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-)

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Winter/Spring 2024

Tues. January 9: Jennifer West (DRAO), in person Recording

Progress on understanding the Galactic magnetic field
The pursuit of understanding the structure and origin of the Milky Way’s magnetic field is a central science question driving several radio polarization surveys, including the Polarization Sky Survey of the Universe’s Magnetism (POSSUM) from the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, the VLA Sky Survey, and The Global Magneto-Ionic Medium Survey (GMIMS). These surveys are poised to give us an unprecedented view of the polarized radio sky. I will give an overview of the expected science, status of progress we have made towards unraveling the answers to these questions, and discuss strategies for the management and analysis these massive datasets.

Tues. January 16: Trystyn Berg (DAO), in person

Astrophysics in absorption — exploring galaxy evolution with quasar absorption lines
Quasar absorption line systems are excellent probes of chemical enrichment across time. In particular, absorption line systems tracing the interstellar (ISM) and circumgalactic media (CGM) of galaxies are ideal environments for both measuring chemical enrichment and constraining the baryon cycle within galaxies. Furthermore, detailed comparison of abundance patterns to observed stellar abundances and predicted yields can provide insights into the progenitor stellar populations that made up these galaxies. In this talk, I will first highlight how the metal enrichment of galactic gas reservoirs provides a key constraint on feedback processes using data from the XQ-100 survey. Using these results to identify where to look for the most metal-poor environments, I will then shift to more detailed chemical abundances studies of the most metal-poor gas reservoirs in order to pinpoint the properties of the first stars to form in the Universe. I will then conclude with what the future holds for pushing our knowledge of chemical enrichment with quasar absorbers.

Tues. January 23: Chris Mann (DAO), in person Recording

TESS exoplanet candidate follow-up with ground- and space-based instruments 
The Transiting Exoplanet Survey Satellite (TESS) mission has done a fantastic job of discovering exoplanets over the last few years by detecting the small drop in stellar brightness as a planet passes between us and the star (i.e. a transit).  Not only has it found many thousands of candidates, it has focused on bright and nearby targets that are well suited for detailed spectroscopic follow-up.  However, every one of these candidate exoplanets requires follow-up observations to rule out false positives and constrain physical and orbital parameters.  This huge effort is coordinated by the TESS Follow-up Observing Program (TFOP), a collection of professional and amateur researchers and telescope operators around the world.  
Posing a particular challenge, are the “long-period” TESS candidates (~100+ days) that show few or singular transits in the TESS data.  Without accurate period information, further characterization of the system grinds to a halt.  This is unfortunate, as these “temperate” exoplanets can provide unique insight on atmospheric structures and processes.  My Ph.D. research involved contributing to the general TFOP effort, and also providing dedicated follow-up effort on some of these challenging long-period targets.

Tues. January 30: Paul Ripoche (UBC), in person Recording

Insight into the faint galactic white-dwarf population: calibration tools for future deep synoptic surveys
In addition to studying galaxies and the early Universe, future deep synoptic surveys will be essential to our understanding of the structure and history of the Milky Way (MW) by providing unprecedented data on the faint galactic stellar populations. However, observing stars at unrivalled depths presents challenges in identification and calibration.
Using the unprecedented 27.1-mag median depth of the Canada-France-Hawai‘i Telescope Large Area U-band Deep Survey (CLAUDS), we precisely studied the galactic stellar populations among over 15 billion of astronomical objects, over 20 square degrees.
In the absence of parallax measurements in the CLAUDS data, we devised photometric methods relying solely on colours to select stars in a survey designed to observe galaxies. We demonstrated the usefulness of precise measurements in determining properties of foreground stars, particularly white dwarfs (WDs).
Furthermore, through the analysis of WDs in the CLAUDS data, we developed a powerful technique for reducing systematics in synoptic surveys, such as the Legacy Survey of Space and Time (LSST). This technique could significantly enhance data quality in future deep synoptic surveys.

Tues. February 6: Ryley Hill (UBC), in person Recording

Resolving the cosmic infrared background with JWST and ALMA
I will talk about combining archival ALMA data targeting the Hubble Ultra Deep Field to produce the deepest currently attainable 1-mm map of this key region. 45 galaxies are detected, 39 of which have JWST counterparts. A stacking analysis on the positions of about 2000 ALMA-undetected JWST galaxies yields a surprising amount of additional information – I will talk about the fraction of the cosmic infrared background resolved by JWST and ALMA, and argue that essentially all of the galaxies that contribute to the cosmic infrared background have now been accounted for.

Tues. February 13: Vincent Chambouleyron (UCSC), in person (not recorded)

Tues. February 20: Kristine Spekkens (RMC), in person Recording

Gas-Rich Galaxies as Cosmological Probes
The atomic gas (HI) content of nearby galaxies provides important insight into how they form and evolve within the standard cosmology. In this talk, I will highlight some of my group’s recent progress towards building statistical samples of gas-rich galaxies to use as cosmological probes. I will first focus on how sensitive HI searches in optically-selected dwarf satellite galaxy candidates are a powerful tool for measuring their structure and constraining formation models. I will then describe how the widefield WALLABY survey, now underway on the Australian SKA Pathfinder (ASKAP), is delivering the first spatially-resolved populations of HI disks for comparison with cosmological predictions. This work paves the way for probing HI disks across cosmic time using the SKA when it comes online towards the end of this decade.

Tues. February 27: Lisa Kewley (CfA Harvard), in person (no recording)

A new vision for the Center for Astrophysics | Harvard & Smithsonian
I will present the latest discoveries and developments at the Center for Astrophysics | Harvard & Smithsonian (CfA).  Our discoveries cover solar astrophysics, star formation and evolution, galaxy formation & evolution, extrasolar planets, black holes, and cosmology.  I will describe the latest ground and space-based technological developments at the CfA, including new space satellites, and compelling new instrumentation for current and future ground-based telescopes in the optical, infrared, IR, and X-rays, as well as for climate science.  I will discuss our challenges with Petabyte scale datasets and the application of AI to astronomical problems.  Finally, I will provide an overview of the diversity, inclusion and culture initiatives that are being implemented at the CfA, using evidence-based studies from the literature.

Tues. March 5: Matt Taylor (Calgary), in person Recording

Do Massive Black Holes Come in Small Packages? The Search for Central Black Holes in Low-mass Stellar Systems.
The black hole (BH) mass function in the local universe is a fundamental measure of the origin and growth of BHs over Cosmic time. Specifically, the occupation fraction of massive black holes in low-mass stellar systems such as dwarf galaxies is an important ingredient for competing theories on supermassive BH (SMBH) formation. Observational challenges limit available avenues to search for BHs in dwarfs, but one under-utilized strategy is to infer the presence of central BHs by their dynamical influence on the cores of compact stellar systems (CSSs) including nuclear star clusters, ultra-compact dwarf galaxies (UCDs), and compact elliptical galaxies. CSSs have interconnected (often galactic) formation histories, and this technique has recently been successful in detecting SMBHs in several nearby UCDs, thereby proving their galactic origins as the remnant nuclei of former dwarf galaxies tidally stripped of their outer stellar sheaths. With the launch of JWST a systematic census of BHs in CSSs is possible, as similar studies are prohibitively expensive using current ground or space-based facilities. We are capitalizing on JWST’s capabilities by using NIRSpec+IFU to obtain spatially resolved stellar kinematics of 18 carefully selected CSSs crossing nearly four decades in mass, all located in a single galaxy cluster environment (Virgo). In this talk, I will give an overview of these efforts including primary science aims and preliminary results based on JWST Cycle 1 operations.

Tues. March 12: Sarah Tuttle (Washington), in person Recording

Here Be Dragons: Exploring the galaxy/CGM interface through data and instrumental adventures
The circumgalactic medium has gone from barely an idea to a crucial part of our understanding of the regulatory galactic environment in about 20 years. I will present a variety of projects and approaches that my group is using to gain an understanding of the evolution of galaxies. We will start with some unusual local galaxies that seem to be quenching, and the steps we’ve taken to try to determine the source of that regulated star formation. We will explore HETDEX and in particular several projects exploring extended emission from Lyman Alpha emitters. And we’ll wrap up by discussing a proposed UV cubesat called Maratus where we are attempting to map the CGM in the FUV at relatively large scales, and some discussion about what comes next on larger scales in the UV from the US. 

Tues. March 19: Marta Reina-Campos (McMaster), in person (no recording)

Beacons in the sky: star clusters from UNIONS and JWST
Stellar clusters are found in virtually all galaxies in the Local Universe, and the JWST is quickly revealing them also at different cosmic epochs. The high-surface brightness of these gravitationally-bound stellar systems makes them excellent beacons across galactic environments, revealing the underlying distribution of mass around their host galaxy. In this talk, I will discuss the demographics of massive star cluster populations both in the Local Universe from the UNIONS survey, and located 3 Gyr in the past from public JWST data. I will discuss our selection procedure of massive clusters in elliptical galaxies within the UNIONS dataset, and show preliminary results of their luminosity, colour and radial distributions. I will also review recent results on the star cluster population in the galaxy cluster Abell 2744, and I will end by discussing how we are correlating these detailed observations with the distribution of mass around this cluster. 

Tues. March 26: Kristy McQuinn (Rutgers), virtual (no recording)

Resolved Stellar Populations Studies with the Hubble and James Webb Space Telescopes
The study of resolved stellar populations in nearby galaxies was transformed by the high sensitivity and spatial resolution of the Hubble Space Telescope (HST), and is now being transformed again with the more powerful James Webb Space Telescope (JWST). The results have far-reaching impacts ranging from providing the most detailed and quantitative measures of the star formation histories of low-mass galaxies and insights into their growth across cosmic timescales, to determining precise distances to galaxies throughout the Local Volume which place galaxy properties on an absolute scale and are essential inputs for measuring of the local value of the Hubble Constant.
In this talk, I will (i) highlight recent results from resolved star studies on very low-mass galaxies with HST data, (ii) present the first star formation history of a galaxy with JWST data alongside a comparison to HST data of the same field, and (iii) introduce new tip of the giant branch (TRGB) distance calibrations derived from HST infrared data and JWST data on nearby galaxies, the latter of which paves the way for a JWST-based measure of the Hubble Constant.
I’ll also provide highlights on the upcoming Nancy Grace Roman Space Telescope.

Tues. April 9: Siddarth Mishra-Sharma (MIT / CCA) virtual Recording

Illuminating the Dark Universe with Probabilistic Machine Learning
The coming years will witness an influx of astrophysical data that will enable us to map out the distribution of matter in the Universe, image billions of stars and galaxies, and create maps of the Milky Way to unprecedented precision. These observations will contain significant potential for challenging our current picture of cosmology and particle physics, e.g. regarding the nature of dark matter. At the same time, the complexity of the data and underlying physical models will present novel challenges. Showcasing applications to a range of different systems (e.g., dwarf galaxies, gravitational lenses, and large-scale structure), I will describe how maximizing the scientific return of astrophysical observations towards fundamental physics discovery will require a qualitative shift in how we interact with the data, centering machine learning as a powerful discovery tool, and an interdisciplinary approach intersecting the particle physics, astrophysics, and machine learning communities.

Tues. April 16: Rowan Smith (U St. Andrews), virtual (no recording)

How galactic environment alters molecular clouds and star formation.
Star forming clouds are not islands in and of themselves, but instead form out of, and are embedded within the ISM of our Milky Way galaxy. Large scale galactic structure such as spiral arms, and clustered supernovae feedback from previous generations of star formation drive large scale turbulence which influences fragmentation within the cloud. Differential rotation within the disc stretches out large filaments of HI, within which molecular filaments are embedded. Magnetic fields generated by the large-scale dynamo thread the galaxy and influence the alignment of filamentary clouds and how they fragment. In this talk I use the CLOUDFACTORY simulation suite of Smith et al. 2020 and its follow-ups to investigate how these factors affect how stars form in galaxies. The CLOUDFACTORY simulations are ideal for this task as they model the large-scale disc (20 kpc) of galaxy, but also go down to extremely high resolution in individual clouds (0.1pc’s) down to the scale of individual star forming cores. Using this we can investigate whether star formation is a purely local process or whether the wider galactic environment alters the efficiency, timescales, and clustering of star formation.

Tues. April 23: William Thompson (NRC HAA), in person (no recording)

Orbit modelling to detect exoplanets
To study exoplanets outside our solar system, we have at our disposal an ever-increasing smorgasbord of techniques. These range from the well-known transit and radial velocity methods, to the up-and-coming direct imaging, GAIA-Hipparcos absolute astrometry, and interferometry methods. Increasingly, these techniques are overlapping in sensitivity, allowing us to detect the same planet using two, or even three different kinds of data.
I will discuss some challenges and opportunities found by combining these disparate kinds of data and present results on several exciting exoplanets: HR 8799, 51 Eri b, Epsilon Eridani b, and more. I will further discuss how this kind of multi-technique modelling is powering three next generation direct imaging surveys, guiding our observations towards the most promising targets. 

Tues. April 30: Daniel Gilman (UToronto), virtual Recording

A guided tour of dark matter physics with quadruply-imaged quasars
The existence of dark matter explains astrophysical phenomena from sub-galactic length and mass scales to the properties of the CMB and large-scale structure. However, we lack a detailed understanding of the particle properties of dark matter, such as its mass, formation mechanism, and possible self-interactions. Strong gravitational lensing provides a purely gravitational means to constrain the particle nature of dark matter through inferences of the properties of dark matter halos. I will discuss how various particle dark matter models affect the abundance and internal structure of dark matter halos, and how we can use strong gravitational lenses observed by HST and JWST to test these predictions on sub-galactic scales.

Tues. May 7: Jamie Bock (JPL / Caltech), virtual Recording

SPHEREx:  An All-sky Infrared Spectral Survey Explorer Satellite
SPHEREx, a mission in NASA’s Medium Explorer (MIDEX) program, is an all-sky survey satellite designed to address three science goals with a single instrument, a wide-field spectral imager.  We will probe the physics of inflation by measuring non-Gaussianity by studying large-scale structure, surveying a large cosmological volume at low redshifts, complementing high-z surveys optimized to constrain dark energy. The origin of water and biogenic molecules will be investigated in all phases of planetary system formation – from molecular clouds to young stellar systems with protoplanetary disks – by measuring ice absorption spectra. We will chart the origin and history of galaxy formation by mapping large-scale spatial power in two deep fields located near the ecliptic poles. Following in the tradition of all-sky missions, SPHEREx will be the first all-sky near-infrared spectral survey, creating spectra (0.75 – 4.2 um at R = 40, and 4.2 – 5 um at R = 135) with high sensitivity using a cooled telescope with a wide field-of-view for large mapping speed.  During its two-year mission, planned to begin in early 2025, SPHEREx will produce four complete all-sky maps that will serve as a rich archive for the astronomy community.  With over a billion detected galaxies, hundreds of millions of high-quality stellar and galactic spectra, and over a million ice absorption spectra, the archive will enable diverse scientific investigations including studies of young stellar systems, brown dwarfs, high-redshift quasars, galaxy clusters, the interstellar medium, asteroids and comets.

Tues. May 14: Nathalie Palanque-Delabrouille (UC Berkley), virtual Recording

Tues. May 21: Cassi Lochhaas (CfA Harvard), virtual Recording

The Non-Equilibrium Circumgalactic Medium
The region of space surrounding galaxies, the circumgalactic medium (CGM), is the site of all gas flows into and out of galaxies and is therefore responsible for regulating or promoting galaxy growth. It has been only in the past couple decades that we have been able to observe this tenuous medium, and even more recently that we have been able to resolve it in cosmological simulations. I will discuss the classical theories for the CGM’s role in galaxy evolution and how new, high-resolution cosmological simulations have shown these theories need serious revision. Using the Figuring Out Gas & Galaxies In Enzo (FOGGIE) simulations that resolve the CGM of Milky Way-like galaxies in exquisite detail, I will describe how dynamic gas motions in the CGM like turbulence, rotation, or bulk radial flows drive the galactic ecosystem out of hydrostatic equilibrium and to lower temperatures than typically assumed, and also disrupt inflowing filaments of gas, ultimately affecting how galaxies accrete new gas to convert into stars. These results suggest that the CGM cannot be well-described using equilibrium assumptions and imply that perhaps the commonly-assumed balance of gas flows in galaxy evolution models need revision as well.

Tues. July 9: Pieter van Dokkum (Yale) (no recording)

Did JWST break the Universe? New insights into the origins of massive galaxies

Fall 2023

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

Why Is Star Formation So Slow?
The problem of slow star formation has been clear for nearly 50 years: simple estimations predict star formation rates more than 100 times what is observed in the Milky Way and other galaxies. Much ingenious theoretical work has been expended to solve this problem, enhancing our understanding of turbulence and feedback in molecular clouds, but the fundamental problem remains. This situation suggests a reconsideration of the basic assumption that underlies the  problem: that molecular clouds are bound entities. In the most complete catalog of structures from CO emission maps, most molecular clouds are unbound, ameliorating the problem. Combining this information with theoretical models of how the star formation rate depends on the initial virial parameter, along with considerations of how metallicity affects the conversion of CO luminosity into mass, leads to correct predictions for the Milky Way, both in total and as a function of Galactocentric radius. Application to other galaxies is also promising but complications exist.

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

Advance in front-end instrumentation at DRAO: from wideband feed antenna to modeling radio telescopes 
In this talk, I am going to cover the advances we have made at DRAO over the past few years at two forefronts of radio astronomical instrumentation: feed antenna development and better understanding our telescopes (in particular, their polarization properties). In the first part, I am going to introduce the development of a new concept of wideband feed antenna, with the product a 400-1800 MHz feed that is soon going to be commissioned on the DRAO Synthesis Telescope, expanding the current frequency coverage by a factor of 38. In the second part, I am going to present a pipeline that we are building to model radio telescopes in order to understand how well they measure partially polarized radio waves.

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

The polarised radio sky at the DRAO and the quest to understand the magnetized Milky Way
The Milky Way hosts an extensive and complicated Galactic magnetic field structure ranging in scale from stellar environments up to the Galactic spiral arms. Establishing a thorough understanding of the present-day three-dimensional magnetic field morphology is instrumental to developing a complete understanding of the physics of the Milky Way. The magnetized interstellar medium imprints signatures of its structure onto polarisation maps of the radio sky, and a wealth of information can be gained by studying how observed Galactic polarised synchrotron emission varies with frequency through the effect of Faraday rotation. In this talk, I present recent progress toward Galactic polarisation maps from the radio telescopes at the Dominion Radio Astrophysical Observatory (DRAO). Specifically, I show results from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope and the DRAO 15-m single-dish telescope. The data sets will form the low-frequency, Northern hemisphere contribution to the Global Magneto-Ionic Medium Survey (GMIMS), an ongoing international effort to map the entire polarised radio sky covering 300 to 1800 MHz that will yield unprecedented spatial coverage and Faraday rotation resolution for studying large-scale structures in the magnetized interstellar medium (ISM). All previously published and new GMIMS data sets are already yielding fascinating insights into the magnetic field structure, and I highlight some of the early science results. These include: differences between the large-scale patterns in the northern and southern Galactic hemispheres, which inform dynamo models of the magnetic field, examples of Faraday complexity towards objects such as HII regions, and potential methods to distinguish the layers of the magnetized ISM through comparisons of data sets probing different depths.

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

Directly imaging protoplanets: the HALPHA survey and beyond.
The direct-imaging detections of accreting young planets herald a new era of planet formation studies, enabling us to witness the process of planet assembly, understand mass accretion mechanisms, and unravel interactions between planets and their birth environment. Ground-based adaptive optics instrument detects planetary accretion by capturing the H⍺ emission from the accretion shock, and the forthcoming ELTs are expected to uncover a large sample of protoplanets. Complementing ground-based telescopes, the Hubble Space Telescope (HST), with its high-Strehl-ratio images and wide spectral range, is invaluable for observing faint host stars and covering ultraviolet wavelengths.
We recently embarked on the Hubble Accreting Luminous Protoplanets in H-Alpha (HALPHA) Survey, which targets accreting planets residing within gaps of transition disks. During my presentation, I will delve into the survey’s motivation, design, and early results. Moreover, I will provide insights into our comprehensive follow-up efforts, employing a combination of time series and multi-wavelength HST data to discern the nature of the protoplanet candidate AB Aur b. Our findings showcase HST’s powerful capability in high-contrast imaging observations and underscore the synergistic potential of space- and ground-based telescopes in directly probing planet formation. Our survey paves the way for characterizations of protoplanets in the ELT era.

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

The POET Mission: A Canadian space telescope for exoplanet astrophysics
POET is a proposed Canadian Microsatellite mission designed to characterize and discover transiting exoplanets.  A 20-cm all-reflective telescope will feed a trio of detectors to obtain simultaneous, high duty-cycle, photometry in the u (300-400 nm), Visible Near-Infrared (VNIR) (400-900 nm) and Short Wave Infrared (SWIR (900-1700 nm) bands to make precision measurements of exoplanet transits for atmospheric characterization and to detect transiting Earth-sized planets.  I will discuss the main science drivers of the mission including the impact of recent JWST observations.  POET was selected as a high priority for a Microsatellite mission by the Canadian community as part of the CASCA Long Range Plan 2020.  Advancement of the payload concept and technology development for the optical telescope assembly (OTA) are currently being carried out through the Space Technology Development Program of the Canadian Space Agency. POET is a collaboration between Bishop’s University, Western University, ABB and SFL-UTIAS.

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

Title: Reshaping Galaxy Zoo for the Deep Learning Era

Abstract: For the last 15 years, Galaxy Zoo has recruited online volunteers to classify the morphology of millions of galaxies. But Galaxy Zoo is changing. In this talk, I introduce the models we now use to predict how volunteers would describe a galaxy, and consider what these models mean for astronomers.
One clear consequence is scale; our latest catalog, Galaxy Zoo DESI, includes all 8.7M well-resolved galaxies in the DESI Legacy Surveys. But models also allow for entirely new science. They are easily adaptable to new surveys and new tasks, allowing anyone to make exactly their own morphology measurements. The models automatically flag unusual galaxies and make personalised suggestions for which unusual galaxies you might be most interested in. And our latest models (first presented here) can identify which pixels belong to spiral arms or bars. Applying our volunteer-powered models to Euclid and Rubin images, and combining the resulting measurements with ancillary data from DESI spectra and IFU surveys, will reveal how morphology influences galaxy evolution.

Tuesday October 10: Break

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

Title: From Little Things Big Things Grow: Tracing the Milky Way’s Assembly History

Abstract: Large galaxies like the Milky Way form hierarchically, with smaller systems merging and accreting to form increasingly massive structures. Evidence for this process can be found all around us: spatially in the form of stellar streams and tidally disrupting satellites in the Galactic halo; dynamically as stellar overdensities in phase space within the Galactic disk; and chemically, in the form of abundance patterns distinct from those of typical stars which formed in situ within the Milky Way. I will briefly outline what we know – or at least, think we know – about the Milky Way’s assembly history, highlighting the work of two ongoing Australian-led spectroscopic surveys, GALAH and S5, in identifying and characterising discrete accretion events. Recent studies with GALAH data clearly show the importance of abundance patterns for detecting accreted structures within the Galaxy, while S5 results I will discuss include evidence for the influence of the LMC on halo stream orbits, an apparent orbital bias in the stellar streams observed, and comparisons with FIRE simulation predictions for detectable streams. I will conclude with a look at the prospects of new facilities such as LSST for driving major advances in our understanding of how galaxies like the Milky Way grow. 

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

Title: Molecular gas in the proto-cluster and Circum-galactic media in the early universe

Abstract: One of the most outstanding questions in contemporary astrophysics is how did the intercluster/circum-galactic medium acquire its mass and how did it develop the multiphase gas phase distribution we observe today. I present several observational results, but focusing on one in particular, showing that the gas in the intracluster medium of distant proto-clusters and around distant galaxies can be partially molecular.  The distribution of the molecular gas is quite varied. Within this context, I discuss how such gas may have cooled to low temperatures and discuss several theoretical implications of our findings on the nature of gas cycle in distant proto-clusters and massive galaxies.

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

Title: High-contrast imaging: the science of detecting faint things next to bright things.

Abstract: High-contrast imaging (HCI) instrument use extreme adaptive optics and coronagraphy to search for faint objects, such as extrasolar planets, around bright stars. To-date, high-contrast imaging has been used almost exclusively to characterize young, self-luminous, massive exoplanets orbiting far from their stars at (near-)infrared wavelengths. However, more mature planets like Earth have cooled off during their evolution and emit little to no thermal emission at optical and near-infrared wavelengths. Starlight that reflects from the top of the atmosphere is the main source of light that will be observable from these planets. Observing reflected light at optical wavelengths will also give us a window into the formation of life in the form of bio-signatures. This signal is very challenging to detect because the Earth-like planets are roughly a billion times fainter than their host star and they also orbit at only a couple times the diffraction-limit. The next generation of extremely large ground-based telescopes (ELTs) and the upcoming habitable worlds observatory (HWO) have the capability to directly image Earth-like planets. However, there is still a major jump in technology that we need to overcome to enable such observations. We need advances in three core technologies: fast and accurate control of the wavefront to counteract dynamic disturbances such as atmospheric turbulence or thermal drifts, optics that block starlight by a factor up to ten billion while letting through planet light at the diffraction-limit and efficient science instruments coupled with advanced data post-processing techniques. I will show how we are using new optics manufacturing technologies to create high-contrast instruments that can reach the fundamental limits of imaging. Such instruments combined with the ELTs will be able to answer a question that has captivated mankind for millennia: are we alone?

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

Title: Too Big to Be?: Searching for the Most Massive Galaxies in the Distant Universe

Abstract: One of the unsolved problems in extragalactic astronomy is understanding the physics of how galaxies grow their stellar mass over cosmic time. Large-scale hydrodynamical simulations have been largely successful in matching the basic properties and number densities of galaxies at z < 2.5 (covering the past 11 Gyr). This has given us confidence in our understanding of the physics thatregulates star formation and quenching over most of cosmic history. However, at earlier cosmic times, simulations underestimate the number densities of massive galaxies by a shocking 1-2 orders of magnitude. While this issue has largely been overlooked for the past decade, recent JWST discoveries of massive galaxies observed at even earlier times than we thought possible have brought this tension with theory back to the limelight. In this talk, I will give an overview of the systematics contributing to this discrepancy between theory and observations, as well as our best attempts at addressing it using (1) medium-band galaxy surveys; (2) novel color-color selection methods; and (3) physically motivated star-formation histories. I will also discuss my upcoming JWST Cycle 2 program and a few others geared at obtaining precise redshifts, stellar masses, and chemical abundances of massive galaxies at z > 3. 

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

Title: The dark side of massive galaxies and new light with JWST

Abstract: Our most powerful telescopes have glimpsed galaxies in their early growth phase only a few billion years after the Big Bang. In past decades, galaxy surveys showed that the most massive galaxies in the Universe must have formed the earliest in cosmic time, in extreme but short-lived bursts of star-formation. However, owing to their optical faintness and extremely red spectral energy distributions, early massive galaxies have historically been challenging to study. JWST is now revealing the early phases of massive galaxy growth at high redshift, and in this talk I will discuss the numerous surprises and new questions raised since JWST launch. Ultimately a more comprehensive census of the early Universe will come from larger area surveys, and I will conclude with preliminary findings about the large-scale abundance of bright massive galaxies from the JWST wide-area PANORAMIC survey.

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

Title: A new concept to measure geometrically the expansion of the universe

Abstract: With several >~4 meter radio dishes in the outer solar system, it appears possible to measure the distances to fast radio bursts that originate hundreds of megaparsecs away and thereby measure the cosmic expansion history using a geometric method that is similar to the trilaterations of global satellite navigation systems (https://arxiv.org/abs/2210.07159
).  The sensitivity scales quadratically with dish separations such that distance measurements to bursts even on the other side of the observable universe may be possible.   Not only could this technique potentially provide a much more precise geometric constraint on the cosmic expansion history, but such
a mission could also provide interesting constraints on micro-Hertz gravitational waves, pulsars, the outer solar system, and the dark matter.   While this idea is ambitious, and there is certainly an argument that we do not need to measure the expansion better than we already have (which I will address), another reason to attend this talk is that this concept involves so much interesting physics related to GPS systems, very long baseline interferometry, plasma propagation effects, gravitational time delays, and
the diffuse outer solar system.

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

Title: Formed too Fast: Massive Galaxies at Cosmic Dawn

Abstract: The pace of galaxy growth in the early Universe offers one of the most accessible tests of the Lambda-CDM cosmological framework. A growing number of surprisingly massive galaxies are now being found in the first billion years after the Big Bang that push the limits of theoretical predictions. Unusually bright high-redshift galaxies discovered by JWST challenge our most fundamental models of how fast stars form.  Some of them contain overly massive black holes whose formation is uncharted. Massive dusty starbursts found with ALMA are requiring new explanations about early dust production.  The spatial distribution of massive galaxies within large scale structure may be more highly clustered than expected, which would impact the timescale and uniformity of reionization — the last major phase change of the Universe from a neutral to ionized medium.  I will present an overview of large, multi-wavelength observational campaigns I lead to place the first comprehensive constraints on the rarest, most massive galaxies to emerge at z>6 and the impact they in turn have on our interpretations of the early Universe. These efforts unite the unprecedented sensitivity of JWST together with ALMA and Keck to work towards the goal of definitively establishing the story of how and when the first galaxies assembled.

Tuesday December 5: Cancelled

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

Title: Our Lonely Sun: How Multiple Star Systems Form (or don’t)

Abstract: Most stars are born with one or more stellar companions. Observational advances over the last decade have enabled high-resolution, interferometric studies of forming multiple systems and statistical surveys of multiplicity in star-forming regions. These have yielded new insights into how such systems form and how multiplicity affects disk evolution and planetary architectures. In this talk, I will review recent observational discoveries of the youngest multiple systems. I will present the results of star cluster simulations modeling the formation and evolution of multiple systems, and I will discuss the role of dynamics and environment in setting stellar multiplicity.  Finally, I will highlight remaining numerical and observational challenges.

Tuesday December 19: Kristen Dage (McGill), remote Recording

Title: Ultraluminous X-ray Sources in Extragalactic Globular Clusters

Abstract: Currently, ultraluminous X-ray sources (ULXs) with globular cluster (GC) counterparts have been identified. This is exciting, as ULXs have been theorized as potential intermediate mass black holes. New black hole mergers detected by LIGO-Virgo may also be associated with GC’s, underscoring the importance of ULXs as a potential linkage between GC electromagnetic and gravitational wave source populations. GC ULXs show a diverse behaviour with regards to temporal variability, both on long (16 years) and short (~hours) timescales, in both the X-ray and optical wavelengths. They can switch on or off over the course of many years or remain at a constant luminosity. Some sources exhibit a long-term change in their luminosity with no discernible variability within the other observations, other sources show a stunning long-term variability while also demonstrating variability on the timescale of around four hours. I will undertake a comprehensive comparison of the temporal variability of the zoo of currently known GC ULXs, discuss the possible origins of some of the extreme variability observed, and how this informs on our knowledge of black hole populations in extragalactic globular clusters.

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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

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

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.

Tues 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

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

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