In light of the COVID-19 pandemic and following the latest advice from the government relating to large gatherings, we have taken the decision to move our seminar series online.  Our seminars are Thursdays from 2-3pm.  For information on how to join virtually, or for any other queries regarding the seminars, please contact Stacy Kim.

Spring seminars

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.

Past seminars

Strange Transients in Strange Times

Speaker:  Morgan Fraser (University College Dublin)
Date:  15 October 2020

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.

Tidally induced morphology of late-type galaxies

Speaker:  Marcin Semczuk (University of Leicester)
Date:  22 October 2020

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. 

Disentangling the early history of the solar system

Speaker:  Simona Pirani (University of Copenhagen)
Date:  29 October 2020

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. 

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

Speaker:  Sebastian Trujillo-Gomez (Heidelberg University)
Date:  12 November 2020

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.  

How can observations teach us about binary evolution?

Speaker:  Gijs Neleman (Radboud University)
Date:  19 November 2020

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.

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

Speaker:  Paul Jofre (Universidad Diego Portales)
Date:  26 November 2020

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.

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

Speaker:  Yumi Choi (Space Telescope Science Institute)
Date:  10 December 2020

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.

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.

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Date: 3 Oct 2019

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