DAO Astronomy Colloquium

DAO Colloquium Schedule

We have a mixture of fully remote and in person talks. In person talks will take place in the LCR and will also be streamed live online via Zoom.

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

Archive of previous seminar schedules (2010-)



Fall 2023

Tuesday August 29: Neal Evans (U. Texas at Austin), in person

Tuesday September 5: Xuan Du (DRAO), in person Recording

Tuesday September 12: Anna Ordog (UBC-O), in person Recording

Tuesday September 19: Yifan Zhou (University of Virginia), remote Recording

Tuesday September 26: Jason Rowe (Bishop’s Unversity), remote Recording

Tuesday October 3: Mike Walmsley (U. Toronto), in person Recording

Tuesday October 10: Break

Tuesday October 17: Dan Zucker (Macquarie U.), in person Recording

Tuesday October 24:Mathew Lehnert (Centre de Recherche Astrophysique de Lyon), remote Recording

Tuesday October 31: Sebastiaan Haffert (U. Arizona), remote Recording

Tuesday November 7:Jacqueline Antwi-Danso (U. Toronto), remote Recording

Tuesday November 14: Christina Williams (U. Arizona), remote

Tuesday November 21: Matt McQuinn (U. Wash.), in person

Tuesday November 28: Caitlin Casey (U. Texas at Austin), remote

Tuesday December 5: Rowan Smith (Manchester), remote

Tuesday December 12: Stella Offner (U. Texas at Austin), remote

Tuesday December 19: Kristen Dage (McGill), remote



Winter/Spring 2023

Tues January 17, 11am, Norm Murray (CITA), in person Recording

Why the day is 24 hours long
The length of the day and the month are seen to be increasing, due to gravitational tidal torques. Geologic data suggests, however, that between about 2,000 million years ago (Ma) and 1,000 Ma, the length of day (LOD) was fixed at about 19.5 hours, while the length of the month was increasing. Following a suggestion by Zahnle and Walker in the 1980’s, I and my co-workers explore the hypothesis that the fixed LOD results from the Solar thermal atmospheric tide, which was stronger in the past, due in part to a resonance in Earth’s atmosphere. Absent this resonance, the LOD today would be around 60 hours. We use two global circulation models (or GCMs), PlaSim and LMD, to estimate the frequencies of normal modes (or free oscilliations) in Earth’s atmosphere, finding excellent agreement with recent measurements. Using the GCMs, we show that an atmospheric resonant period of 19.5 hours corresponds to a mean global surface temperature T in the range 40-50 C; the GCMs show that T could have been that high despite the lower Solar flux 2,000 Ma, if the partial pressure of CO2 was of order a tenth of a bar, compared to the present day value of 0.0004 bar. This is at the upper range of estimates 1,500 Ma based on geochemical and paleosoil evidence. Thermal tides are likely to have affected the length of day of many exoplanets.

Tues January 24, 11am, Kevin Casteels (HAA), in person Recording

Complex Spacetime and Luminosity Driven Expansion
The two pillars of modern physics, Quantum Mechanics and General Relativity, interpret reality in very different ways, the former as purely statistical, and latter purely geometrical. This apparent incompatibility can be reconciled through the use of complex math to describe spacetime. We will explore previous work done on Complex Spacetime including recent studies which show imaginary numbers aren’t merely a mathematical convenience, but essential to describe observation. A new model will be described which treats real spacetime as composed of two imaginary parts, or layers, which mix to create real space. One of the predictions of this model is that when mass is converted to energy, the real space metric expands by an amount proportional to the photon’s half wavelength. We will explore the consequences of this prediction, and calculate expansion rates for various objects. It is found that the Local Group of Galaxies produces a luminosity driven expansion rate of ~70 km/s, which is comparable to measurements of the Hubble flow. When considering galaxy clusters, the luminosity driven real space expansion would act to increase the observed velocity dispersions, giving virial mass estimates several times greater than the observed baryonic mass, potentially removing the need for large dark matter components in these systems.

Tues January 31, 11am, Melissa Graham (UWash), in person Recording

Supernova Science with Large Sky Surveys
As the endpoints of stellar evolution, sources of dust, and cosmological standard candles, supernovae are a useful and important astrophysical phenomenon to understand. In this talk I will describe how I use large sky surveys and targeted follow-up to constrain the physical nature of Type Ia supernovae (SN Ia), the thermonuclear explosions of carbon-oxygen white dwarf stars. I will also provide a look towards the future with the Rubin Observatory, which will detect millions of supernovae over its 10-year Legacy Survey of Space and Time (LSST). I am currently a research staff scientist at the University of Washington in Seattle, and I hold the roles of Data Management Science Analyst and Lead Community Scientist for the Rubin Observatory. This talk will also cover how individuals without Rubin data rights can participate in LSST science, and provide an inside look at how Rubin staff are preparing themselves — and the science community — for the data revolution of the LSST.


Tues February 7, 11am, Joan Najita (NOIRLab), on zoom Recording

Clues to Our History from Debris Disks and the Dynamics of Andromeda’s Halo

Abstract: The patterns we notice in astronomical data can provide simple but valuable clues to how systems evolve and to our own origins. I’ll share two examples. (1) The similar sizes of the spectacular rings observed in protoplanetary disks and debris disks suggest that they share a common origin. New calculations of the evolution of rings of pebbles and planetesimals lead us to a simple picture in which large protoplanetary disks evolve into the known bright debris disks, with our Solar System following a distinct evolutionary path that originates in compact disks. (2) Data recently obtained by DESI, the highly multiplexed multi-object spectrograph on the 4m Mayall Telescope on Kitt Peak, reveal delicate structures—streams, wedges, and chevrons—in the positions and velocities of individual stars: evidence of a recent galactic immigration event in exquisite detail. The observations may open a window onto our past, offering a view of what our own galaxy may have looked like billions of years ago. 

Tues February 14, 11am, Jessie Christiansen  (Caltech), in person Recording

Towards an Exoplanets Demographics Ladder
The NASA Kepler mission has provided its final planet candidate catalogue, the K2 mission has contributed another four years’ worth of data, and the NASA TESS mission has been churning out new planet discoveries at a rapid pace. The demographics of the exoplanet systems probed by these transiting exoplanet missions are complemented by the demographics probed by other techniques, including radial velocity, microlensing, and direct imaging. I will walk through the progress of the Kepler occurrence rate calculations, including some of the outstanding issues that are being tackled. I will present our new results from K2 and TESS, and outline how K2 and TESS will able to push the stellar parameter space in which we can explore occurrence rates beyond that examined by Kepler. Finally, I will highlight some of the pieces of the larger demographics puzzle – occurrence rate results from the other techniques that probe different stellar and exoplanet regimes – and progress to be made working to join those pieces together.

Thursday March 9, 11am, Mark Voit (MSU), in person Recording

Black Hole Masses in Massive Halos
Whether or not the mass of a supermassive black hole (MBH) is causally linked to the mass of the halo around it (Mhalo) has been hotly debated for at least two decades. I will present evidence supporting a direct proportionality between MBH and the binding energy of the halo’s baryons that extends from Milky Way scales up to at least Mhalo ~ 10^14 MSun. That relationship is consistent with models of black-hole feedback that rely on cumulative kinetic energy injection to quench star formation by lifting the halo’s baryons. I will also show why the IllustrisTNG simulations do not reproduce the observed MBH-Mhalo relationship, even though  baryon lifting through cumulative kinetic energy injection appears to be what quenches star formation in those simulations.


Tues March 14, 11am, Alex Cameron (Oxford), in person Recording

Chemical evolution of galaxies from the early Universe to the present day
Rest-frame optical emission lines offer one of our most powerful and well-studied probes of chemical abundances in the ISM of galaxies. With the advent of JWST/NIRSpec, we can now study rest-optical emission spectra at z>9, within the first 0.5 Gyr after the Big Bang, allowing us to measure metallicities in the interstellar medium (ISM) of galaxies over almost the entirety of cosmic history.
Meanwhile, integral-field unit (IFU) spectrographs on current and future ground-based telescopes are enabling new constraints to be placed on chemical abundances in different phases of galaxies, such as from emission lines in extended diffuse emission. The combination of these cutting-edge facilities presents an opportunity to track chemical enrichment throughout the Baryon cycle in unprecedented detail.
In this talk I will present early results from the JWST Advanced Deep Extragalactic Survey (JADES), a combined NIRSpec + NIRCam large GTO program studying galaxy evolution from z~2 out to the highest redshifts observable with JWST. In particular, I will focus on what these (and other) early JWST observations are already telling us about the ISM conditions in high redshift galaxies, including some very intriguing findings from a recently published spectrum of GN-z11 (an ultra-luminous galaxy at z=10.60).
I will also present results from the DUVET survey, an IFU survey targeting low-redshift starburst galaxies, in which we have present direct measurements of outflow metallicity, affording new constraints on how these galaxy-scale gas flows are shaping the chemical evolution of galaxies.
However, deriving chemical abundances is notoriously susceptible to systematic uncertainties.
I will discuss our recent efforts to model the emission lines measurements from outflows, which are helping to unpick the systematic uncertainties involved in making these novel outflow metallicity measurements.

Tues March 28, 11am, Tarraneh Eftekhari (Northwestern), in person

Uncovering the Elusive Origin of Fast Radio Bursts
Upgrades in multiple fast radio burst (FRB) experiments have led to a growing sample of precisely localized events, enabling host galaxy associations and detailed observations of the immediate environments surrounding FRBs. Such observations play a key role in elucidating the stellar populations that give rise to FRB progenitors. Indeed, host galaxy demographics, as well as the spatial offsets of FRBs from their host galaxy centers, can be used to inform progenitor channels. The localizations of two repeating FRBs to dwarf galaxies, their coincidence with persistent radio sources, and their large observed excess dispersion measures (DMs) stand in stark contrast to other localized events, which generally reside in more massive galaxies and exhibit modest excess DMs. Understanding this dichotomy among FRB hosts will provide critical insight into the stellar populations prevalent in FRB host galaxies and hence their likely progenitors. In this talk, I will review our current knowledge of FRB progenitors based on the properties of a small, but growing sample of host galaxies. I will also outline key follow-up observations that will lend to detailed characterizations of the galactic and local environments of FRBs, thereby shedding light on their progenitor channels. 

Tues April 4, 11am, Kartheik Iyer (Columbia), on zoom Recording

From cosmic dawn to cosmic noon: studying galaxy evolution using star formation histories in the era of JWST
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 these processes are thought to have characteristic timescales, studying their imprints on galaxy star formation histories (SFHs) helps quantify their contributions for a population of galaxies. In this talk, I will describe a framework for modeling galaxies as Gaussian processes, that can be applied to both theory and observations to (i) establish a formalism to study the variability of star formation across different timescales and analyze cosmological galaxy evolution simulations, and (ii) reconstruct the star formation histories of galaxies observed using HST and JWST (from the CANUCS and CEERS surveys). Taken together, simulations and observations leverage predictive power against observational constraints, and allow us to develop a fuller picture of how galaxies evolve over time.

Tues April 11,
11am, Melissa Amenouche (HAA), in person Recording

Type Ia Supernovae (SNeIa) Cosmology era with the Zwicky Transient Facility (ZTF)
 The accelerated expansion of the Universe was discovered in the late 90s with Type Ia Supernovae (SNe Ia), unveiling for the first time the existence of Dark Energy (DE). Multiple precise measurements have validated its existence since. It has led to the emergence of the standard model of cosmology (ΛCDM), which describes the content and evolution of the Universe with only six free parameters. The model is an excellent fit to early and late Universe observable like the Cosmic Microwave Background (CMB) data, Baryon Acoustic Oscillations (BAO) and SNe Ia measurements. However, unresolved tensions persist between early and late probes data, like the Hubble constant H0 tension, the structure growth parameter f𝝈8 and the yet unexplained nature of DE. SNe Ia are key to contribute to alleviate these tensions, they are the sole cosmological probe able to precisely map the late Universe (z<0.3). In the past two decades,  the number of SNe Ia has drastically increased along with our understanding of these objects and their ability to indicate distances. With ~3700 observed and spectroscopically classified SNe Ia, the Zwicky Transient Facility (ZTF, 2018-2024) provides the largest nearby (z < 0.1) SNeIa existing sample. I will present the ongoing efforts to unlock cosmological measurements with ZTF SNeIa sample and focus on sample selection. It is crucial to understand how SNe Ia are selected and ensure that their distances are unbiased. I will show how with a forward model that includes our understanding of SNe Ia and the real-time cadence of the survey we reproduced every SN Ia observed by ZTF in  the first three years along with the selection function characterisation of the next golden nearby SNeIa sample.

Tues April 18,
11am, Leonardo Ferreira (UVic), in person Recording

The Optical Morphologies of Galaxies beyond z~3 with JWST
The launch of JWST with its powerful infrared capabilities has opened up a new window for studying the resolved optical morphologies of galaxies beyond z~3. Prior to this, morphology studies in the early Universe were limited to the ultraviolet portion of the electromagnetic spectrum. This narrowed our ability to track the bulk of stellar masses in high redshift galaxies, as the UV mainly captures clumpy star formation and other energetic phenomena. We present our findings from a visual classification effort of 4265 galaxies observed with JWST at $1.5 < z < 8$ in the JWST CEERS observations that overlap with the CANDELS EGS field. By employing a simple classification scheme to produce disk/spheroid/peculiar classifications, we find a strong dominance of morphologically selected disk galaxies up to z~8, a far higher redshift than previously thought possible. Our results suggest that most stars in the Universe were likely formed in a disk galaxy, and that these morphological populations are consistent with predictions from cosmological simulations.

April 25, 11am, Hao He (McMaster), on zoom Recording

Evolution of Giant Molecular Clouds in Nearby Starburst Mergers
Starburst mergers represent a common population of galaxies in early universe and hence are great laboratories to study the star formation (SF) in these most extreme environments. Due to the rarity of mergers in the local universe, we do not have large observed samples that are close enough to study individual giant molecular cloud (GMC) properties. In this work, we employ idealized galaxy merger simulation based on the Feedback In Realistic Environments (FIRE-2) physics model to study how the properties of GMCs evolve at different merging stages. We conduct a pixel-by-pixel analysis of molecular gas properties in both the simulated control galaxies and galaxy major mergers. The simulated GMCs in the control galaxies follow a similar trend in a diagram of velocity dispersion (σv) versus gas surface density (Σmol) to the one observed in local spiral galaxies in the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For GMCs in simulated mergers, we see a significant increase of factor of 5  10 in both Σmol and σv during the second passage along with the fact that more than 50% of molecular gas is concentrated in the central 1 kpc region. GMCs during the second passage also exhibit much higher virial parameters (αvir) reaching 10  100, which suggests they are much less gravitationally bound compared to GMCs in normal spiral galaxies. Furthermore, we find that the increase in αvir happens at the same time as the increase in global star formation rate (SFR), which suggests stellar feedback is responsible for dispersing the gas. We also find that the gas depletion time (tdep) is significantly lower for high αvir GMCs during a starburst event. This is in contrast to the simple physical picture that low αvir GMCs are easier to collapse and form stars on shorter depletion times. This might suggest that some other physical mechanisms besides self-gravity are helping the GMCs in starbursting mergers collapse and form stars. Instead, we find significant anti-correlation between average Σmol for GMCs and tdep, which might be due to higher fraction of dense gas that leads to shorter tdep. In the future, we will gather a larger sample of observed starburst mergers (~40) with GMC-resolution CO 2-1 ALMA archival data to perform a comprehensive comparison between observation and simulation.

Friday April 28,
11am, Katherine de Kleer (CalTech), in person Recording

The Surface Environments of the Galilean Satellites
The galilean satellites of Jupiter provide a laboratory for studying geological activity and its role in surface-interior exchange on planet-scale bodies. The differing degrees of tidal heating in these four satellites play out clearly in the level and style of their activity, from the intense volcanism of Io through the elusive plumes of Europa to the remnants of ancient activity on Ganymede. Callisto orbits outside the orbital resonance, presenting a control case of a long-inactive moon. Active internal processes manifest on the surface in the form of heat and materials delivered from the interior, providing specific detectable signatures that act as windows into the interior. High-resolution multi-wavelength telescope datasets from visible and near-IR (HST/Keck) through millimeter (ALMA) wavelengths sense different depths, from the subsurface up through the tenuous exosphere. This talk will discuss how such datasets are helping us understand the surface environments of these objects and the links between their interiors, surfaces, and atmospheres, and will present some outstanding questions that may be addressed by upcoming missions and observatories.

Tues May 9,
11am, Pallavi Patil (NRAO), in person Recording

Multiwavelength Investigations of WISE-NVSS Selected Heavily Obscured Quasars at Cosmic Noon
AGN feedback at z ~ 1-3 is believed to occur in the presence of thick columns of gas and dust, leading to heavily obscured systems that are challenging to detect at optical/X-rays but are transparent at radio and MIR wavelengths. By combining MIR and radio diagnostics, we have identified a sample of 155 ultra-luminous and obscured quasars (0.4 < z < 3) selected to have extremely red MIR colors in WISE and compact,
bright radio emission in the NVSS/FIRST. I will present our ongoing multiwavelength efforts to understand the nature of this unique sample in the context of jet-ISM feedback. High-resolution studies with VLA and VLBA confirm that most of the sample is compact with angular scales <0.2” (1.7 kpc at z~2). A radio spectral analysis reveals many sources show peaked/curved spectra consistent with being young radio AGN. A snapshot imaging survey with the VLBA at 5 GHz of 90 sample sources reveals a range of small jet structures and also unresolved sources. I will also discuss our follow-up study that includes deep multi-frequency VLA imaging and 870μm ALMA continuum and line (CO and HCN) observations, LBT imaging and spectroscopy, and NuSTAR data revealing a Compton-thick AGN.  Overall, our sample is consistent with a population of recently triggered, young radio jets caught in a unique evolutionary stage in which they reside in a dense ISM. Finally, I discuss the implications of our study for understanding the impact of young jets on the ISM and star formation rates in powerful young AGNs.

Tues May 16,
11am, Alberto Saldana-Lopez (University of Geneva), in person Recording

A low-redshift look to reionization with star-forming galaxies
One of the current hot topics in galaxy evolution is the so-called Cosmic Reionization, the last major cosmic phase by which the Universe transitioned from a neutral to a (mostly) ionized state, happening around z = 6-9. The main responsible sources for reionization are still under debate, but star-forming galaxies seem to be the likeliest due to their high number density at early epochs respect to Active Galactic Nuclei.
However, directly probing ionizing (also called Lyman Continuum, LyC) radiation from galaxies at the Epoch of Reionization is difficult, since the escaping photons are likely absorbed by the remaining neutral gas in the intergalactic medium (IGM). Therefore, in order to study the role of star-forming galaxies during Reionization, we need to rely on indirect indicators of such emergent LyC radiation. The low-redshift Universe offers a unique window to study the properties of LyC-emitting galaxies, since the IGM attenuation is negligible and, at the same time, a full suite of multi-wavelength observations is available both from space and the ground.
Recent HST campaigns such as the Low-Redshift Lyman Continuum Survey (LzLCS) has nearly tripled the number of LyC detections at low-redshift. The LzLCS has revealed that compact star formation, high ionization parameters, strong and narrow LyA emission, low dust-attenuation and the presence weak absorption lines seem to characterize the spectra of the strongest LyC emitters. In this talk, I will present some of the main results of the LzLCS and explain how this data set has changed our understanding of the mechanisms for LyC escape. We will also discuss the implications of these results for the high redshift community in the JWST era.

Tues June 6,
11am, Vincent Henault-Brunet (St Mary’s), remote Recording

New Constraints on the Stellar Initial Mass Function of Globular Clusters
The initial mass function (IMF) of stars and its variation with environment is key to understanding star formation and star cluster evolution. Given their high densities, old ages, and large statistically significant numbers of stars of similar distance, age and composition, nearby globular clusters (GCs) represent a unique opportunity to study the IMF. In this talk, I will summarize recent results on the IMF of Milky Way GCs, both in the low-mass regime (<1 solar mass) thanks to HST stellar counts over the radial extent of clusters, and at higher masses (>1 solar mass) based on dynamical modelling of clusters that allows us to constrain the distribution of dark stellar remnants having higher-mass stars as progenitors. Using an extensive collection of locally measured mass functions, kinematic data, and dynamical models, we determined present-day global mass functions for a large sample of clusters. We find a strong correlation of the low-mass global mass function slopes of GCs with both their internal relaxation times and their lifetimes. Once dynamical effects are accounted for, we find evidence for a bottom-light IMF (a power-law slope shallower than the canonical Kroupa IMF below 1 solar mass). Above 1 solar mass, our inferred global mass function slopes do not depend on dynamical age, and our results suggest an IMF in this regime that is consistent with a canonical IMF. I will discuss implications of these results for the environmental dependency of the IMF, the use of GCs as calibrators for distant stellar populations, the self-enrichment of GCs, and the evolution of black hole populations in these systems.

Tues June 20, 11am, Matías Bravo (McMaster), In person Recording

New Constraints on the Stellar Initial Mass Function of Globular Clusters
Both environment and AGN play important roles in shaping the evolution of galaxies, though the relative importance of these mechanisms remains an area of active research. Less explored has been how the galaxy-AGN and galaxy-environment connections impact each other, due to current observational limitations. The combination of large surveys from upcoming observational facilities, both space-based (e.g, Athena, CASTOR, Roman) and ground-based (e.g., 4MOST, PFS, Rubin), promises to greatly expand the current range of measured redshift, environments, and AGN/galaxy properties. These surveys will not only enable a deeper understanding of the galaxy-environment and galaxy-AGN connections but also offer a view into the overall galaxy-AGN-environment interplay. Theoretical predictions for the evolution of the galaxy-AGN-environment scaling relations will allow us to best exploit the results from these surveys. In this talk, I will introduce the upcoming version 2.0 of the SHARK semi-analytic model, with a special focus on the new AGN models implemented, and explore some preliminary predictions for galaxy/SMBH/AGN properties across cosmic time.