Seminars

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 roughly every other week.  For information on how to join virtually, or for any other queries regarding the seminars, please contact Stacy Kim.

Summer seminars

23 July:  Kate Daniel (Bryn Mawr)

Dynamical Chaos near Corotation: Limits on cold radial migration
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.

20 August:  Azadeh Fattahi (Durham University)

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

3 September:  Catherine Heymans (University of Edinburgh)

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

10 September:  Keith Hawkins (University of Texas, Austin)

Galactic Archaeology with Gaia and Large Spectroscopic Surveys
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.

24 September:  Richard Stancliffe (University of Hull)

The intermediate neutron capture process
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.

Fall seminars

15 October:  Morgan Fraser (University College Dublin)

Past seminars

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