Astrophysics Colloquium

Colloquium Meetings are held in the Physics and Astronomy Building (PAB) in Room 1-434A from 3:30-4:30pm every Wednesday of the Academic Year.
Coordinator: Tuan Do

Winter 2020

1/15/20 Keith Hawkins, UTA
Title:  Galactic Archaeology in the Era of Gaia and Large Spectroscopic Surveys 
Abstract:  One of the key objectives of modern astrophysics is to understand the formation and evolution of galaxies. In this regard, the Milky Way is a fantastic testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical  chemical, and age distributions of its stellar populations.  Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography and the prospects of chemical tagging. 

1/22/20 Gregory Herczeg (Kavli Institute) 
Title:  From protostars to adolescence:  A tour of young stellar systems
Abstract:  While the stages in the formation of stellar systems are now well charted, uncertainties in the initial conditions and evolution lead to stellar systems with a diverse array of architectures.  In this talk I will discuss the major stages in the evolution of young stellar objects, starting from the young protostars and ending in stars that have dispersed all circumstellar material.  At each step I will describe insights into some of the relevant processes that are being obtained from ongoing observational programs.  For protostars, we are pursuing the first long-term monitoring program in the sub-mm to establish the role of accretion variability during the main phase of stellar growth.  The next stage, protoplanetary disks, is now being revolutionized by exquisite ALMA images of substructures, which point to the presence of hidden planets.  Finally, Gaia observations of young stars that have shed their disk promise to reveal the recent star formation in our local neighborhood, although this will require improved measurements of stellar properties.

1/29/2020 Xiaohui Fan (University of Arizona )
Title:  Extreme Quasars During the Cosmic Dawn
Abstract: The most distant quasars provide unique probes to the formation of the earliest supermassive black holes, the co-evolution of early massive galaxies and their central black holes and the reionization of the intergalactic medium. More than 100 quasars have been discovered at z>6. I will present progress on surveys of the most distant quasars, focusing on three recent discoveries: (1) the most distant quasars known to date at z~7.5-7.6 and the constraint on cosmic reionization history; (2) the most luminous quasar at z>6 powered by a twelve billion solar mass black hole and its implications on the seed of the earliest supermassive black holes; and (3) the first gravitationally lensed quasar during the epoch of reionization and the prospect of direct black hole mass measurements at cosmic dawn. 

2/5/2020 Will Farr (StonyBrook/Flatiron)
Title:  Cosmography and Black Hole Spectroscopy with Gravitational Waves
Abstract:  In this seminar I will describe recent results in gravitational wave observations of binary black hole mergers.  By tracking a feature in the binary black hole mass spectrum across cosmic time it will be possible for Advanced LIGO and Virgo to measure the expansion history of the universe to few-percent accuracy at redshifts z ~ 0.7.  Measurements at these redshifts are particularly interesting because they correspond to the transition from a matter-dominated to dark-energy-dominated universe; in concert with other percent-level cosmographical measurements, binary black hole observations could constrain the dark energy equation of state parameter to better than 10%.  Because binary black hole mergers are standard(izable) sirens, these measurements are independent of any of the other distance ladders or standard rulers employed for cosmography.  Binary black hole mergers also enable precision tests of general relativity as a theory of gravity.  One such test is black hole spectroscopy---measurement of the normal modes of the spacetime near a black hole horizon through their gravitational wave emission---which is analogous to the use of atomic spectral lines to test quantum mechanics.  I will explain the first-ever measurement of multiple modes of oscillation from a black hole spacetime (the remnant black hole from the first binary black hole merger observed by LIGO, GW150914) and discuss the future of such measurements and the constraining power they have over general relativity.  In both black hole cosmography and spectroscopy, advanced-era gravitational wave detectors are delivering precision and power at a level not anticipated until the next generation ("3G") of gravitational wave detectors.

2/12/2020 Vikram Ravi (Caltech)
Title:  Missing matter, missing mass

Abstract:  Although 85% of the mass of the Universe is identified with dark matter, only 10-20% of baryons are observable at low redshifts. The remaining baryon density is understood to be in a hot, diffuse phase surrounding galaxies and in the cosmic web. I will show how observations of fast radio bursts (FRBs) are beginning to identify the locations of these missing baryons. I will also detail how FRBs together with a suite of novel tracers can evince the physical conditions in galactic halos, and provide clues towards the nature of dark matter. I will conclude by describing the relevant observational programs underway at the Owens Valley Radio Observatory, including the 110-dish Deep Synoptic Array.  

2/19/2020 Judit Szulágyi (ETH)
Title:  The key for planet formation: The Circumplanetary Disk

Abstract:  Nascent massive planets are surrounded by their own disk, the so-called circumplanetary disk. This channels material to the forming planet, serves as a birthplace for moons to grow, and affects the observational signatures of forming planets. The circumplanetary disk composition and chemistry will naturally affect that of the forming planet and of the moons. So understanding its role and characteristics is bringing us closer to understand planet- and moon-formation as a whole. Our knowledge is still very limited on circumplanetary disks, as they are hard to resolve in computer simulations. We are just entering an era when the observations of these disks are possible, as the first observational evidence for their existence just came in May 2019. I am carrying out sub-planet resolution thermo-hydrodynamical simulations of planet formation, trying to understand what are the characteristics of the circumplanetary environment, how we can detect forming planets and their circumplanetary disks in near-infrared, sub-millimeter and radio wavelengths or with hydrogen recombination lines, such as H-alpha. In my talk I will show mock observations in order to discuss which wavelength-range is the best to detect forming planets and what H-alpha fluxes we can expect from the circumplanetary environment. Finally, I will discuss how the circumplanetary disk alters the accretion rate and what does that mean for the timescales of planet-formation.

2/26/2020 Jo Bovy (University of Toronto)
Title:  The Milky Way in the era of large surveys
Abstract:  For over a hundred years, the Milky Way has been the nexus between many fields of astrophysics, linking together investigations into the formation of planetary systems and stars to studies of galactic evolution, cosmology, and astroparticle physics. Obtaining a detailed understanding of our Galaxy’s structure, formation, and evolution is therefore crucial to the advancement of the whole of astrophysical knowledge. Long thought to be a simple spiral galaxy with a simple disk-plus-bulge structure leading a relatively unperturbed life, the advent of large surveys such as SDSS, Gaia, and soon LSST has breathed new life into the field of galactic structure. I will discuss the new view of the Milky Way—complex, dynamic, and very much in the process of evolving—and what it implies about galaxy formation, galaxy evolution, and the nature of dark matter.

3/4/2020 Jim Fuller (Caltech)
Title:  Surprising impacts of gravity waves

Abstract: Gravity waves are low frequency fluid oscillations restored by buoyancy forces in planetary and stellar interiors. Despite their ubiquity, the importance of gravity waves in evolutionary processes has only recently been appreciated. Gravity modes are resonantly excited by tidal forcing in stellar binaries, and their observed amplitudes in heartbeat stars indicate a resonance locking process operates in some systems. A similar resonance locking process appears to be at work in the Saturn system, as evidenced by the rapid outward migration of its largest moon, Titan. Gravity waves asteroseismically measure the core rotation rates of red giant stars, informing new angular momentum transport models that predict slow rotation rates for white dwarfs, neutron stars, and black holes. In the late phase evolution of massive stars approaching core-collapse, vigorous convection excites gravity waves that can carry huge amounts of energy within the star. The wave energy redistribution can drive outbursts and enhanced mass loss in the final years of massive star evolution, with important consequences for the appearance of subsequent supernovae.

3/11/2020 Lori Lubin (UCD) 
Title: Understanding Cluster Formation and Galaxy Evolution ... ORELSE

Abstract:  The Observations of Redshift Evolution in Large Scale Environments (ORELSE) Survey is a systematic photometric and spectroscopic searchfor structure on scales > 10 Mpc around 18 known clusters at 0.6 < z <1.3.  The survey covers 5 square degrees, all targeted at high-densityregions, making it comparable in area and spectral coverage to field surveys such as DEEP2.  The goal of survey is to study galaxy evolution across all scales -- from dense cluster cores to infall/intermediate-density regions to the field. In this talk, I describe the survey design, the galaxy sample, and our novel environmental metrics. I present some recent results on using the large galaxy sample to create a quantifiable cluster catalog, measure galaxy properties as a function of stellar mass, environment, and redshift, and constrain the nature of the active galaxy population. Finally, I describe our latest program to combine ORELSE with the higher-redshift survey VUDS to chart cluster formation and its effect on member galaxies over the last 12 billion years.

Astrophysics Colloquium Archive