Department of Astronomy & Physics

Colloquia & Current Events 2018 - 2019

Colloquia Abstracts
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The intergalactic medium at the end of reionization

Speaker: Laura Keating (CITA, University of Toronto)
Time: October 26, 2018 - 3:00 PM
Location: Loyola 171

 

Studying the luminous sources responsible for reionizing the Universe directly is difficult, as they are challenging to identify. An alternative method is to instead look for signatures of reionization in the diffuse gas between these sources. In this talk I will discuss my work modelling the intergalactic medium during the epoch of reionization, as seen through quasar absorption lines. I will make comparisons between high resolution cosmological radiative transfer simulations and observations of the high-redshift Lyman-alpha forest. I will show that the current data seem to point towards a very late reionization, with large islands of neutral gas still found below redshift 5.5.

 

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Speaker: Suresh Sivanandam (Dunlap Institute, University of Toronto)
Time: November 2, 2018 - 3:00 PM
Location: Atrium 101

 

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White Dwarfs in Globular Clusters and the Field

Speaker: Jason Kalirai (STScI)
Time: November 9, 2018 - 3:00 PM
Location: Atrium 101

 

The lifecycles of most stars in the Universe end in the white dwarf phase, where stars have exhausted their nuclear fuel and fade over time by radiating their stored thermal energy into space.  The simplicity of these stars and their straightforward cooling physics provides remarkable insights into their properties, and a link to the evolution of their parent populations. Without knowing the distance, a spectrum of an individual white dwarf can reveal its temperature, surface gravity, mass, cooling age, and luminosity.  Through a combination of high-performance space-based imaging and high-throughput ground-based spectroscopy, we've now probed the properties of white dwarfs in a wide range of populations, from the field Milky Way to open and globular clusters spanning a broad range of ages.  Interpretation of these data are providing new insights on stellar mass loss, stellar evolutionary time scales, the chronology of the Milky Way, the dynamical evolution of clusters, the initial mass function, and more.  In this talk, I'll summarize the latest results from observations of white dwarfs in the Milky Way, and offer some thoughts on how we can best leverage the next generation large ground-based telescopes and JWST to extend this work into new dimensions.

 

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Speaker: Gregory Sivakoff (University of Alberta)
Time: November 23, 2018 - 3:00 PM
Location: Atrium 101

 

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Speaker: Anna Sajina (Tufts University)
Time: November 30, 2018 - 3:00 PM
Location: Atrium 101

 

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Going wide and deep with the Hyper Suprime Camera Survey

Speaker: Dr. Jenny Greene (Princeton University)
Time: September 14, 2018 - 3:00 PM
Location: Atrium 101

 

Our ongoing imaging survey with the Hyper Suprime Camera (HSC) on the Subaru Telescope provides a rich data set for studying galaxy evolution, from the most massive elliptical galaxies to ultra low surface-brightness dwarfs. I will discuss ongoing projects that utilize the deep and wide HSC imaging to address the role of merging in the growth of black holes and galaxies, as well as our search for some of the most extreme low surface-brightness galaxies.

 

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Jetted Outflows Powered by Supermassive Black Holes

Speaker: Dr. Matthew Lister (Purdue University)
Time: September 21, 2018 - 3:00 PM
Location: Atrium 101

 

In the heart of many galaxies lie supermassive black holes responsible for launching extremely powerful relativistic plasma outflows that emit radiation strongly from radio to TeV gamma-rays. Because of the effects of special relativity, these jets from active galactic nuclei (AGN) can easily outshine their entire host galaxy if they are pointed
nearly directly at us. As a result, they dominate the sky away from our galactic plane, as catalogued by large surveys in gamma-rays and at short radio wavelengths. The properties of AGN jets have been studied extensively with the Very Long Baseline Array, which provides sufficiently high angular resolution that their sub-light year scale
evolution can be studied via time-lapse imaging. I will present results from the largest such study, MOJAVE, which has monitored over 400 AGN jets to date, and has revealed apparent superluminal motion, standing features, and slow precession-like behavior of energized channels within the flows. MOJAVE has also found strong correlations with AGN gamma-ray emission detected by NASA's Fermi observatory, which has found distinct sub-divisions within the AGN jet population, based on the peak frequency of their synchrotron radiation output. I will discuss current controversies regarding the high-peaked jet population, whose slow apparent speeds and lower synchrotron output appear to be inconsistent with the high Lorentz factors inferred from emission modeling of rapid variability seen at gamma-ray energies.

 

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Galaxies in the Early Universe: The view from the newest observations with Spitzer, ALMA, and HST

Speaker: Andreas Faisst (IPAC/Caltech)
Time: September 28, 2018 - 3:00 PM
Location: Atrium 101

 

Less than 1 billion year after the Big Bang (< 8% of today’s age of the Universe), the cosmic star formation rate and stellar mass density of galaxies increased by more than one order of magnitude, a gradient steeper than at any other time. Hence it is expected that galaxies during this early rapid growth phase show significantly different spatial and physical properties compared to galaxies at later times. I present how observations with Spitzer, the Atacama Large Millimeter Array (ALMA), and the HST help us to study and understand this important corner-stone phase of galaxy evolution. With forward modeling of the Spitzer broad-band colors, we measure the fluxes of optical emission lines in galaxies out to z = 6. This allows us to witness the rapid growth of these infant galaxies, to study (on a basic level) their inter-stellar medium (ISM) conditions, and to quantify their contribution to the reionization of hydrogen at z > 6. Observation with ALMA of the C+ emission at 158μm and the continuum bracketing it, enables a deep insight into the dust and far-infrared properties of these galaxies. Our very recent observations of a dozen galaxies at z = 6 reveal a population that has a large range in evolutionary stages already 1 billion years after the Big Bang. With our 70h ALMA program to extend these measurements to >100 galaxies at z = 4-6 (ALPINE) we will study this population in more detail.

 

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Rare Isotope beams (RIB) at TRIMF

Speaker: Friedhelm Ames
Time: October 4, 2018 - 3:00 PM
Location: Atrium 305

 

Rare isotopes are used in many scientific fields starting from basic nuclear physics to applications in material science and medicine. TRIUMF’s ISAC facility is one of the worldwide leading facilities to produce rare isotopes and providing rare isotope beams (RIB) to a variety of users. The isotopes are produced by nuclear reactions in solid targets which are bombarded by a high energy proton beam. The products are ionized in a variety of ion sources and separated by mass. Experiments can be performed either at low energies up to 60 keV or with further acceleration up to 10 MeV/u. The presentation will describe the technologies and processes used. Future improvements and  possibilities arising from the new ARIEL and CANREB projects will be discussed.

 

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Unveiling the secrets of quasar feedback: searching for multi-scale quasar winds with obscured and red quasars

Speaker: Rachael Alexandroff (Dunlap Institute, University of Toronto)
Time: October 5, 2018 - 3:00 PM
Location: Atrium 101

 

Black holes of masses one million to one billion times that of the sun are now believed to reside at the centers of almost all massive galaxies in the universe. Though small on the scale of a galaxy and negligible for the overall galaxy dynamics, these supermassive black holes are now thought to play a crucial role in the evolution of their host galaxy. Feedback from the actively accreting supermassive black hole or ‘quasar’, in the form of galaxy-scale winds or jets, is necessary to shut-off star formation and thus inhibit the growth of very massive galaxies and to drive correlations between black hole and galaxy properties.  Nevertheless, such a mechanism is not yet well supported by observational evidence, especially at the peak of galaxy formation (z~2.5).  I will present new multi-wavelength results from a sample of quasars at the peak of galaxy formation and black hole growth that may represent the “blowout phase” of AGN evolution where we see quasar feedback in action.  The combination of various observational techniques allows us to probe, if indirectly, scales from the broad-line region all the way to the host galaxy and to understand the nature of the multi-component outflow (both ionized and molecular components).  Keck spectropolarimetry results argue for the presence of dusty outflows on scales of the emission line region while near-infrared spectroscopy reveals some of the most extreme ionized gas velocities observed (> 5000 km/s), indicating wind speeds too large to be contained by the galaxy potential. Finally, intriguing new results from the Very Large Array (VLA) at lower redshift open the radio regime as a new front for studying quasar feedback using synchrotron emission from the shocks generated by quasar winds.  Taken together, these results provide a robust observational window into important mechanisms of galaxy growth and quenching via quasar feedback. 

 

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PhD Thesis Defense: Integrated Light Stellar Population Synthesis of Globular Clusters Using Non-Local Thermodynamic Equilibrium Modelling

Speaker: Mitchell Young
Time: October 18, 2018 - 1:00 AM
Location: Loyola 188

 

We present an investigation of the globular cluster (GC) stellar population syn-thesis method of McWilliam & Bernstein (2008), focusing on the impact of stellar atmospheric non-local thermodynamic equilibrium (NLTE) modeling effects.  For this purpose, we have generated comprehensive, fully NLTE libraries of both individual stellar spectra and continua and GC integrated light (IL) spectra and continua.  The stellar library spans large ranges in Teff, log g, and [M=H], and is reproduced for 0.5 and 1 M_Sun and two degrees of alpha-enhancement, covering the parameter space of GC population members.  The IL library spans 9 to 15 Gyr in age, and -1.790 to -0.253 in [M=H], covering the full range of Galactic GC ages, and the majority of the Galactic GC metallicity distribution.  The IL spectral library is used to investigate Johnson-Cousins-Bessel UBV IJK IL colours, sensitivity of IL spectral features to cluster age or metallicity, and for deriving the ages and metallicities of 10 Galactic and one extragalactic GC for which IL spectra are acquired from Colucci, Bernstein & McWilliam (2017).  The IL colours cofirm previously reported trends of GC reddening with increasing age or metallicity, and demonstrate that NLTE colours to be bluer than LTE by up to a few tens of millimagnitudes, as does increased alpha-enhancement. We find a dependence of a few millimagnitudes on the discretization resolution of the population CMDs when using the 25-30 boxes suggested in the literature.  This dependence is minimized when the number of boxes increased to 40-50.  We found 240 spectral features sensitive to either cluster age or metallicity, of which 209 are newly identified as GC diagnostic features.   We determine the best fit to the observed GC IL spectra, deriving ages for six of the 11 clusters, and metallicities for all of them.  The uncertainties of both the ages and metallicities are reduced by a factor of two to three times when fit with NLTE IL spectra when compared with those from fitting LTE spectra.

 

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