DAO Astronomy Colloquium


DAO Astronomy Colloquium Schedule

Online via Zoom, Victoria

Tuesdays at 11am unless otherwise indicated with (***)

Archive of previous seminar schedules (2010-)
Link to remote connection information

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Winter/Spring 2022

Tues February 8th, 11am,  Zoom: Simon Blouin (U. of Victoria) Recording

White dwarf crystallization as revealed by Gaia: White dwarf evolution is often described as a simple cooling process that naturally provides accurate age determinations. In reality, fundamental aspects of white dwarf cooling remain poorly understood. The extent of current uncertainties has been clearly demonstrated by Gaia DR2, which revealed that some white dwarfs undergo multi-Gyr cooling delays not predicted by current cooling models. In this talk, I will present my recent theoretical work aimed at improving the constitutive physics of white dwarf cooling models (in particular the physics of core crystallization) and discuss outstanding uncertainties that should be addressed before white dwarf cosmochronology can reach its full potential.

Tues February 15th, 11am,  Zoom: Katja Fahrion (ESA) Recording

Deciphering the formation of nuclear star clusters: Nuclear star clusters (NSCs) are dense, massive star clusters found in the centres of at least 70% of all galaxies. NSCs are known to co-exist with central black holes (BHs), our own Milky Way being the most prominent example, and are known to follow the same scaling relations with host galaxy properties, suggesting a connected evolution. To understand the detailed buildup of galactic nuclei we can study NSCs that still contain records of their formation and evolution imprinted in their stellar populations and kinematics. Generally two main scenarios are discussed for NSC formation: in-situ from gas at the galactic centre or via the dissipationless accretion of globular clusters (GCs) that spiral inwards due to dynamical friction. Most likely, a mixture of both pathways is realized in nature, but the dominant channel nor the mass fractions are unknown. I will present recent results from integral-field spectroscopy of NSCs hosted by dwarfs and massive early-type galaxies that allow to constrain NSC formation for individual galaxies. These results indicate a clear dependence of the dominant NSC formation channel from GC-accretion to in-situ formation with increasing galaxy and NSC mass.

Tues February 22th, 11am,  Zoom: Freeke van de Voort (Cardiff U.) Recording

Cosmic gas flows in the circumgalactic medium: Galaxies are intimately connected to the environments they live in. The haloes around them contain the gas reservoir from which the galaxies grow, while galactic outflows heat and enrich this ‘circumgalactic medium’ (CGM). Using ‘zoom-in’ cosmological, magnetohydrodynamical simulations, I will discuss some of the physical and observable properties of the gas around galaxies, focusing on Milky Way-like systems. The simulations use a new refinement technique to reach orders of magnitude higher resolution in the CGM than the current state-of-the-art. These spatially refined simulations show that the CGM has more ‘cool’ gas than previously thought, which strongly affects predicted observables in the CGM: The neutral hydrogen column densities are greatly enhanced, more in line with observations. Furthermore, I will show how the presence of magnetic fields alters the gas flows into and out of galaxies, which results in less (metal) mixing and higher gas fractions inside the halo and changes the properties of the CGM. I will also briefly discuss the effects of the enhanced resolution and presence of magnetic fields on the galaxies themselves.

Tues March 8th, 11am,  Zoom: Bala Chaudhary (Dartmouth) Recording

Strategies for antiracist action in STEM

Tues March 15th, 11am,  Zoom: Steven Goldman (STScI) Recording

Probing the Evolution of Stars, Dust, and Galaxies using Evolved Stars: Asymptotic Giant Branch stars play a number of roles in the evolution of dust, galaxies, and the ISM. This complex and short-lived phase of evolution is the key to calibrating stellar and chemical evolutionary models, dust budget calculations, and understanding the regeneration of the Universe. AGB feedback is an important parameter in cosmological simulations and has also been invoked to explain the multiple stellar populations in globular clusters. Despite their importance, their mass-loss and dust production mechanisms remain unclear. I will discuss the results of recent large observing programs with the Hubble and Spitzer Space Telescopes (PHAT, SAGE, DUSTiNGS). These programs have allowed us to study the impact of AGB stars on their environment and vice-versa; how the late stages of all low-to-intermediate mass stars might impact massive galaxies, dwarf galaxies, and high-redshift galaxies. I will summarize the results of these observing programs, discuss the recent advancements in AGB modeling, and advocate for the future prospects of AGB studies in nearby galaxies.

Tues March 22nd, 11am,  Zoom: Victoria Strait (DAWN) Recording

What have we learned about early galaxies before JWST?: Constraints on physical properties of early galaxies in the redshift range z ~ 6-10 (just a few million years after the Big Bang) are key for a full understanding of the process of reionisation and early galaxy evolution, including the onset of star formation. I will present results on the highlights from my study of ~200 z ~ 6-10 galaxy candidates from the Reionisation Lensing Cluster Survey (RELICS) survey which utilises galaxy clusters as comic lenses to magnify faint sources. This will include a variety of results about specific exciting galaxies using data from the Hubble Space Telescope, Spitzer Space Telescope, and the Keck Telescopes. Additionally, I will discuss some exciting prospects for the recently launched James Webb Space Telescope. 

Tues March 29th, 11am,  Zoom: Alessandro Lupi (U. of Milan-Bicocca) Recording

Massive black holes in high-redshift quasar hosts: formation and evolution: The observations of high redshift quasars up to z~7 tell us that massive black holes (MBHs) were already in place, with masses well above 10^9 solar masses, when the Universe was less than 1 Gyr old. According to Soltan’s argument and the evolution of the quasar luminosity function, MBHs gain most of their mass via radiatively efficient accretion, hence we expect they formed early in the Universe as smaller seeds. To date, the common formation mechanism advocated to explain the most massive MBHs at high redshift is the direct collapse scenario, which leads to the formation of seed MBHs of about 10^4-5 Msun. However, because of the peculiar conditions required by this formation mechanism, its plausibility is still debated. After highlighting the main conditions required by this scenario, I will discuss whether the peculiar environment in which high-redshift massive galaxies evolve provides ideal conditions for the formation of such massive seeds, and the processes that may potentially inhibit the process. I will also discuss the subsequent evolution of these protogalaxies and their central MBHs up to the observed masses, the importance of the galaxy-BH interaction, and how the MBH obesity found by observations is not necessarily real.

Tues April 5th, 4pmZoom: Luca Cortese (ICRAR) Recording

Galaxy Transformation in the Local Universe

Tues April 12th, 11am,  Zoom: Tim Davis (Cardiff U.) Recording

Tracing out the darkness with cold gas: dynamically probing galaxy evolution: In this talk I will describe how mapping the dynamics of gas clouds in the centre of galaxies can help us to constrain a wide range of astrophysical problems. From the enigmatic relation between galaxies and their supermassive black holes, to the suppression of star-formation in dying galaxies, the dynamics of cold gas provides an ideal probe that can help us make progress. I will show how parsec resolution observations can be used to estimate the masses of supermassive black holes in galaxies across the Hubble sequence, and describe the WISDOM project, that aims to use this technique to constrain the importance of accreting SMBHs in galaxy quenching. I will go on to show that, contrary to expectations, molecular gas appears to be an important source of fuel for accretion in even low-luminosity, low-excitation active SMBHs. Finally, I will show that the deep potential wells of massive galaxies change the small-scale structure of the ISM, suppressing star formation, and helping to keep these objects quenched.

Tues April 26th, 11am,  Zoom: Chris Matzner (U. of Toronto) Recording

Astronomy in the Anthropocene: Rapid climate change, due primarily to burning fossil fuels, poses existential risks to humanity and the world’s ecosystems that demand a swift transition in all aspects of society.   As the most public-facing and publicly-engaged of sciences, astronomy bears a special responsibility to proactively face these complex challenges.   How should researchers and institutions respond?  I will highlight some emerging answers to this question.  I will also argue that being open-eyed about the forces shaping public perception and government policy, and bringing this perspective into our education, outreach, and communications, is just as important as curtailing our professional climate impacts.

Tues May 3rd, 11am,  Zoom: Shantanu Basu (Western U.) Recording

Supermassive Black Holes: The Mass Function and a Formation Channel: One of the ideas to explain the existence of supermassive black holes (SMBH) that are in place by z ~ 7 is that there was an earlier phase of very rapid accretion onto direct collapse black holes (DCBH) that started their lives with masses ~10^5 M_solar.  Working in this scenario, we show that the mass function of SMBH after a limited time period of growing formation rate paired with Eddington or super-Eddington accretion can be described as a broken power-law with two characteristic features. These features identify the growth rate and duration of the DCBH era. A lingering concern about the DCBH scenario however is the angular momentum barrier.  Using a set of three-dimensional magnetohydrodynamic (MHD) simulations, we show that the magnetic field efficiently extracts angular momentum from accreting gas and also enhances the coalescence rate of fragments. Almost all the fragments coalesce to the primary star. We conclude that the magnetic field strongly aids the direct collapse scenario of supermassive star formation.

Tues May 10th, 4pmZoom: Kathryn Grasha (ANU)