Colloquia & Current Events 2020 - 2021

Colloquia Abstracts
SUBJECT TO CHANGE! Be sure to check back often

A&P Colloquium series

To obtain the Zoom link for any online event, please email the Astronomy and Physics Department Secretary, Shannon Rhode, at


Friday 25 March 2021 @3pm AST, via Zoom

Speaker: Tyrone Woods (NRC Herzberg Astronomy & Astrophysics)

The First Stars and the Birth of the Most Massive, High-redshift Quasars 

Abstract: The discovery of billion-solar-mass quasars at redshift ~ 7 challenges our understanding of the early Universe — how did such massive objects form in the first billion years, and what can this tell us about their environments at Cosmic Dawn? Observations and theory increasingly favour a "heavy seed" or "direct collapse" scenario, in which the rapid accretion possible in some primordial halos leads to the formation of uniquely supermassive stars, which collapse to form the initial seeds of supermassive black holes. In this talk, I'll present systematic, self-consistent simulations of the evolution of these objects under realistic formation conditions, and propose observational diagnostics to decisively test the origin of high-z quasars using forthcoming next generation electromagnetic and gravitational wave observations. I'll also discuss the expected multiplicity of such supermassive stars and their subsequent interactions, as well as the unique observational signatures of primordial stellar populations which are intermediate in mass between supermassive objects and "typical" Pop III stars.



Friday, March 19 @3pm AST, via Zoom

Speaker: Dr. Sandro Tacchella (Harvard-Smithsonian Center for Astrophysics)

The diversity of building up the quiescent sequence at redshift z~1

Abstract: How and why galaxies grow in stellar mass and cease their star formation are key open questions of galaxy formation and evolution. I present evidence for a diversity of pathways for building up the quiescent galaxy population at early cosmic times. Specifically, I will present observational constraints on star-formation histories and quenching timescales by combining Keck DEIMOS spectroscopic data with >10-band photometry. I will discuss how one can self-consistently fit both photometric and spectroscopic data together with the tool Prospector, which allows fitting for non-parametric star-formation histories and complex stellar, nebular, and dust physics. Despite the apparent diversity, we find that the most massive, compact galaxies have formed their stars the earliest and most rapidly. Furthermore, from the star-formation history constraints, I will discuss how galaxies evolve about scaling relations (such as the star-forming main sequence) with cosmic time. Finally, I will relate these findings to numerical simulations (in particular IllustrisTNG), showing that the large diversity of quenching epochs and timescales challenge numerical models and point toward a combination of internal and external quenching mechanisms.


Friday, 12 March 2021, @3pm AST, via Zoom

Dr. Allison Man (University of British Columbia)

The rise and fall of star formation in galaxies

Abstract: Star formation and supermassive black hole growth were most active at cosmic noon (z~2). Massive galaxies appear to experience accelerated growth at early cosmic times, and eventually quench their star formation and become more bulge-dominated. Exactly why these transformations take place is unclear. Mergers and active galactic nuclei feedback are often invoked as explanations, but a consensus is yet to be reached. I will discuss how multiwavelength observations of stellar populations and multiphase gas of distant galaxies can shed new light on the complex problem of galaxy evolution.


Friday, 5 March 2021, @3pm AST, via Zoom

Dr. Johannes Zabl, Saint Mary’s University

Flows of gas around z~1 galaxies  

Abstract: Galaxies are embedded in a complex circumgalactic medium (CGM). Any valid model of galaxy evolution needs to include the exchanges of gas between the CGM and the galaxy at the centre of the halo. Unfortunately, it is observationally extremely challenging and to directly measure these flows. While the low-density gas can be studied through the absorption which it imprints on bright background sources, large samples of galaxy-absorber pairs with well-constrained galaxy properties and kinematics are required to interpret the absorption measurements. Fortunately, thanks to the panoramic IFU spectrograph MUSE, it has become feasible to collect such samples at z≈1, a time when the Universe was less than half as old as it is today. In this talk, I will present statistical results for distribution, properties, and kinematics of the cool (10^4 K) CGM gas (probed by MgII), as obtained from the MusE GAs FLOw and Wind (MEGAFLOW) survey. Further, I will show that deep MUSE observations begin to directly map the CGM around typical individual galaxies through MgII emission. I will conclude my talk with a peek at my other research interests. 


Friday  26 February 2021, @11:30am AST note unusual time

Dr. Junjie Mao (University of Strathclyde)

Ionized winds driven away from supermassive black holes

Active Galactic Nuclei (AGN) are the observed manifestation of inflow of matter onto supermassive black holes. Ionized winds driven away from black holes have also been observed, which might play an important role in the evolution of black holes and their host galaxies. In the X-ray band, three types of ionized winds have been observed so far. First, classical warm absorbers are identified with multiple narrow absorption lines with a typical outflow velocity of <1000 km/s. Second, ultrafast outflows with an outflow velocity up to a quarter of the speed of light are inferred from highly ionized Fe absorption lines in the hard X-ray band. Third, in the past few years, obscuring winds have been reported in a few nearby Seyfert galaxies. The putative disk winds lead to a pronounced flux drop in the soft X-ray band without any discrete absorption line features. It can last for several weeks (e.g., NGC 3783 and NGC 985) or years (e.g., NGC 5548 and Mrk 335). Contemporaneous observation features found in the UV and NIR bands might be associated with the soft X-ray obscuration. We still have many gaps in our understanding of AGN winds. More high-resolution and/or multi-wavelength observations are certainly required to unveil their nature.


Friday 19 February 2021– NO COLLQIUM (Winter break)


Friday12 February 2021, @3pm AST, via Zoom

Dr. Nicholas Martis (Saint Mary’s University/NRC-Herzberg)

Properties of Dusty Galaxies at Cosmic Noon

Motivated by earlier findings that dusty star-forming galaxies become more prevalent both with increasing stellar mass and increasing redshift, I investigate the stellar and dust properties of a stellar mass complete sample of massive and dusty galaxies at 1<z<4 by modelling their UV-to-infrared spectral energy distributions (SEDs) obtained from the combination of UltraVISTA DR3 photometry and Herschel PACS-SPIRE data using MAGPHYS. I evaluate the ability of the rest-frame UVJ color-color diagram to determine the star formation and dust obscuration properties for our sample. I construct median SEDs of massive, dusty galaxies as a function of redshift and star-formation activity (quiescent vs star-forming). Simultaneous modelling of the panchromatic SED allows us to quantify the contribution to the IR emission from dust heated by star formation rather than evolved stellar populations, which we find to be a crucial element in characterizing these galaxies correctly. I also investigate in more detail the properties of a subset of the most heavily dust-obscured sources in the UltraVISTA DR3 to determine their relationship to IR-selected and sub-millimeter galaxies (SMGs).  Finally, I will propose how this line of research can be extended through my current involvement with the CANUCS program using the James Webb Space Telescope.


Friday, 5 February 2021 @3pm AST, via Zoom

Dr. Marie-Lou Gendron-Marsolais (ESO/ALMA)

A journey into the Perseus cluster of galaxies: radio lobes, mini-halo and bent-jet radio galaxies

Jets created from accretion onto supermassive black holes release relativistic particles on large distances. These strongly affect the intracluster medium when located in the center of a brightest cluster galaxy. On the other hand, the hierarchical merging of subclusters and groups, from which cluster originate, also generates perturbations into the intracluster medium through shocks and turbulence, constituting a potential source of reacceleration for these particles. In this talk, I will present an overview of the deep multi-scale low radio frequency observations of the Perseus cluster obtained from the Karl G. Jansky Very Large Array, probing the non-thermal emission from the old, les energetic, particle population of these outflows. Our observations of this nearby relaxed cool core cluster have revealed a multitude of new structures associated with its radio lobes, mini-halo as well as the bent-jet radio galaxies harboring the cluster. These results show how such high-quality images at low radio-frequencies can bring a whole new dimension to our understanding of a galaxy cluster and add new constraints on the complex nature of diffuse radio emission in these environments.


Friday, 29 January 2021 @3pm AST, via Zoom

Dr. Thomas Wevers (European Southern Observatory, Santiago)

From the Eddington limit into quiescence with tidal disruption events 

Tidal disruption events have been heralded as probes of extreme accretion regimes, in particular as super-Eddington accretors, since the seminal papers over 30 years ago. However, observationally this field has only started to blossom in the last 5-10 years, with the advent of wide-field optical surveys and coordinated multi-wavelength follow-up facilities, particularly at X-ray and UV wavelengths. These efforts are now producing a steady stream of exquisite observational constraints for the formation and evolution of the accretion flows, including the disk, soft excess and the corona, in the aftermath of disruption. I will discuss TDE observations in the framework of accretion states as developed for X-ray binaries and show that they are in excellent agreement with predicted properties. I will also present new results that probe, for the first time, accretion state transitions in an individual supermassive black hole, from the Eddington limit into quiescence. These results open up a new avenue to study accretion physics and its scale (in)variance across the mass scale in the near future. 


Friday, 22 January 2021: **1-2pm AST** (note unusual time)

Dr. Trisha Bhowmik (Universidad Diego Portales, Chile)

High-contrast imaging of exoplanets and disks.

Planets are formed in the circumstellar environment when dust grains start accreting material from an optically thick circumstellar disk leaving behind young planets and occasionally a debris disk. Several techniques such as radial velocity, transit, and direct imaging techniques have become popular to discover and confirm such exoplanets. The advantage of direct imaging is in observing wide orbit planets and doing spectral characterization of an exoplanet atmosphere. Yet, an Earth-like exoplanet would be 10e8 to 10e9 times fainter than its Sun-like star, in near-infrared wavelengths. This requires extremely high contrast techniques. The backbone of high-contrast imaging is the use of adaptive optics, advanced coronagraphy, and post-processing techniques. Throughout the years this technique has been very successful in imaging circumstellar disks and recently has also started discovering circumplanetary disks.  In this talk, I will present the basic working principle of high-contrast imaging and the scientific advancements that have been done with this technique.


Friday, 15 January 2021 @3pm AST, via Zoom 

Lamiya Mowla (Dunlap Institute/UofT)

Breaking the Law and Getting it Right: A Revised View of the Relation between the Sizes and Masses of Galaxies

Galaxy morphology is one of the fundamental and oldest observational tools used to study the formation and evolution of galaxies. Decades of observations from the ground and thousands of orbits of extragalactic imaging by the Hubble Space Telescope (HST) have constructed a picture for the relation between the sizes and masses of galaxies and how it has evolved in the last 10 Gyrs. However, the origin of the size-mass relation is impossible to comprehend using observations alone, making today's semi analytic models and cosmological hydrodynamical simulations with multi-scale models of physical processes critical deciphering tools. Using semi-analytic models, I will demonstrate that the observed size-mass relation of galaxies can be derived from the halo mass - stellar mass relation. This suggests a straightforward relation between the size scale of dark matter halos and that of galaxies, with the effects of dust and young stars being the main remaining uncertainty. I will then present results from state-of-the-art cosmological hydrodynamical simulation SIMBA with dust radiative transfer package Powderday, used to disentangle the effect of dust attenuation on the sizes of galaxies, which remarkably changes the picture of the size-mass relation as painted by HST. Resolved stellar population synthesis modelling augmented by upcoming JWST observation will be needed to verify our revised view of the size-mass relation of galaxies.


Friday, 4 December @3pm AST, via Zoom 

Prof. Louise Edwards (CalPoly)

Galaxies Evolution in Cluster Cores and Outskirts.

When and how does environment impact the evolution of galaxies? We will approach this question by considering two extreme environments. First, the cores of massive clusters. Here, the largest, reddest galaxies of the local universe are found, Brightest Cluster Galaxies (BCGs). These galaxies are found mixed with diffuse intracluster light (ICL) and often on top of the cooling intracluster medium (ICM). We'll explore recent results from an integral field unit survey of Local cluster cores which provides photometric and spectroscopic evidence of a break in age, between the old red and dead BCG cores, and the ICL that surrounds them. Second, cluster scale filaments. Here, galaxies find themselves in moderate density environments, where they are potentially able to interact with each other, and the intracluster medium, initiating starbursts and undergoing quenching. Recent SITELLE observations from CFHT provide clear examples of merging filament galaxies. I will also show how bent jets from supermassive black holes can measure the density of the filament medium. We'll discover how within these environments, the galaxy’s ability to interact with the surrounding medium and surrounding galaxies has the potential to drive its own evolution.


Friday, 27 November @3pm, via Zoom 

Prof. Harvey Richer (UBC)

Gaia, Star Clusters and Massive White Dwarfs

In this talk I’ll present some very recent work we have been doing on trying to understand the formation of massive white dwarf stars. The route to this will be exploitation of data from the recent Gaia satellite, which I’ll discuss, and the Gemini telescope in Hawaii. The results have implications for massive star evolution. the supernova rate in the galaxy, the formation rate of neutron stars and black holes and the chemical evolution in galaxies.


Friday, 20 November @ 3pm

Dr. Kartheik Iyer (Dunlap Institute/UofT)

Constraining galaxy evolution timescales with simulations and observations

A diverse range of physical processes are responsible for regulating star formation across galaxies. Understanding their relative contributions to galaxy growth and quenching at different epochs is one of the key questions in galaxy evolution today. Since processes like mergers, winds, and feedback from supernovae and active galactic nuclei (AGN) are thought to have characteristic time-scales, studying the strength of star formation rate (SFR) fluctuations on these time-scales allows us to disentangle their relative contributions for a population of galaxies. In this talk, I will give a brief summary of current work focusing on (i) establishing a formalism to study the stochasticity of star formation at a given time-scale and analyzing a variety of cosmological galaxy evolution simulations using this formalism, and (ii) observational methods of reconstructing star formation histories, which yield constraints on the time-scales of galaxy growth, morphological transformations, and quenching. Taken together, simulations and observations leverage the predictive power against observational constraints to obtain a fuller picture of how galaxies evolve over time.


Friday, 6 November @ 3pm

Dr. J Antonio Vazquez (UNAM, Mexico)

A new detailed visual morphological classification for galaxies in the MaNGA survey and a general characterisation.

Within the current theoretical scenario of galaxy formation and evolution, the interplay between cosmic cool gas accretion onto galaxies and galaxy mergers, and other internal processes, give rise to the observed morphological diversity of galaxies (e.g., Dubois et al. 2016). In this context, a reliable morphological classification becomes crucial for understanding the properties and formation mechanisms of galaxies, and constraining models and simulations of galaxy evolution.

In the last years surveys like MaNGA (an IFU survey), offer the possibility to connect physical and morphological properties of galaxies with their spatially-resolved properties at the kpc scales (e.g., Cano-Diaz et al. 2019). This positions the study of the origin of the Hubble sequence to a new level.

In this talk, I will present the results of a direct visual morphological classification to 4600 MaNGA galaxies, the identification of tidal debris (reported in the MVM-VAC, SDSS), and a general characterisation of some MaNGA properties with morphology. The classification and identification of tidal debris are based on a new uniform treatment of the SDSS and DESI Legacy Surveys images, which allows us to identify various internal structures and low surface brightness features for a more definite morphological classification. This classification recovers morphological trends and bimodalities observed by other authors, (e.g. Nair & Abraham 2010), and its reliability is good enough to be used to improve automatic classifications based on Machine Learning (e.g., Dominguez-Sanchez et al. 2018).


Friday, 30 October @12:30 pm **note unusual time**

Dr. Helena Domínguez Sánchez (Institute of Space Sciences, Barcelona)

Deep learning algorithms for morphological classification of galaxies

Galaxies exhibit a wide variety of morphologies that are strongly related to their star formation histories. Having large samples of morphologically classified galaxies is fundamental to understand their formation and evolution. I will present recent results on morphological classifications for SDSS and DES surveys obtained with Deep Learning (DL) algorithms using convolutional neural networks (CNN). Supervised DL algorithms are fast, accurate and efficient but they rely on large training sets (~5000 ) of pre-labelled galaxies.  I will show how transfer learning (i.e., the ability of CNNs to export knowledge acquired from an existing survey to a new dataset), helps reducing by almost one order of magnitude the necessary training sample for morphological classification. Another important caveat is that visually classified galaxies are usually very bright. We model fainter objects by simulating what the brighter objects with well determined classifications would look like if they were at higher redshifts. The CNNs reach 97% accuracy to mr ~ 21.5, suggesting that they are able to recover features hidden to the human eye. Where a comparison is possible, our classifications correlate very well with Sérsic index, ellipticity and spectral type, even for the fainter galaxies. We provide classifications for ~27 million galaxies, the largest multi-band catalog of automated galaxy morphologies to date.


Friday, 23 Oct @3pm, via Zoom 

Dr. Sebastian Kamman (Liverpool John Moores University)

Stellar Clusters as the Nurseries of Black Holes

The Local Group galaxies host massive star clusters across all ages, from ancient Galactic globular clusters to young massive clusters found in the Magellanic Clouds. Such clusters are the natural habitat of massive stars, which collapse into black holes once their atomic fuel is exhausted. While it is evident that many black holes will be formed in star clusters, little is known about the subsequent evolution of the newly formed black holes. Are they ejected from the clusters following natal kicks or interactions with each other? Or can they survive inside the clusters for a Hubble time? Answering these questions is of fundamental importance for interpreting the wealth of gravitational wave data expected in the coming years.

In my talk, I will describe how integral-field spectroscopy has opened a new window to search for black holes inside star clusters. In particular, I will show results from the ongoing MUSE survey of massive star clusters, which has already lead to the detection of several black holes in the Galactic globular cluster NGC3201. Such observations, in combination with sophisticated dynamical models, promise to provide us with a census of black holes in star clusters of all ages. They may also be key to finally answer the question of whether the enigmatic intermediate-mass black holes reside in the centres of massive clusters.


Friday, 16 Oct @3pm, via Zoom 

Dr. Nienke van der Marel (University of Victoria) 

On the origin, diversity, and fate of gapped protoplanetary disks

Protoplanetary disks with large inner cleared dust gaps are thought to host massive planetary companions. These gapped disks show a range of structures in the millimeter dust continuum, including asymmetries and one or multiple rings, caused by dust trapping in pressure bumps, and potentially vortices. However, it remains unclear why these asymmetric features appear in some disks and not in others. I will present a possible explanation for this phenomenon, based on the analysis of a sample of 16 disks with large scale dust rings and asymmetries using the local gas surface density profile. Second, I will explain which planets are likely responsible for clearing these wide gaps in addition to other features such as spiral arms, warps, and misalignments. In the second half of my talk, I will discuss the fate of gapped protoplanetary disks in comparison with disks without gaps. Disks without gaps are found to evolve faster, as dust drifts inwards. The structure of protoplanetary disks tells us directly about what kind of planets are formed and I have found a remarkable correlation between disk morphology and exoplanet demographics. I will highlight the efforts of graduate and undergraduate students in the UVic Protoplanetary disk group from the past year in the context of this work. 


Friday, 9 Oct – NO COLLOQUIUM


Friday, 2 Oct @3pm, via Zoom 

Dr. Darko Donevski (SISSA)

Knocking on Giants’ Doors: The evolution of the dust-to-stellar mass ratio in distant galaxies

Since their initial discovery 20 years ago, the distant, dusty star-forming galaxies possess a serious challenge to the theory of galaxy formation and evolution. On one hand, these massive systems form prodigious amounts of young stars but their light is heavily absorbed by dust and re-emitted in far-infrared. On the other hand, a significant number of these dusty “Giants” have been formed in the time when the Universe was very young (e.g. < 1 billion years), questioning how such a large amount of dust has been produced so rapidly. One way of answering this question is by analysing the ratio between the dust and stellar mass in distant galaxies. In this talk, I will present how to link the state-of-the-art simulations and the observations of 300 galaxies detected with ALMA, and apply the dust-to-stellar mass ratio as a tool to understand the complex processes involved in the production of dust, metals and stars in galaxy evolution.


Friday, 25 Sep, 3-4 pm, on Zoom

Astronomy & Physics Faculty Jamboree

Presenting (not necessarily in this order):

  • Dr. Greg Christian, “Stellar explosions in the lab: Understanding the influence of nuclear physics on galactic nucleosynthesis and stellar evolution.”
  • Dr. Vincent Hénault-Brunet, “Globular clusters inside out: from central black holes to the Milky Way halo”
  • Dr. Luigi Gallo, “The Ins and Outs of Black Hole Accretion”
  • Dr. Ritu Kanungo, “Unveiling a new era in nuclear science with rare isotopes”
  • Dr. Marcin Sawicki, “Probing galaxy evolution from space and from the ground”
  • Dr. Ivana Damjanov, “Galaxy evolution in the last 7 Gyr of cosmic time”



Knocking on Giants’ Doors: The evolution of the dust-to-stellar mass ratio in distant galaxies

Speaker: Dr. Darko Donevski (SISSA)
Time: October 2, 2020 - 3:00 PM
Location: Zoom


A&P Colloquium series


Friday, 2 Oct @3pm, via Zoom 


To obtain the Zoom link for this online event, please email the Astronomy and Physics Department Secretary, Shannon Rhode, at


Dr. Darko Donevski (SISSA)


Title: Knocking on Giants’ Doors: The evolution of the dust-to-stellar mass ratio in distant galaxies

Since their initial discovery 20 years ago, the distant, dusty star-forming galaxies possess a serious challenge to the theory of galaxy formation and evolution. On one hand, these massive systems form prodigious amounts of young stars but their light is heavily absorbed by dust and re-emitted in far-infrared. On the other hand, a significant number of these dusty “Giants” have been formed in the time when the Universe was very young (e.g. < 1 billion years), questioning how such a large amount of dust has been produced so rapidly. One way of answering this question is by analysing the ratio between the dust and stellar mass in distant galaxies. In this talk, I will present how to link the state-of-the-art simulations and the observations of 300 galaxies detected with ALMA, and apply the dust-to-stellar mass ratio as a tool to understand the complex processes involved in the production of dust, metals and stars in galaxy evolution.


<< Back

Undergrad Career Seminar I

Speaker: Jean-Marc Samson and Fernando Pena-Silva
Time: January 17, 2020 - 3:00 PM
Location: AT 101


Jean-Marc Samson (high school science teacher) and Fernando Pena-Silva (data scientist, Load Forecaster for Nova Scotia Power) will share their professional experience following studies in physics and astronomy, respectively. You can find their short bio below. There will be time for students to ask questions and also an opportunity for them to meet with the speakers for individual/small group discussions after the presentations.

Jean-Marc Samson started his physics training at Mount Allison university completing a BSc Honours in Physics, then continued his studies at Dalhousie University studying models of decoherence of spin states in quantum dots. Shortly after passing his doctoral candidacy exam, he decided to change careers and become a high school teacher. He travelled to Korea to teach ivy league bound students physics. He then came back to Canada and has taught a variety of math and science course at the junior high and high school level. He is currently teaching Math 10, Pre IB Math 10 and Physics 11 and 12.

Fernando Pena-Silva is originally from Chile, where he completed his BSc in Astronomy before coming to Canada for graduate studies, obtaining his PhD in Astrophysics from the University of Toronto. He was then a Postdoctoral fellow in Astrophysics at Saint Mary’s University and a Postdoctoral fellow in Medicine at Dalhousie University before moving to industry, first as a Senior Consultant at Ernst & Young Center for Advanced Analytics in Halifax, and now as a Load Forecaster for Nova Scotia Power. He has a passion for statistics, data science, math, research, electromagnetism, data visualizations… and soccer!


<< Back

Satellite Galaxies as Tracers of Dark Matter Halos

Speaker: Dr. Tereasa Brainerd (Boston University)
Time: November 22, 2019 - 3:00 PM
Location: Atrium 101


Small, faint satellite galaxies, orbiting within the gravitational potentials of large, bright “host” galaxies have the potential to place powerful constraints on the relationship between dark and luminous matter on scales < 500 kpc.  It has long been hoped, and a number of simulations have strongly suggested, that the satellites of isolated host galaxies are “fair tracers” of the dark matter distribution that surrounds the hosts.  Over the past decade or so, observational studies of the satellites of isolated host galaxies have led to a number of interesting conclusions.  In terms of their spatial distributions, satellite galaxies are, on average, distributed anisotropically with respect to their hosts’ major axes.  In addition, some observational studies have concluded that the radial density profiles of satellite galaxies (i.e., the number of satellites per unit area on the sky) are consistent with the expectations of the Cold Dark Matter (CDM) model, but others have suggested that the profiles are inconsistent with CDM.  Here I’ll discuss a number of recent observational and theoretical studies that are aimed at answering two key questions: [1] In the context of CDM, should we actually expect satellite galaxies to be fair tracers of the dark matter distribution around isolated host galaxies? and [2] Is the CDM model able to reproduce all of the observations of satellite galaxies that we currently have? 


<< Back

Uncovering the nature of dark matter with stellar streams in the Milky Way

Speaker: Dr. Ana Bonaca (Harvard-Smithsonian CfA)
Time: November 8, 2019 - 3:00 PM
Location: Atrium 101


Stars escaping globular clusters form thin, long and kinematically-cold tidal streams. In pristine conditions, these streams have nearly uniform density, however, new Gaia observations of one such structure in the Milky Way halo have revealed a likely site of perturbation. In this talk, I will show that the on-sky morphology suggests a recent, close encounter with a massive and dense perturber. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. This discovery opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Milky Way halo. I will discuss the observational and modeling efforts I am leading to follow up this feature, review prospects for discovering evidence of similar events in other stellar streams, and forecast the sensitivity of stellar streams to low-mass perturbers in the era of LSST. 


<< Back


Speaker: Dr. Priyamvada Natarajan (Yale University)
Time: November 1, 2019 - 3:00 PM
Location: Atrium 101


Nearly all galaxies appear to harbor a central supermassive black hole. The origin and properties of initial black hole seeds that grow to produce the detected supermassive black hole population are poorly constrained at present, as actively growing seeds are not directly observable near their birth epochs. Despite many open questions about the formation, fueling and feedback from accreting black holes in the universe, with multi-wavelength data, we have been able to successfully model the growing black hole population over cosmic time. I will present the current status of our understanding of the assembly history of supermassive black holes and the prospects of constraining our best-to-date models with data from upcoming missions.


<< Back

The Launching and Evolution of the Relativistic Jets of SS 433

Speaker: Dr. Herman L. Marshall (MIT Kavli Institute)
Time: October 25, 2019 - 3:00 PM
Location: Burke 221


In the binary system SS 433, oppositely directed, precessing jets emit line emission from highly ionized plasma moving at 0.26c from the compact object.  In high resolution spectra of SS 433 taken with the Chandra X-ray Observatory, we found a large Doppler shift change on a time scale of 20 ks, a time much shorter than the known dynamical times. The rapid change could be related to the formation and ejection of a jet knot, as observed in VLBI observations, perhaps as a leptonic jet impinges on a disk wind and shock heats it.  During eclipses of the jets by the companion star, we can estimate the length of the jet during its hot phase, cooling via expansion and X-ray line emission. Chandra imaging also shows X-ray emission well beyond the scale of the binary, after the jet has cooled to produce optical emission.  We address a model in which this emission results from reheating due to jet velocity variations.


<< Back

Studying Star Formation from the Stratosphere

Speaker: Dr. Laura Fissel (Queen’s University)
Time: October 11, 2019 - 3:00 PM
Location: Atrium 101


The conversion of interstellar molecular gas into stars is an extremely inefficient process, due to regulation from a combination of turbulent gas motions, magnetic fields, and feedback from young stars.  Of these the role played by magnetic fields is particularly poorly understood, largely because of the difficulty of making direct observations. In this talk I will discuss what we have learned about magnetic fields in star-forming regions using the Balloon-borne Large Aperture Sub-mm Telescope for Polarimetry (BLASTPol). BLASTPol operated from 38km above the Earth’s surface (above 99.5% of the atmosphere), mapping polarized radiation at sub-mm wavelengths from dust grains aligned with their local magnetic field. By statistically comparing BLASTPol-inferred magnetic field maps of the nearby giant molecular cloud Vela C with simulations, we find that magnetic fields play an important role in the formation of both low- and high-density molecular gas sub-structures.  I will finish by presenting our next-generation balloon-borne polarimeter, BLAST-TNG, which is scheduled for a first Antarctic flight this December. With BLAST-TNG we will map dozens of molecular clouds at 5x better resolution and quantitatively determine the extent to which magnetic fields affect star formation efficiency.


<< Back

At the crossroads of galaxy evolution and cosmology: multi-wavelength studies of galaxy clusters

Speaker: Dr. Stefania Amodeo (Cornell University)
Time: October 4, 2019 - 3:00 PM
Location: Atrium 101


In the nodes of the cosmic web, galaxy clusters retain a wealth of information on the evolution of cosmic structures, that are accessible through a variety of observables across the electromagnetic spectrum.

A key ingredient in cosmological studies that use the cluster mass function is a well calibrated relation between the observables and the mass. I will discuss the techniques used to estimate the cluster masses and their systematics, and I will present a mass calibration for a sample of Planck-selected clusters, using galaxy dynamics from Gemini optical spectroscopy.

I also will discuss the baryonic processes that govern the evolution of galaxies in clusters like star-formation, feedback and non-thermal pressure support, using observations of the intra-cluster gas through the Sunyaev-Zeldovich effect with the Atacama Cosmology Telescope and SDSS observations of the large-scale structure.

I will conclude moving to the high-redshift frontier to explore the role of the environment on galaxy evolution, and I will present results on the stellar populations of galaxies in protoclusters at z~2 from the CARLA (Clusters Around Radio-Loud AGN) survey.


<< Back

Searching for the smallest-mass supermassive black holes

Speaker: Dr. Holger Baumgardt (University of Queensland, Australia)
Time: September 26, 2019 - 3:00 PM
Location: **special time and place** Burke 218


Supermassive black holes are thought to exist in the centres of most massive galaxies and their masses have been found to correlate strongly with the properties of their host galaxies like overall luminosity or central velocity dispersion. Yet it is unknown what processes have established these correlations and if they continue towards lower mass systems. In my talk I will present results from our search for massive black holes in ultra-compact dwarf galaxies in nearby galaxies and in massive globular clusters of the Milky Way. I will also briefly discuss constraints on the retention fraction of stellar mass black holes in these systems.


<< Back

HectoMAP Galaxy Cluster Surveys

Speaker: Dr. Jubee Sohn (Harvard-Smithsonian Center for Astrophysics)
Time: September 20, 2019 - 3:00 PM
Location: Atrium 101


HectoMAP is a dense redshift survey covering a 53 deg^2 of the sky enabling various studies of galaxies and galaxy systems. We survey galaxy clusters based on the dense spectroscopic data. We first test photometrically identified redMaPPer clusters with spectroscopic data. More than 90% of redMaPPer clusters in HectoMAP have at least 10 spectroscopic members even at the low richness. We then construct galaxy clusters based on the HectoMAP and ROSAT All-Sky Survey (RASS) data. We identify 15 galaxy clusters (7 newly discovered) to the X-ray flux limit of the RASS. Three X-ray clusters covered by Subaru/Hyper Suprime-Cam survey are impressive; one of them shows strong lensing arcs. We predict that there are ~ 12000 detectable X-ray clusters in the RASS. I will discuss how the dense spectroscopic data contribute to expand our knowledge on galaxy clusters. 


<< Back

Narrow-line Seyfert 1 galaxies - young rebels of the AGN family

Speaker: Dr. Emilia Jarvela (UC Santa Barbara)
Time: September 13, 2019 - 3:00 PM
Location: Atrium 101


Narrow-line Seyfert 1 galaxies (NLS1s) are young active galactic nuclei (AGN). They harbour low-mass black holes accreting close to or above the Eddington limit and are preferentially hosted by spiral galaxies. So far ~20 NLS1s have been detected at gamma-rays, confirming the presence of powerful relativistic jets in them. This discovery contradicts the conventional view that only supermassive black holes residing in massive ellipticals are able to launch relativistic jets, and urges us to revise the evolution and unification schemes of AGN. On the other hand, NLS1s offer us an unprecedented opportunity to study the first stages of the AGN evolution.


Our knowledge of NLS1s is still very limited, excluding some outstanding individuals. Studying the whole population is challenging due to its seemingly heterogeneous nature; NLS1 class includes sources with powerful relativistic jets, as well as sources that are totally radio-silent. So far it has remained unclear what causes this disparity. To fully exploit their extraordinary nature we need to understand them better as a class.


In this talk I will give an overview of our latest studies aimed at starting to unveil the nature of the NLS1 population, and to clarify their place in the big picture of AGN. To this end, we have performed statistical studies of large samples, concentrating on their large-scale environment properties, and multiproperty principal component analysis. In addition, we have completed targeted studies of smaller samples, investigating, for example, their radio continuum and morphology properties, and host galaxies.


<< Back

MSc Defence: Investigating X-ray variability of AGN through spectral and temporal analyses: Mrk 530 & Mrk 335

Speaker: Ms. Hannah Ehler (SMU)
Time: August 16, 2019 - 10:30 AM
Location: Atrium 305


X-ray observations of two Seyfert AGN were analysed in an effort to characterise the nature of their variability. A spectral analysis was performed on a Suzaku observation of the Seyfert 1.5 AGN Mrk 530, and a timing analysis on XMM-Newton observations of the narrow-line Seyfert 1 Mrk 335. Mrk 530 was found to exhibit two distinct spectral states over the course of a single observation, as well as apparent periodic variability in the light curve. Three physical models (blurred reflection, partial covering, and soft Comptonisation) were explored to characterise the spectrum however the spectral fits were indistinguishable.  The timing analysis of Mrk 335 revealed evidence of non-stationary behaviour using multiple timing methods, though it is difficult to determine if these results are real or artifacts of undersampling. As such, further testing is required to confirm these results.


<< Back

Contour advection tracer method in smoothed particle hydrodynamics

Speaker: Ms. Tiffany Fields (SMU)
Time: August 15, 2019 - 9:30 AM
Location: Atrium 305


Studying the evolution of galaxies is important in understanding many other astrophysical phenomenon such as star formation history, the interstellar medium, and more. However, this process required numerical work due to the long timescales over which evolution occurs. To study the effects of chaotic mixing within a galactic disk, we implemented a contour advection method using tracers to follow the flow of the gas within a galactic disk. In this thesis, we examined the effects of integration properties such as timestep normalization, tracer mass, artificial viscosity felt by the tracers, temperature ceiling, and more to determine the ways that our tracer method could produce errors during evolution. We found that the primary issue with the tracer method was how the tracers experienced shock regions, or regions of steep density contrasts. When tracers experienced too much shock heating, they overheated and were ejected from the disk. When the tracers experienced too little shock heating, however, they flowed through the shock without feeling the effects. Both of these instances led to incorrect following of the gas flow.


<< Back

Diagnosing Light Pollution Sources: the physics and potential remediation

Speaker: Dr. Robert Stencel (Department of Physics and Astronomy, University of Denver)
Time: August 19, 2019 - 11:00 AM
Location: Loyola 171


In 1879, Thomas Edison was granted a patent for an improved version of an incandescent light bulb using a tungsten filament. Subsequently, improvements have enhanced the efficiency of lighting, most recently with the light emitting diode [LED, ref.1]. However, mis-aimed and reflected light at night waste energy, produce glare and trespass, and culminate in urban skyglow. Over metro Denver, for example, skyglow is measured to be more than 100X natural background, and increasing. This equates to hundreds of megaWatts of electrical power being wasted, a non-trivial fraction of the several gigaWatts consumed locally [2]. Considering the transition toward renewable energy sources and the growing demand for recharging electric vehicles, the inefficiency in night lighting energy can represent a substantial quantity of future energy needs. In addition, the spectrum of skyglow is trending toward shorter wavelengths as a result of widespread adoption of high color temperature (bluer) LEDs [3]. In this talk, I will describe some of the impacts of excess lighting, using local examples, touch upon the known and suspected health hazards of excessive blue/white lighting, and outline some cost effective solutions that can mitigate light pollution sources [4,5].



<< Back

Number Density Distribution of Satellite Galaxies around Massive Central Galaxies in CLAUDS + HSC at 0:3 < z < 0:9

Speaker: Mr. Lingjian Chen (SMU)
Time: August 19, 2019 - 1:00 PM
Location: Atrium 305


Satellite galaxies around massive central galaxies can provide important information forenvironmental effect of galaxy evolution in groups or clusters. In this paper, we selected thousands of massive (log(Mcen) > 11.15) central galaxies using 20 square degree deep (iAB = 27) imaging data from Subaru-HSCSSP and CFHT-CLAUDS at 0.3 < z < 0.9 and investigated radial number density distribution of the satellite galaxies around them with appropriate corrections applied. We found that the number density of satellites can be described by the NFW profile at large projected radii from central but deviate from it in inner regions (within 100 kpc). We also found that redshift evolution of number density enhances number density in inner regions more significantly than outer regions.  We found that central mass does not have a strong effect on slope of radial distribution of satellites, only the total number of satellites is different for different central masses. Galaxy conformity signal was found when separating star-forming and quiescent centrals. By dividing the satellite galaxy sample, we found that low massive satellite galaxies (9.5 < log(Msat) < 10.2) have more significant deviation from the NFW profile in inner regions than high mass satellites (log(Msat) > 10.2). Additionally, we compared our satellite distribution with IllustrisTNG simulation. Our results support the satellite population formation scenario in which the specific shape of the distribution is related to migration
of satellites, which involves various environmental effects such as dynamical friction and the tidal stripping.


<< Back

Knocking on giants' doors: Evolution of dust-to-stellar content of dusty high-z galaxies

Speaker: Dr. Darko Donevski
Time: March 13, 2020 - 3:00 PM
Location: Atrium 101


The pace of progress of our understanding of the physical nature of dusty star-forming galaxies (DSFGs) has been made in the last several years. However, linking their dust, stellar and SFR properties over cosmic time is extremely challenging task. On one hand, infrared selections of DSFGs in larger fields are restricted only to sources from the brightest-end of the luminosity function. On the other hand, due to the lack of information from shorter wavelengths, some important quantities such as stellar mass and AGN-fraction remain unconstrained. In this talk I will present how to benefit from rich panchromatic data available in deep fields, in order to estimate and interpret the cosmic evolution of dust-to-stellar content of the most massive star-forming galaxies.


<< Back

Observing planet formation in action in protoplanetary disks: current and future possibilities

Speaker: Dr. Nienke van der Marel
Time: March 13, 2020 - 3:00 PM
Location: Atrium 101


Previous Years' Abstracts


Share: Page Feedback