astro seminar

Introduction to the SDSS-V Milky Way Mapper

Date: 
Wednesday, November 10, 2021 - 3:00pm
Location: 
Blazer Dining 339 (on zoom)
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The fifth incarnation of the Sloan Digital Sky Survey (SDSS-V) began taking data last year and is in the process of transitioning to the use of a robotic positioning system. I will described the SDSS-V Milky Way Mapper program and its goals. The first of its major goals is to understand the history and structure of the Milky Way. Following upon work done with the APOGEE-1 and 2 surveys the Milky Way Mapper will use approximately 6 million stars to trace out the detailed structure of the Galaxy. The second major goal is to understand stellar astrophysics. The Milky Way Mapper contains several smaller programs called cartons, whose goals cover a wide variety of stellar candidates including white dwarfs, binary stars, young stellar objects, planet hosts, asteroseismology targets and x-ray binaries. These cartons will allow us to explore all sorts of interesting topics that can only be done with a large-scale spectroscopic survey. I will give updates on the current progress of the survey, and lay out our plans for the future.

Multi-messenger Observations of the Most Relativistic Cosmic Bangs: from Outflows to Remnants

Date: 
Wednesday, October 27, 2021 - 3:00pm
Location: 
Blazer Dining 339 (on zoom)
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The deaths of massive stars seed our universe with black holes and neutron stars - the most exotic objects of the stellar graveyard. The births of these stellar remnants, as well as their mergers when paired in binaries, power explosions that can launch the most relativistic jets we know of in the universe (gamma-ray bursts) and shake the very fabric of space-time via ripples called gravitational waves. GW170817, the merger of two neutron stars witnessed through both its gravitational wave siren and its glow at all wavelengths of light, represents the first multi-messenger detection of one such extreme cosmic bang. Starting from the example of GW170817, in this talk I will discuss how radio light in particular, and gravitational waves, can be used in tandem to unveil the physics of relativistic transients. I will also highlight opportunities and challenges that lie in front of us, as improvements in detectors’ sensitivities will transform a trickle of multi-messenger discoveries into a flood.

H2 modelling in galaxies at high redshift

Date: 
Wednesday, October 13, 2021 - 3:00pm
Location: 
Blazer Dining 339 (on zoom)
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Modelling the molecular gas that is detected through CO observations of high-z galaxies constitutes a major challenge for ab initio simulations of galaxy formation. I will present recent numerical work aimed at studying the formation and evolution of the simplest and most abundant molecule, H2. Our model fully solves the out-of-equilibrium rate equations and accounts for the unresolved structure of molecular clouds. We apply our model to two types of cosmological simulations: a) the formation of a Milky Way-sized galaxy at z=2 and b) a small cosmological box in order to obtain some statistical results. The results are compared to those obtained from two different approximations commonly used in the literature and for numerical convergence. Our results indicate that independently of the model, robust results (H2 masses) can only be obtained for galaxies that are suffiiciently metal enriched in which H2 formation is fast. However, their morphology differ from model to model. Furthermore, the cosmological H2 mass function derived from the non-equilibrium model agrees well with recents observations that only sample the high-mass end. Extensions of our model towards including other molecules, such as CO, and species, in particular C and its derivatives, will also be discussed.

Where Do Galaxies End?

Date: 
Wednesday, September 8, 2021 - 3:00pm
Location: 
on zoom
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Abstract: I will review recent observations and theoretical estimates of the spatial extent of galaxies, defined as systems of stars and gas embedded in extended halos of dark matter and hot gas. Formed by the infall of smaller systems, their sizes are determined by gravitational assembly, gas dynamics, and chemical enrichment in heavy elements blown into extragalactic space by galactic winds. But the full extent of galaxies remains poorly determined. The “virial radius” and “splash-back radius” approximate the separation between collapsed structures and infalling matter. Other measurements include X-ray emission and ultraviolet absorption lines from metal-enriched gas in galactic halos. Astronomers have identified large reservoirs of baryonic matter in the circumgalactic medium (CGM) and intergalactic medium (IGM) that contain 50-70% of the cosmological baryons formed in the Big Bang. Investigations of physical processes at the “edge of galaxies” help define the importance of this gas in sustaining the star formation in galaxies.

Tracing the physics of the neutral and ionized ISM with the HI-MaNGA survey

Date: 
Thursday, September 3, 2020 - 4:00pm to 5:00pm
Location: 
Online by Zoom
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The HI-MaNGA survey is an HI (21cm line) follow-up program for the SDSS-IV MaNGA survey.  I will describe the HI-MaNGA survey, its progress to date, and future plans.  I will then present new results where we combine HI-MaNGA and MaNGA data to investigate how the global HI content of star-forming galaxies relates to the mean properties of their ISM derived from optical emission lines, including integrated equivalent width, metallicity, ionization parameter, and the relative strength of low-ionization lines such as [SII] and [OI]. This analysis allows us to understand if and how the properties of the ISM vary between the most gas-rich galaxies to the most gas-poor, and how such variations may affect their evolution.  I will also discuss how gas content relates to the nuclear ionizing source (e.g., Seyfert, LINER, HII regions) and whether we find any evidence that AGN contribute to gas deficiency in the galaxy population.

Zoom Recording: https://uky.zoom.us/rec/share/xGrDewYs7rui5ao4A-_hr_N5r8_c6mkGkiksm--I61WP-hQ8VhJn9HM8fTadgDUG.M9oUhbMc-oa1r2Nr

Estimating Stellar Atmospheric Parameters for the MaStar Stellar Library

Date: 
Thursday, April 30, 2020 - 4:00pm to 5:00pm
Location: 
https://uky.zoom.us/j/94381145379?status=success
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Stellar parameter determination represents a critical step in the procedure of Simple Stellar Population Synthesis (SSPS), of which the aim is to produce realistic composite spectra for model stellar populations, such as galaxies and star clusters. As SDSS-IV nears completion, the MaStar stellar library, an unprecedented large and comprehensive collection of medium-resolution stellar spectra from ~10,000 unique stars, will be made available to the public in its entirety. In order to make proper use of such a library in SSPS, accurate stellar parameter estimates (Teff, log g, [Fe/H], and [\alpha/Fe]) associated with the spectra are a necessity. Here, I present a new approach to stellar parameter estimation that we in the MaStar team have developed in recent years, which combines the spirit of conventional methods with the goal of obtaining an efficient and versatile software pipeline for delivering stellar parameter estimates, as well as other important data such as extinction estimates. This method makes use of the continuum-normalized stellar spectra and the spectral continua themselves (often characterized by photometric measurements in previous efforts) to match the MaStar spectra to models derived from the ATLAS9-based BOSZ model set via reduced-\chi^2 fitting. Carefully combining these two approaches allows us to avoid many of the problems that afflict each approach separately, and obtain reliable estimates throughout parameter space, including notoriously difficult regimes, such as at higher temperatures (> 7000 K). Steps are also taken to account for extinction and flux calibration residuals present in the data. Since this project is ongoing, focus will be placed on recent progress, preliminary results, and plans for the near future.

Hui Li

Date: 
Thursday, March 26, 2020 - 4:00pm to 5:00pm
Location: 
BL 339
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A New Mask for An Old Suspect -- Testing the Sensitivity of the Galactic Center Excess to the Point Source Mask

Date: 
Thursday, December 19, 2019 - 4:00pm to 5:00pm
Location: 
BL 339
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The Galactic center excess has lingered as a possible, but ambiguous, signal of new physics for several years. It has previously been argued that certain details of the excess emission imply that it likely originates from a population of point sources, but this remains a topic of vigorous debate. In this talk, I will report on my recent work, relying on a new point source catalog (obtained by the Fermi-LAT collaboration), that sheds light on this controversial topic. After giving some background on the excess, I will discuss various metrics that have been used to try to understand its true nature. I will show that the large majority of bright sources that were previously suggested to be members of the excess are indeed contained in the new Fermi-LAT point source catalog -- and yet, despite masking out these sources (so that they cannot contribute to the excess), the excess remains just as bright in our new fit to the data. I will go on to discuss the implications of our findings for the two most popular interpretations of the excess.

Astro Seminar: A Post-Mortem on Post-Starburst Galaxies

Date: 
Thursday, October 19, 2017 - 4:00pm to 5:00pm
Location: 
CP 179
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While much effort has been expended to detect the earliest galaxies and to follow their evolution to z~0, astronomers remain baffled by the present-day dichotomy between disky, star forming (aka late-type) galaxies and quiescent, spheroidal (aka early-type) galaxies. Finding galaxies in transition from one class to the other, whose spectra indicate intense recent star formation that has now ended, is key.  We have identified thousands of such "post-starburst galaxies" and discovered that they are often the products of late-type galaxy-galaxy mergers. Their current kinematics, stellar populations, and morphologies are consistent with late- to early-type galaxy evolution. I will discuss recent work that further demonstrates the importance of these galaxies in the study of galaxy evolution. In particular, we have shown that their molecular gas fractions decline rapidly in time after the starburst ends and in a manner consistent with feedback processes. Furthermore, we have determined that tidal disruption events, in which a star is disrupted by the supermassive black hole in a galaxy's center, favor post-starburst galaxies by factors of tens to hundreds. Like the well-known black hole-bulge mass correlation, this surprising connection between the properties of a galaxy on kpc scales and its supermassive black hole on pc scales requires explanation. 
 

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