Seminars

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.

Autumn Seminars

This schedule is currently under construction, but will be updated as we fill out our line-up for the fall.

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

Past seminars