellar tidal streams outside the local group

Speaker: David Martinez Delgado (University of Heidelberg)

Date: 10 Oct 2019

Mergers and tidal interactions between massive galaxies and their dwarf satellites are a fundamental prediction of the Lambda-Cold Dark Matter cosmology. These events are thought to influence galaxy evolution throughout cosmic history and to provide important observational diagnostics of nonlinear structure formation. Thin stellar streams in the Milky Way and Andromeda galaxies are spectacular evidence for satellite disruption at the present day. However, constructing a statistically meaningful sample of tidal streams beyond our immediate cosmic neighborhood has proven a daunting observational challenge, and their potential for deepening our understanding of galaxy formation has yet to be realized. In this talk, I present the results of the Stellar Tidal Stream Survey in the last decade, a project devoted to undertake a systematic deep imaging survey of stellar tidal streams with amateur telescopes (and recently with deep archival DECam data) around a sample nearby galaxies, including many Milky Way analogues. The goal of our survey is to constrain the rate of occurrence of tidal streams in the local Universe and construct robust distributions of their mass, morphology and colour. These data will allow us, as part of this project, to make the first direct, quantitative comparisons of tidal stream statistics to predictions from state-of-the-art Lambda-CDM cosmological simulations.

Speaker: Andres Piatti (Astronomical Observatory of Cordoba)

Date: 3 Oct 2019

Speaker: Else Starkenburg

Date: 26 Oct 2017

Speaker: Mike Shara

Date: 1 Nov 2017

Speaker: Glenn Van de Ven

Date: 3 Nov 2017

Speaker: Chris Evans

Date: 9 Nov 2017

Speaker: Victor Debattista

Date: 23 Nov 2017

Speaker: Sergio Martinez

Date: 14 Dec 2017

Date: 22 Sep 2016

Speaker: Douglas Heggie (Edinburgh)

Date: 6 Oct 2016

Speaker: Luis Aguilar (IAE-UNAM)

Date: 2 Nov 2016

Speaker: Lucio Mayer (University of Zurich)

Date: 3 Nov 2016

Speaker: Rodrigo Ibata (Strasbourg)

Date: 10 Nov 2016

Speaker: Taysun Kimm (Cambridge)

Date: 24 Nov 2016

Speaker: Jackie Hodge (Leiden)

Date: 16 Feb 2017

Speaker: Clare Dobbs (Exeter)

Date: 2 Mar 2017

Speaker: Alis Deason (Durham)

Date: 16 Mar 2017

Speaker: Lia Athanassoula (Marseille)

Date: 18 Apr 2017

Speaker: Glenn van de Ven (MPIA)

Date: 18 May 2017

Speaker: Hans-Walter Rix (MPIA)

Date: 1 Jun 2017

Speaker: Chris Evans (ROE)

Date: 15 Jun 2017

Speaker: Paul Clark (Cardiff)

Date: 22 Jun 2017

Speaker: Jean Brodie

Date: 24 Sep 2015

Speaker: Holger Baumgardt

Date: 15 Oct 2015

Speaker: Peter Jonker

Date: 22 Oct 2015

Speaker: Chris Conselice

Date: 29 Oct 2015

Speaker: Greg Ruchti

Date: 5 Nov 2015

Speaker: Richard Alexander

Date: 19 Nov 2015

Speaker: Pierre-Alain Duc

Date: 26 Nov 2015

Speaker: Filippo Fraternali

Date: 3 Dec 2015

Speaker: Benjamin Moster

Date: 21 Jan 2016

Speaker: Melissa Ness

Date: 4 Feb 2016

Speaker: Stephen Wilkins

Date: 25 Feb 2016

Speaker: Nadine Neumayer

Date: 10 Mar 2016

Speaker: Alberto Sesana

Date: 7 Apr 2016

Speaker: Mark Wilkinson

Date: 21 Apr 2016

Speaker: Rosemary Wyse

Date: 5 May 2016

Speaker: Eugene Vasiliev

Date: 19 May 2016

Speaker: Denis Erkal

Date: 21 Feb 2015

Speaker: Alan Heavens

Date: 6 Mar 2015

Speaker: Monica Valluri

Date: 13 Mar 2015

Speaker: Claudia Maraston

Date: 3 Apr 2015

Speaker: Andrea Maccio

Date: 8 May 2015

Speaker: Jorge Penarrubia

Date: 15 May 2015

Speaker: Michela Mapelli

Date: 22 May 2015

Speaker: Daisuke Kawata

Date: 29 May 2015

Speaker: Laura Greggio

Date: 12 Jun 2015

Speaker: David Bacon

Date: 19 Jun 2015