DAO Astronomy Colloquium


DAO Astronomy Colloquium Schedule

Online via Zoom, Victoria

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

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

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Fall 2020

Tues September 15th, 11am, Zoom: Silvia Toonen (U. of Birmingham) Recording

Stellar interactions and transients: In order to understand the Universe we live in, we must understand the diverse lives of stars, the fundamental building blocks of galaxies and stellar clusters. However, most stars do not evolve in isolation; they have one or more stellar companions that they can interact with. These interactions give rise to some of the most energetic events in the universe, e.g. stellar mergers, supernovae Type Ia explosions and gravitational wave sources. The last decade has revealed the existence of a large and diverse zoo of transients, but their origin or progenitor evolution is often unknown. In this talk I will show novel channels to induce stellar interactions and subsequent transients. I will present our latest results regarding the evolution of white dwarf binaries and their mergers, with implications for Galactic archaeology. Secondly, I will discuss how triple stars can evolve differently from binary stars, and show their potential as transient progenitors. While triple star systems are common, our understanding of their evolution has lagged behind compared to single and binary stars. I will conclude with revealing a new primary channel of binary evolution towards supernova type Ia events.

Tues September 22nd, 11am, Zoom: Fabio Pacucci (Harvard)  Recording

Detecting the Dawn of Black Holes: In this talk I discuss the quest to unravel two undetected populations of high-z black holes. The first population of black holes formed when the age of the Universe was less than 500 Myr, while by z=7 we have observations of fully-fledged, shining quasars. Shedding light on this cosmic period is one of the key tasks that the astronomical community will focus on in the next decade, due to its crucial importance for the early evolution of the Universe.The observational signatures of black hole seeds remain largely unexplored, and we are yet to detect these sources. I present detailed predictions for the spectra of different categories of seeds, and a photometric method to identify them in surveys. I explore the role that future facilities (e.g., JWST, Athena, Lynx, LISA) will play in this quest, by detecting both black hole accretion and mergers. To conclude, I show a detailed analysis of the requirements and the expectations for a comprehensive search of z > 10 black hole seeds. Moving at slightly lower redshifts, the discovery of the first strongly lensed quasar at z > 6 (J0439+1634, Fan et al. 2018) represents a breakthrough in our understanding of the early Universe. Pacucci & Loeb (2018) predict that the observed population of z > 6 quasars should contain many mildly magnified sources, with image separations below the resolution threshold. Additionally, current selection criteria should have missed a substantial population of lensed z > 6 quasars: WFIRST will likely play a crucial role in revealing this population. I estimate the fraction of undetected quasars as a function of the slope of the bright end of the quasar luminosity function. For steep values of this parameter, the vast majority of the z > 6 quasar population is lensed and still undetected. These “phantom quasars” would be misclassified and mixed up with low-z galaxies. I conclude by addressing how gravitational lensing can affect the inferred black hole mass distributions at z > 6, possibly having important implications for current theories of growth for early quasars.

Tues September 29th, 11am, Zoom: Benne Holwerda (U. of Louisville) Recording

A Search for High-Redshift Galaxies And An Accidental Galactic Survey of Dwarf Stars with Hubble: Current searches for high-redshift galaxies use a combination of near-infrared filters with one or more optical filters to check for contaminants. One of these contaminants are faint dwarf stars in the disk and halo of our own Milky Way Galaxy. Thus in the search for the galaxies that caused the Reionization of the Universe, an accidental census of Dwarf stars in the Milky Way was conducted as well. I report on one observational approach with the Hubble Space Telescope; the Brightest Origin of Reionizing Galaxies (BORG) survey. I will discuss the identification of the distant high-redshift galaxies, their unique properties, the faint brown and red dwarf stars (M-dwarfs) and our results on characterizing the shape of the Milky Way from their number. We identified stellar objects in the BoRG survey and mapped their distribution onto the Milky Way. Results are a thin disk (300pc), the rediscovery of the Sagittarius stream, a total count of 58 billion M-dwarfs in the Milky Way of which 7% reside in the halo. We also found a population of very bright, redshift eight galaxies (“Super-Eights”) that show evidence for strong nebular emission lines and direct observations of Lyman-alpha. The Super-Eights appear to reveal a very specific mode of star-formation that may have played a critical role in the Reionization of the Universe.

Tues October 6th, 11am, Zoom: Vivienne Baldassare (Washington State U.) Recording

Searching for active galactic nuclei in low-mass galaxies via optical variability:  The present-day population of supermassive black holes in low-mass galaxies offers a window into massive black hole formation in the early universe. While we cannot yet observe the formation of “black hole seeds” at high redshift, the fraction of small galaxies that host a supermassive black hole — and the properties of those black holes — are thought to depend on the mechanism by which these they form. However, black holes in the smallest galaxies can be difficult to find, requiring creative new approaches. I will discuss recent work showing that long-term optical photometric variability in low-mass galaxies can identify active galactic nuclei that are missed by other selection techniques. I will present an analysis of the nuclear variability of more than 70,000 nearby galaxies and discuss our sample of low-mass, variability-selected supermassive black holes. Using this sample, we are also able to place meaningful constraints on the present-day black hole occupation fraction at low galaxy stellar masses. 

Tues October 13th, 3pm***, Zoom: Madeline Marshall (HAA) Recording

The Host Galaxies of High-Redshift Quasars: Studying the host galaxies of high-redshift quasars provides vital insights into the early growth of supermassive black holes and the black hole—galaxy connection. Here I present an analysis of the properties and environments of the host galaxies of z=7 quasars from the BlueTides hydrodynamical simulation. I will also show our latest work using HST to obtain upper limits on the UV emission of z~6 quasar host galaxies. In combination, these two techniques will help to pave the way for making the first near-infrared detections of high-redshift quasar hosts with JWST.

Tues October 20th, 11am, Zoom: Greg Mosby (NASA) Recording

Estimating star formation histories from galaxy spectra and the path to life finding NIR detectors: The evolution of galaxies can be conveniently broken down into the evolution of their contents. We focus on the stellar content that can be observed, as the stars reflect information about the galaxy when they were formed. We approximate the stellar content and star formation histories of unresolved galaxies using stellar population modeling. We can use stellar population modeling of galaxies to test galaxy evolution and formation models. However, in the limit of low galaxy surface brightness, integrated spectra often have such low S/N that it hinders analysis with standard stellar population modeling techniques. To address this problem, we have developed a method that can recover galaxy star formation histories (SFHs) from rest frame optical spectra with S/N ~ 5 Å^-1 with a specific application to quasar host galaxies. We use the machine learning technique diffusion k-means to tailor the stellar population basis set, composed of 4 broad age bins, and it is successful in recovering a range of galaxy SFHs. Our method has the advantage in recovering information from quasar host galaxies and could also be applied to the analysis of other low S/N galaxy spectra such as that typically obtained for high redshift objects and integral field spectroscopic surveys. I have now begun using diffusion k-means to generate a multi-metallicity basis set to estimate the stellar mass and chemical evolution of unresolved galaxies. In addition, I have begun work to fully characterizing today’s HgCdTe photodiode arrays to lay the foundation for future near infrared detector development. Low read noise and well-characterized detectors are crucial in the emerging search for biosignatures in exoplanet atmospheres.

Tues October 27th, 11am, Zoom: Monika Soraisam (U. of Illinois at Urbana-Champaign) Recording

Big-Data Rush: Mining Time-Domain Data of Local Stellar Populations: With several large-scale sky surveys ongoing and recently completed, the era of Big Data has dawned on optical time-domain astronomy. Abundant archival data provide an opportunity to perform systematic studies of stellar variability in nearby galaxies, and thereby yield constraints on the effect of environmental factors, such as metallicity. I will present results from mining the data from the Palomar Transient Factory survey for photometric variability of luminous stars in the Andromeda Galaxy. On the other hand, the enormous volume and rate of data bring their own challenges for real-time applications. I will highlight recent progress in the development of infrastructure and analysis tools to tackle those challenges, with a focus on event-broker systems (e.g., ANTARES), which allow timely discovery and characterization of interesting short-lived astrophysical events by automatically triaging the overwhelming time-domain alert-data. As part of the broker, an efficient and effective algorithm for selecting rare and novel events is crucial in the big-data era. I will briefly describe such an algorithm I have designed in preparation for VRO/LSST alerts.

Tues November 3rdNRC Virtual Town Hall

Tues November 10th, NRC Awards Ceremony

Tues November 17th, 11am, Zoom: Davide Farnocchia (JPL/CalTech) Recording Unavailable

OSIRIS-REx and impact hazard assessment for asteroid (101955) Bennu: The OSIRIS-REx mission is part of NASA’s New Frontiers Program and is the first U.S. mission to retrieve a sample of an asteroid and carry it to Earth for further study. The target of OSIRIS-REx (101955) Bennu, a 500 m near-Earth asteroid that is expected to have organic compounds and water-bearing materials. OSIRIS-REx launched in September 2016 and rendezvoused with Bennu in late 2018. After two years of proximity operations, on 2020 October 20 OSIRIS-REx successfully executed the sample collection sequence and is now preparing for the return cruise to Earth to deploy the sample in 2023. In this talk, I will present the current status of the mission, highlighting some the main challenges faced by the mission and the most relevant scientific results. In particular, I will focus on the impact hazard problem. Bennu is a potentially hazardous asteroid and its trajectory is deterministic until the close approach with Earth in 2135. After this encounter, the orbital uncertainty of Bennu increases to a level that its trajectory can only be analyzed in a statistical sense, and impacts with Earth cannot be ruled out in the second part of the next century. Key to the modeling of Bennu’s trajectory is the Yarkovsky effect, a subtle nongravitational perturbation due to non-isotropic thermal radiation that causes a semimajor axis drift. The OSIRIS-REx tracking data provide useful constraints on this perturbation and on the Bennu hazard assessment.

Tues November 24th, 11am, Zoom: Ian Roederer (U. of Michigan) Recording

The astrophysical r-process: what we are learning from gravitational waves, dwarf galaxies, and stellar archaeology: Understanding the origin of the elements is one of the major challenges of modern astrophysics. The rapid neutron-capture process, or r-process, is one of the fundamental ways that stars produce the heaviest elements, but key aspects of the r-process are still poorly understood. I will describe four major advances in the last few years that have succeeded in confirming neutron star mergers as an important but perhaps not exclusive site of the r-process. These include the detection of freshly produced r-process material powering the kilonova associated with the merger of neutron stars detected via gravitational waves (GW170817), the identification of a dwarf galaxy where most of the stars are highly enhanced in r-process elements (Reticulum II), new connections between the r-process and its Galactic environment (thanks to data from the Gaia satellite), and advances in deriving abundances of previously-undetected r-process elements (such as Se, Te, Pt) in ultraviolet spectra of metal-poor stars. I will highlight opportunities to connect these research directions with future facilities (like FRIB, CETUS, and HabEx) to associate specific physics with specific sites of the r-process.

Tues December 1st, 11am, Zoom: Feige Wang (U. of Arizona)

Exploring Reionization-Era Quasars: The most distant luminous quasars provide unique probes to the formation of the earliest supermassive black holes (SMBHs), the assembly of massive galaxies and the reionization of intergalactic medium. In this talk, I will review the ongoing quasar surveys which yield a sizable sample of quasars at redshift z~7 and several luminous quasars at redshift beyond 7.5, deep into the epoch of reionization. These quasar surveys allow the first measurement of quasar luminosity function and the characterization of quasar evolution in the reionization-era. The existence of billion solar mass black holes at redshift beyond 7.5 posts stringent constraints on the theory of black hole formation and growth in the early Universe. I will also present the on-going projects for investigating quasar host galaxies using high resolution ALMA observations and studying the large-scale environment of the earliest SMBHs with deep wide field imaging.

Tues December 8th, 11am, Zoom: Stanimir Metchev (Western U.)