We host a mix of virtual and hybrid seminars every other Thursday from 1:30-2:30pm.  For information on how to join them, or for any other queries regarding the seminars, please contact Stacy Kim.

Spring Seminars

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

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

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

Autumn 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.

Past seminars