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 2025:

Upcoming: 

Tues. Dec: Hank Corbett (UNC), remote

Tues. Dec: Marta Spinelli (OCA), remote

Tues. Dec : Jerry Xuan (UCLA), TBD

Tues. Nov 25th: Alexandra Tetarenko (Lethbridge), local, in-person

Tues. Nov 18th: Rosa Merida Gonzalez (Saint Mary’s), local, in-person

Tues. Nov 4th: Roland Bacon (CRAL), local, in-person

WST – The Wide Field Spectroscopic Survey Telescope

The WST project aims to design and construct an innovative 10-metre class wide-field spectroscopic survey telescope (WST) in the southern hemisphere. It will feature the parallel operation of two cutting-edge instruments: a high-multiplex (30,000), large field-of-view (3 square degrees) multi-object spectrograph (MOS) operating in both low- and high-resolution modes, and a giant panoramic integral field spectrograph (IFS).WST’s ambitious top-level requirements place it well ahead of all existing and planned facilities. In its first five years of operation, the MOS is expected to observe 300 million galaxies, 25 million stars at low resolution, and 2 million stars at high resolution, while the IFS will deliver 4 billion spectra. These capabilities will enable transformative science across a wide range of astrophysical domains. A key strength of WST lies in the synergy between its MOS and IFS instruments, offering highly complementary spectroscopic survey modes. This dual approach is central to the project’s scientific ambition. WST is envisioned as the next major facility for ESO following the ELT, and a proposal will be submitted in response to the upcoming ESO Call for Ideas in 2027. Given the project’s early stage, there is an opportunity to broaden the current collaboration—comprising leading institutes from nine European countries and Australia—by engaging additional communities interested in contributing to its development and scientific exploitation.

Tues. Oct 28th: Kaho Morii (CfA), remote 

Unveiling Early Stages of High-Mass Star Formation: Insights from Infrared Dark Clouds

High-mass star formation, a key process governing the energy budget of galaxies, remains one of the outstanding problems in astrophysics due to observational challenges arising from its short evolutionary timescales and the large distances at which it forms compared to low-mass stars. My research addresses these challenges by utilizing the Atacama Large Millimeter/submillimeter Array (ALMA) to probe the earliest stages of star formation, with a focus on infrared dark clouds (IRDCs), ideal laboratories for investigating the initial conditions of high-mass star formation. By constructing and analyzing the large, homogeneous sample of 839 cores embedded in IRDCs, I have obtained insights into the earliest stage of high-mass star formation. These findings provide strong support for the clump-fed scenario, implying that high-mass stars form through external gas accretion onto initially low- to intermediate-mass cores, in contrast to the traditional monolithic collapse model. In this talk, I will especially present the results about the physical properties of cores, including the fragmentation process and the core mass function, as well as the gas dynamics around cores. 

— Past —

Tues. Oct 21st: Laya Ghodsi (UBC), local, in-person (Recording)

The Multi-phase baryon cycle in massive cluster environments

Galactic star formation, and consequently the evolutionary path of galaxies, are shaped by their gas content within the interstellar medium (ISM), which directly fuels the formation of stars. The ISM is influenced by the interactions of galaxies with their close environment, the circumgalactic medium (CGM), a diffuse gas reservoir surrounding them. The CGM acts as a bridge between the ISM of galaxies and their cosmological environment, enabling gas flows that fuel star formation and redistribute metals throughout the medium. I will discuss the multi-temperature molecular baryon cycle of the brightest cluster galaxy (BCG) in the cool-core cluster MACS1931-26 at z=0.35. This galaxy hosts one of the largest known H₂ reservoirs, exhibits elevated star formation, and contains a radio-loud AGN. We trace cold gas (10-100 K) in MACS1931-26 using multiple CO and CI emission lines with ALMA and warm H₂ (100-1000 K) with JWST, spatially coincident with CO with similar kinematics. Intriguingly, the CGM shows a higher H₂ excitation temperature than the ISM, suggesting the presence of more energetic heating mechanisms, including shocks and AGN-driven X-ray emission. This highlights the CGM as a key site of feedback-regulated gas transformation. Moreover, I will discuss our plans to use upcoming JWST Cycle 4 NIRCam + MIRI spectroscopy (2–28 μm) to perform comprehensive radiative transfer and shock modeling, aiming to constrain heating sources and baryon cycle in the CGM and ISM. This pilot study lays the groundwork for a broader framework to trace baryon cycling in cool-core BCGs, leveraging the synergy of JWST and cold gas tracers as a transformative tool for CGM studies.

Tues. Oct 14th: Adam Johnson (HAA, Victoria), local in-person (Recording)

Beyond the Speckles: New Horizons in High-Contrast Imaging for Exoplanet Science

Direct imaging of exoplanets offers a uniquely powerful way of characterizing planetary atmospheres, surface conditions, and potential biological markers beyond our solar system. However, the extreme contrast and small angular separations involved present significant observational challenges that demand continual innovation in instrumentation and observing strategies. The work presented (my PhD dissertation) addresses these challenges through a coordinated set of developments in instrumentation for focal plane wavefront sensing, post-processing, calibration systems, and atmospheric tomography. The first contribution presents the design, implementation, and deployment of the SPIDERS instrument for the Subaru Telescope. This compact, modular platform integrates the fast atmospheric self-coherent camera technique (FAST), a Lyot-based low-order wavefront sensor (LLOWFS), and an imaging Fourier transform spectrometer (IFTS) to validate advanced wavefront control, spectral retrieval, and post-processing techniques. A custom ultra-low speed optical chopper (ULSOC) enables stable fringe modulation at subhertz frequencies, supporting focal plane wavefront sensing and coherent differential imaging (CDI). A dedicated motion control system further supports full-field broadband spectral acquisition through the IFTS. The second major contribution focuses on the CAL 2.0 upgrade for the Gemini Planet Imager (GPI). The system incorporates adapted versions of the focal plane mask (FPM) wheel and polarization modulator (PolMod) from the original CAL architecture, along with a facility-class ULSOC, enabling real-time focal plane wavefront correction and enhanced post-processing. Informed by lessons from SPIDERS, these upgrades are expected to improve contrast by up to two orders of magnitude beyond current state-of-the-art systems for bright stars at small angular separations. Finally, STARLITE proposes a constellation of small satellites carrying laser beacons in highly elliptical orbits, periodically aligning with ground-based observatories to enable tomographic reconstruction of atmospheric turbulence. A feasibility study is presented, evaluating orbital mechanics, optical performance, and reconstruction fidelity, demonstrating STARLITE’s potential to support diffraction-limited, high-contrast imaging for small separation targets. Together, these contributions advance the capabilities of direct imaging systems and establish a technical foundation for future instruments to probe terrestrial exoplanets and explore their potential for life with unprecedented precision.

Tues. Oct 7th: Jessica Lu (University of California, Berkeley), remote (Recording)

Milky Way Black Holes and the Tech to Find Them

The landscape for studies of stellar-mass black hole origins, evolution, and demographics has expanded dramatically not only with the detection of gravitational waves; but also the explosion of EM photometric and astrometric time domain surveys. Time domain microlensing surveys are particularly valuable for finding isolated black holes in our Milky Way. Microlensing probes black holes across the mass spectrum in a relatively unbiased manner. I will present observational results, including the first detection of a free-floating black hole, and population simulations that show how sensitive microlensing surveys are to the black hole mass function, binary fraction, and velocity distribution. I will also discuss the photometric and astrometric technologies needed to expand the sample of stellar mass black holes know in the Milky Way, including next-generation adaptive optics systems for precise astrometry, small space satellites for precise photometry over wide fields, and large space observatories, such as the Nancy Grace Roman Space Telescope, that simultaneously deliver photometry and astrometry for microlensing events. These technologies and observatories will likely expand the sample of known Milky Way black holes by 100x in the coming decade.

Tues. Sep 23rd: Gustavo Medina Toledo (University of Toronto), in-person (Recording)

The structure and accretion history of the Milky Way from the photometry and spectroscopy of halo RR Lyrae stars 

In the Lambda CDM framework, the extended halo of the Milky Way retains key information to place our Galaxy in a broad cosmological context. RR Lyrae stars (RRLs) are pulsating variable stars among the most used outer halo tracers, as they are intrinsically bright distance indicators ubiquitous in the halo. Therefore, studying their chemo-dynamics with dedicated and large surveys (photometric and spectroscopic) is crucial to reconstruct the accretion history of our Galaxy, its mass distribution, and the properties of the halo progenitors.

I will discuss the outcomes of a search for faint RRLs using Dark Energy Camera (DECam) data, as part of the Halo Outskirts With Variable Stars (HOWVAST) survey. We identify over 500 RRLs in ~400 sq. deg and report the tantalizing detection of 27 stars beyond 100 kpc from the Galactic center (out to ~300 kpc). With our sample, we infer the shape of the halo, contrast accretion models with observations, and check for asymmetries resulting from the dynamical response of the Galaxy to the infall of massive satellites. I will also report the results of our spectroscopic follow-up campaigns and what we learnt from the Milky Way accretion history from a multi-dimensional data analysis. Additionally, I will discuss a recent exploration of the rich dataset provided by the Dark Energy Spectroscopic Instrument (DESI) survey, that contains ~6,000 RR Lyrae stars with homogeneously-derived spectroscopic properties. We use the reported properties to investigate the physics of their pulsation, and the nature of galactic accretion events. Lastly, I will summarize our measurement of the Milky Way mass out to 250 kpc with DESI. I will highlight the tantalizing potential of DESI and its complementarity with other large spectroscopic and photometric surveys, all of which will allow us to shed new light on the formation and evolution of our Galaxy.

Tues. Sep 9th: Ryan Cloutier (McMaster University, Hamilton ON), in-person (Recording)

Aligned Stellar Obliquities for Two Hot Jupiter-hosting M Dwarfs: Implications for Hot Jupiter Formation

Do you like Rossiters? Of course you do! Rossiters are terrific. I like them too! I will present early results from our multi-year observing campaign to measure the Rossiter-McLaughlin (RM) effect of hot Jupiters around low-mass stars known as early M dwarfs. The RM effect–seen during a planetary transit–is sensitive to the host stellar obliquity relative to the planet’s orbital plane such that the RM effect informs our understanding of a planetary system’s dynamical history, and hence its formation/migration process. I will present detections of the RM effect using the Gemini-North/MAROON-X spectrograph for two hot Jupiters around M dwarfs (HJMDs), which represent just the second and third such detections in the literature for these types of planets. I will discuss the implications of our results on the formation of HJMDs compared to around more massive stars, around which hot Jupiters are 2-3x more common and therefore may form via alternative pathways.

Previous — Spring 2025

Tues. June 17: Bradley Meyers (Curtin University, Perth), in-person (Recording)

Science with the AusSRC: Building on the Precursors Towards the SKA

The Australian SKA Regional Centre (AusSRC) is Australia’s portion of the international SKA Regional Centre network, which aims to establish and build capability to support the Australian and international SKA science communities, thereby ushering in the next era of astronomical discovery and advanced data processing. In this presentation I will cover the broad and diverse scientific projects that the AusSRC facilitates, with particular focus on our current support for the Australian SKA Precursors, the Murchison Widefield Array (MWA) and the Australian SKA Pathfinder (ASKAP), and several large surveys within the domains of galactic and extragalactic astronomy, fast and slow transients, cosmology, and more. I will cover AusSRC’s future plans to transition into full support of the SKA science, building on top of our experience with the SKA precursors to develop the capabilities essential to facilitating science with the SKA.

Tues. June 10: Akshara Viswanathan (UVic), in-person (Recording)

Mapping the ancient Milky Way through metal-poor stars and stellar streams

The stellar halo of the Milky Way is a relic of cosmic archaeology—a diffuse, ancient structure shaped by billions of years of mergers and interactions. Among its stars, the most metal-poor populations and coherent tidal streams offer uniquely sensitive tracers of the Galaxy’s earliest building blocks and the dark matter-dominated halo that surrounds them.

In this talk, I will present a multi-faceted investigation of the Milky Way halo, uniting kinematic, chemical, and spatial information to reconstruct its formation history. I begin by mapping the halo with a novel catalogue of main-sequence stars selected via reduced proper motion—without the need for reliable parallax or radial velocity—allowing us to trace retrograde stellar streams like GD-1 and Jhelum with unprecedented details. These structures, remnants of long-disrupted satellites, illustrate how cold stellar debris encodes the dynamical evolution of the halo. To probe the halo’s early chemical evolution, I construct detailed metallicity distributions using Pristine photometric metallicities and Gaia astrometry. These reveal a sharp rise in extremely metal-poor stars with distance, suggesting variations in the mass spectrum and chemical yields of the earliest accreted systems. Expanding this view to the outer halo, I will present bright red giant catalogues reaching out to $\sim$100 kpc, uncovering both a growing metal-poor fraction and evidence of non-linear substructures, including a metal-poor stellar counterpart to the Magellanic Stream—tracing tidal disruption at the Milky Way’s edge.

The second half of the talk focuses on very and extremely metal-poor stars as chemical fossils of the early Galaxy. Using Gaia’s radial velocity spectrometer and the Pristine-Gaia synthetic catalogue, I assemble large samples of bright metal-poor stars ideal for spectroscopic follow-up. These stars not only populate the debris of major accretion events like Gaia-Enceladus and Sequoia but also help refine the chemodynamical signatures of ultra-faint and the most metal-poor stream such as the C-19 stream. Finally, I explore how the high-redshift Milky Way progenitor gradually spun up into a rotating, high-α disc, connecting metal-poor chemistry with early Galactic dynamics.

Together, these efforts map the faint, ancient skeleton of our Galaxy—where stellar streams trace the scars of disruption, and the chemistry of the most metal-poor stars preserves the memory of galaxies long since absorbed.
Most of the work I will present are in my recently published PhD thesis available here: https://research.rug.nl/en/publications/mapping-the-ancient-milky-way-through-metal-poor-stars

Tues. May 27: Yuki Okoda (NRC-HAA), in-person (Recording)

Dynamic Nature of Early Star Formation

Exploring the early stage of low-mass protostellar evolution is crucial for understanding the processes of star and planet formation, and ultimately, the origin of our solar system. Key questions include: How does a young protostar grow? When does disk formation begin? And how does the disk evolve—both physically and chemically? In this talk, I will present intriguing observational features during the early stage, such as drastic changes in outflow directions in a source with an early disk structure; an accretion shock located outside an expectedly tiny disk; and chemical diversity within disk/envelope systems. These findings suggest that early star formation is more dynamic and complex than previously thought.

Tues. May 20: Alex Riley (Durham University, UK), in-person (Recording — Audio starts at 10:35, sorry. You mostly miss only some introduction material)

The tangled web of disrupting satellites surrounding Milky Way-mass galaxies

Stellar streams provide a record of their host’s merger history and the opportunity to study stellar populations of older, disrupting galaxies. As more of these structures have been uncovered in wide-area digital sky surveys, there has been a push to characterize the full system of tidally disrupting satellites in simulations at Milky Way-mass and smaller scales. I will present my efforts to do so in two regimes: fully hydrodynamical simulations (Auriga) and “painting” stars onto the highest resolution cosmological simulation ever run of a Milky Way-mass halo (Aquarius). These simulations predict that (i) most satellites are disrupting to form faint tidal streams, (ii) many of these streams are detectable in Rubin/LSST-depth photometry, and (iii) most Milky Way satellites have their own accreted stellar haloes formed from mergers with smaller galaxies. These results suggest that upcoming surveys (DESI, LSST, ARRAKIHS) will unlock new tests of dark matter and hierarchical galaxy formation at the smallest scales.

Tues. May 6: Chris Martin (CalTech), in-person (Recording)

Imaging the Cosmic Web

The intergalactic medium (IGM) represents the dominant reservoir of baryons at high red- shift, traces the architecture of the cosmic web dominated by dark matter, and fuels on-going galaxy evolution. The IGM has been studied using Quasi-Stellar Objects (QSO) absorption lines including the Lyman alpha forest (LAF), which are unable to provide the information that emission maps would give. But because of the low surface brightness and extended, diffuse distribution, direct detection of an emission equivalent to the absorption LAF has not been possible with existing instrumentation and observational approaches. Using a purpose-built instrument, with nod-and-shuffle and dual-field subtraction, we have detected, for the first time, an emission Lyman α forest (ELAF). The emission forest is highly extended, shows filamentary morphology with filaments connecting galaxies, exhibits statistics like the ab- sorption Lyman α forest, displays spectra resembling the absorption forest, and is correlated with galaxy-traced overdensities consistent with bias like dark matter. We conclude that the ELAF may provide a new tool for tracing a significant fraction of the cosmic web of baryons and dark matter. Finally, I will present status of the Super-pressure STABLE Cosmic Web Imager (SSCWI) program, a Brinson Exploration Hub balloon experiment, focused on emission from the Circum-QSO, the Circum-Galactic Medium, and the cosmic web. SSCWI offers the opportunity to image the cosmic web in the local universe for the first time, and compare its properties to those at high redshift.

Tues. Apr 22: Joanna Woo (SFU), in-person (Recording)

The Diverse Lives of Galaxies

It has been long established that galaxies can be divided into those that star-forming spiral discs and those that are spheroids that have stopped forming stars long ago. Despite decades of attempts to fit galaxy evolution into a cookie-cutter model, it is becoming evident that galaxies grow and evolve from one class to the other through numerous evolutionary pathways. From passive evolution to the dramatic influence of black hole activity driving gaseous outflows, to the influence of the vast cosmic web, I will review the different drivers of galaxy evolution in light of the latest observational and theoretical discoveries. In particular I will highlight new research from my group that explores the influence of the cosmic web on the evolution of galaxy structure.

Tues. Apr 15: Cail Daley (CEA Saclay, France), in-person (Recording)

Cosmology with SPT-3G

The 10-meter South Pole Telescope (SPT) is designed to observe the Cosmic Microwave Background (CMB) and astrophysical foregrounds with observing bands at 95, 150, and 220 GHz. The third-generation SPT-3G experiment began its Main 1500 deg² survey in 2019, and the inclusion of SPT-3G’s Summer and Wide fields brings the combined Ext-10k survey area above 10,000 deg² (25% of the sky). A suite of primary anisotropy and lensing analyses using data from the 2019 and 2020 Main observing seasons is almost complete, and the first Bayesian polarization-only results were presented in Ge et al. (2024). Combined constraints from this dataset are comparable to those from the Planck satellite, and the Ext-10k survey will improve constraints by up to a factor of 2 relative to Planck. SPT-3G’s lensing maps will also be used to delens polarization data from the BICEP-Keck experiment in the search for primordial gravitational waves, and cross-correlations between polarization-based lensing maps and external mass tracers will provide insights into the growth of structure robust to foregrounds that can bias temperature-based lensing reconstructions. Finally, SPT-3G has conducted dedicated observations of the Euclid Deep Field South to be publicly released on a similar timescale to Euclid Q1, providing a rich testbed for cross-correlations between deep CMB and galaxy surveys. In this talk I will discuss recent and upcoming cosmology results from SPT-3G that are largely independent of Planck, allowing for consistency tests between the two data sets and potentially revealing evidence of new physics.

Tues. Apr 8: Maya Fishbach (UofT), in-person (Recording)

Gravitational Waves from the Stellar Graveyard

The LIGO-Virgo-KAGRA Collaboration has observed hundreds of gravitational-wave sources to date, including mergers between black holes, neutron stars, and mixed neutron star–black holes. These neutron stars and black holes connect many astrophysical puzzles, including the lives and deaths of stars, star cluster dynamics, cosmic chemical enrichment, and the expansion history of the Universe. I will describe some recent astrophysical lessons from gravitational-wave discoveries.

Tues. Apr 1st: Matt Holman (Harvard/CfA), in-person (Recording)

A Pan-STARRS Search for Distant Planets

I present a search for distant planets in Pan-STARRS1 data. This search has been calibrated by injecting an isotropic control
population of synthetic detections into Pan-STARRS1 source catalogs, providing a high-fidelity approximation to injecting synthetic sources at the image level. The search method is sensitive to a wide range of distances, as well as all rates and directions of motion. The search discovered and recovered 692 solar system objects, including 642 TNOs, 23 of which are dwarf planets. By raw number of detections, this is the third most productive Kuiper Belt survey to date, in spite of the fact that distances closer than 80 au were not explicitly searched. Although the search did not find Planet Nine or any other planetary objects, to date, it shows that the remaining parameter space for Planet Nine is highly concentrated in the galactic plane. A catalog-based approach to characterizing searches will be increasingly important for surveys such as Rubin, Euclid, and Roman, for which injecting synthetic moving sources directly into the images will be even more challenging.

Tues. Mar 25th: Michelle Kunimoto (UBC), in-person (Recording – apologies, missed first few minutes)

Exploring Exoplanet Demographics with Kepler, TESS, and Beyond

Exoplanet surveys have been spectacularly successful in identifying thousands of planets with breathtaking diversity. These discoveries help place the Solar System in context and inform our understanding of how planets form and evolve. Finding large numbers of planets also enables statistical studies of the exoplanet population, through which we can uncover which types of planets are more common than others and find correlations between planet abundance and the properties of stars. Exoplanet demographic studies are key for constraining planet formation and evolution theories. I will highlight contributions to the field of demographics, from large and small planet populations with NASA’s Kepler mission and its implications for the search for other Earths, to the exciting potential of NASA’s TESS mission to significantly expand our understanding of planet populations around more diverse stellar samples than were possible before. Finally, I will identify a set of important open questions that remain to be answered and outline future goals to push the field of demographics to new frontiers.

Winter 2025

Tues. Mar 11th: Christine Wilson (McMaster), in-person (Recording)

A tale of two mergers: molecular gas and star formation in extreme environments

The merger of two gas-rich galaxies can produce some of the highest rates of star formation found in the present-day Universe. Understanding how this burst of star formation is triggered has implications for understanding star formation at higher redshift, where star formation rates of 100 solar masses per year were much more common than they are today. I will present our recent work on measuring molecular gas properties at 100 pc resolution in two nearby mergers, the Antennae and NGC 3256. We find that molecular clouds in these galaxies have turbulent line widths and column densities that are significantly higher than those found in normal spiral galaxies. These different cloud properties likely play a key role in enabling rapid star formation in these galaxies.

Tues. Feb 25th: Zhibo Hau, in-person (Recording)

Development of Data Reduction Pipeline and Calibrators for High-Precision RV Measurements with 216-HRS

Radial velocity technique for exoplanet detection has made great achievements. It discovered the first exoplanets beyond the solar system, which was awarded the Nobel Prize in 2019, and has confirmed 1,100 exoplanets by now. The recent 30 years witnessed the sensitivity of RV measurement progress from several tens of m/s (ELODIE) to 0.4 m/s (ESPRESSO), with the aid of improvements in spectrograph optics, stability, and the application of new technologies, such as new wavelength calibrators and data reduction techniques. In recent years, an upgrade project for the high-resolution spectrograph (216-HRS) at the Chinese Xinglong 2.16-m Telescope has been implemented, aiming to enable rocky planet detection around M-dwarfs – whose typical RV signal is 1 m/s – by transforming it from an iodine-technique-based spectrograph to a HARPS-like fiber-fed spectrograph. During my doctoral research, I engaged in this project, focusing on addressing the challenges in calibration and data reduction. I developed a data reduction pipeline software for 216-HRS. This software processes raw images into high-quality spectra and enables calibration with both traditional calibrators, the Thorium-Argon lamp, and newly developed advanced calibrators, the astro-comb and the Fabry-Perot etalon (FPE). The pipeline achieved a photon-noise-limited 0.1 m/s calibration precision with the astro-comb and sub-1-m/s calibration precision with the temperature-stabilized solid FPE. It enabled 216-HRS to achieve the calibration precision necessary for detecting rocky exoplanets.

Tues. Feb 12th: Paul Bennet (STScI), in-person (Recording)

Orbital insights into distant Local Group dwarfs

In general, our current understanding of classical dwarf galaxies is that they are star-forming when in the field and quench via tidal forces and/or ram pressure stripping as they approach massive hosts. However, recent results from the Local Volume, such as the SAGA survey, have shown that our current understanding might be over-quenching dwarf galaxies. Many Milky Way (MW) analog galaxies having more star-forming low-mass satellites than expected. We have also observed quenched dwarf galaxies in the wider LG, distant from either M31 or the MW, contrary to what simulations predict. These quenched dwarfs are often explained as backsplash galaxies that have had a previous encounter with a massive host; but have now moved outside the virial radius. I will present our ongoing efforts to obtain new proper motions with HST for a sample of distant dwarf galaxies in the LG, significantly expanding both the number of systems for which we have full 6D position-velocity information and crucially looking at more distant dwarfs than previous samples. I will also discuss how our work shows that the star formation properties might not be so cleanly divided with quenched dwarfs and star forming dwarfs having separate interaction histories. 

Tues. January 7th: Anna Ho (Cornell), in-person (Recording)

Finding Relativistic Stellar Explosions as Fast Optical Transients

For the last half-century, relativistic outflows accompanying the final collapse of massive stars have predominantly been detected via high-energy emission, as long-duration gamma-ray bursts (GRBs). Yet, it has long been hypothesized that GRBs are the tip of the iceberg of relativistic stellar explosions, because the conditions required to produce and detect a GRB are contrived. I will present results from a search for relativistic stellar explosions using optical time-domain surveys. The emerging zoo includes afterglows at cosmological distances with no detected GRB, supernovae with luminous X-ray and radio emission, and a mysterious class of “fast blue optical transients” with minute-timescale optical flares at supernova-like luminosities. An understanding of the origin of these events and their relation to GRBs will be enabled by upcoming time-domain surveys in other bands, including X-ray, UV, and submillimeter.

Fall 2024

Tues. September 10: Jenny Greene (Princeton), virtual Recording

The Nature of “Little Red Dots” 
One of the most surprising results from JWST has been the discovery of a large population of compact red sources at z>4, with very red rest-frame optical colors, blue UV slopes, and broad Balmer lines. The compact sizes and luminous broad lines strongly suggest these objects are powered by accreting supermassive black holes, but their lack of evidence for X-ray emission or hot dust in the mid-infrared calls that conclusion into question. Regardless, their high number densities (~5% of the galaxy population) makes them an important new contribution to the high-redshift galaxy zoo. I will discuss our ongoing efforts to understand the nature of this population, and what they may teach us about the growth of black holes and galaxies.