Colloquium PhD Thesis Defense: Sten Hasselquist
Apr 6 @ 3:15 pm – 4:15 pm
Colloquium PhD Thesis Defense: Sten Hasselquist @ BX102

Colloquium Title

Sten Hasselquist, NMSU


Colloquium Thesis Proposal: Emma Dahl
Apr 13 @ 3:15 pm – 4:15 pm
Colloquium Thesis Proposal: Emma Dahl @ BX102

Colloquium Title

Emma Dahl, NMSU

Abstract text

Colloquium Thesis Proposal: Caitlin Doughty
May 9 @ 2:15 pm – 3:15 pm
Colloquium Thesis Proposal: Caitlin Doughty @ Science Hall 107

Metal Absorption in the Circumgalactic Medium During the Epoch of Reionization

Caitlin Doughty, NMSU

The characteristics of metal absorption arising from the circumgalactic medium of galaxies have been demonstrated to be related to conditions in the galaxy which sourced them, as well as to the ambient ultraviolet background. I propose a three- pronged thesis in order to better understand and utilize these relationships. First, I will explore whether the spectral energy distributions of binary stars, incorporated into a custom version of GADGET-3, can explain the discrepancy between observed and simulated absorber statistics. Second, I will study the relationship between neu- tral oxygen absorbers and the neutral hydrogen fraction in simulated quasar sight- lines and relate the results to observations of neutral oxygen at z ≥ 4.0. Third, I will study the relationships between the emissive properties of galaxies, stemming from their nebular gas, and the metal absorbers which they source. Taken as a whole, this thesis will improve the ability of cosmological simulations to reproduce realistic metal absorption, probe the local progress and topology of reionization, and under- stand what emissive galaxy traits we expect at z > 5 based on observations of metal absorbers.

Colloquium: Dave Thilker (Host: Rene Walterbos)
Sep 21 @ 3:15 pm – 4:15 pm
Colloquium: Dave Thilker (Host: Rene Walterbos) @ BX102

Fresh Perspectives on Star Formation from LEGUS, the Legacy ExtraGalactic Ultraviolet Survey

David Thilker, Johns Hopkins University

The Legacy ExtraGalactic Ultraviolet Survey (LEGUS) was a Cycle 21 Large Treasury HST program which obtained ~parsec resolution NUV- to I-band WFC3 imaging for 50 nearby, representative star-forming Local Volume galaxies, with a primary goal of linking the scales of star formation from the limit of individual stars, to clusters and associations, eventually up through the hierarchy to giant star forming complexes and galaxy-scale morphological features.

I will review the basics of the survey, public data products and science team results pertaining to clusters and the field star hierarchy.  I will then describe work to optimize photometric selection methods for massive main sequence O star candidates and LBV candidates, in the former case establishing a means to statistically constrain the fraction of O stars in very isolated locales.  I will introduce new ideas on how to quantify the complex spatio-temporal nature of hierarchical star formation using multi-scale clustering methods. The first steps of this work have yielded a landmark OB association database for 36 LEGUS target fields (in 28 of the nearest available galaxies), with tracer stellar populations selected and interpreted uniformly.  I will finish with discussion of a pilot HST program to demonstrate remarkably increased survey efficiency of WFC3 UV imaging enabled by use of extra-wide (X) filter bandpasses.  Such efficiency is required as we move beyond LEGUS and begin to rigorously explore low surface brightness star-forming environments where canonical results for the IMF and cluster formation efficiency are increasingly called into question.


Colloquium: Sanchayeeta Borthakur (Host: Kristian Finlator)
Oct 19 @ 3:15 pm – 4:15 pm
Colloquium:  Sanchayeeta Borthakur (Host: Kristian Finlator) @ BX102

Understanding How Galaxies Reionized the Universe

 Sanchayeeta Borthakur, Arizona State University

Identifying the population of galaxies that was responsible for the reionization of the universe is a long-standing quest in astronomy. While young stars can produce large amounts of ionizing photons, the mechanism behind the escape of Lyman continuum photons (wavelength < 912 A) from star-forming regions has eluded us. To identify such galaxies and to understand the process of the escape of Lyman continuum, we present an indirect technique known as the residual flux technique. Using this technique, we identified (and later confirmed) the first low-redshift galaxy that has an escape fraction of ionizing flux of 21%. This leaky galaxy provides us with valuable insights into the physics of starburst-driven feedback. In addition, since direct detection of ionizing flux is impossible at the epoch of reionization, the residual flux technique presents a highly valuable tool for future studies to be conducted with the upcoming large telescopes such as the JWST.

Colloquium Thesis Proposal: Rachel Marra
Sep 12 @ 1:30 pm – 2:30 pm
Colloquium Thesis Proposal: Rachel Marra @ Jett Hall 210

An Observer’s Examination of the Circumgalactic Medium using Cosmological Simulations

Rachel Marra, NMSU

A significant aspect to understanding galaxy evolution is having an understanding of the intricacies involving the inflow and outflow of baryons onto a galaxy. Gas needs to accrete onto the galaxy in order for star formation to occur, while stellar winds, supernovae, and radiation pressure result in the outflow of gas from the galaxy. The diffuse region around the galaxy that has gas from interstellar medium (ISM) inflows and intergalactic medium (IGM) outflows interacting is the circumgalactic medium (CGM). Studying the CGM will help us learn about the baryon cycle and give us a better understanding of galactic evolution.

The primary method to studying the CGM is through absorption, as the density is too low to detect emission. Studying these absorption features allows us to learn about the physical properties of the gas giving rise to the absorption. Other than through observations, cosmological simulations play a large role in how we learn about the CGM of galaxies. Using MOCKSPEC, the Quasar Absorption Line Analysis Pipeline, to create mock quasar sightlines through the VELA simulation suite of galaxies, we use the absorption features seen in the sightlines to study the CGM in the simulations. While there are many ions that are used to study the CGM, we focus on OVI.

We intend to study how effective our methods are for studying the CGM with both observations and simulations. The covering fraction of OVI for a sample of observed galaxies will be compared with the covering fraction that is found from a selection of LOS that probe simulated, Milky-Way type galaxies. This tells us if the simulations can reproduce the observations, and if they do not, we can gain insights as to why the simulations do not match observed data. We will also investigate if the metallicity calculated from an observed absorption feature reflects the actual metallicity of the probed gas by using mock sightlines through simulations. Additionally, we will do a comparison of different methodologies used to study the CGM in simulations, to determine if using mock quasar sightlines is a more realistic and accurate method to compare to observed data.

Colloquium: Raja GuhaThakurta (Host: Rene Walterbos)
Oct 4 @ 3:15 pm – 4:15 pm
Colloquium: Raja GuhaThakurta (Host: Rene Walterbos) @ BX102

The SPLASH Survey of the Andromeda Galaxy

Raja Guhathurkurta, University of California, Santa Cruz

Our nearest large spiral galaxy neighbor, the Andromeda galaxy (M31),
and its dwarf satellites, offer a panoramic yet detailed view of
galaxy formation and evolution in our astronomical backyard. This
system also serves as an excellent laboratory for the study of stellar
populations because the stars are all practically at the same distance
from us. I will present results from the SPLASH (Spectroscopic and
Photometric Landscape of Andromeda’s Stellar Halo) survey, the
backbone of which was a large Keck DEIMOS spectroscopic survey of
evolved stars in M31. Most of the SPLASH spectroscopic targets in
M31’s disk were selected from the PHAT (Panchromatic Hubble Andromeda
Treasury) survey, a wide-field 6-filter Hubble Space Telescope mosaic
image of a portion of the disk of M31. The talk will cover a range of
science topics including: Local Group dynamics, structure/substructure
and metallicity of M31’s stellar halo, satellite tidal interactions,
disk/halo interface, dynamical heating of the disk, and rare stellar

Colloquium: Elise Boera (Host: Kristian Finlator)
Dec 6 @ 3:15 pm – 4:15 pm
Colloquium: Elise Boera (Host: Kristian Finlator) @ BX102

Revealing reionization with the thermal history of the intergalactic medium

Elisa Boera, SISSA Trieste

During hydrogen reionization the UV radiation from the first luminous sources injected vast amount of energy into the intergalactic medium, photo-heating the gas to tens of thousands of degree Kelvin. This increase in temperature has left measurable `imprints’ in the thermal history of the cosmic gas: a peak in the temperature evolution at the mean density and a smoothing out of the gas in the physical space by the increased gas pressure following reionization (i.e. Jeans smoothing effect). The structures of the HI Lyman-alpha forest at high redshift are sensitive to both these effects and therefore represent a powerful tool to understand when and how reionization happened. I will present the most recent constraints on the thermal history of the intergalactic medium obtained using the Lyman-alpha forest flux power spectrum at z>5. I will show how these results can be used to obtain information on the timing and the sources of the reionization process and I will discuss their consistency with different possible reionization scenarios.
Colloquium Thesis Proposal: Sean Sellers
Feb 26 @ 3:30 pm – 4:30 pm
Colloquium Thesis Proposal: Sean Sellers @ Domenici Hall 006

A Multi-Wavelength Study of the Evolution of Solar Flares

Sean Sellers, NMSU


Remote Colloquium Thesis Defense: Drew Chojnowski
May 29 @ 2:30 pm – 3:30 pm
Remote Colloquium Thesis Defense: Drew Chojnowski @ Online

H-Band Spectroscopy of Exotic, Massive Stars

Drew Chojnowski, NMSU

We report on spectroscopy of exotic B-type emission line (Be) stars and chemically peculiar (CP) stars in the near-infrared (NIR) H-band, using data provided by the Apache Point Observatory Galactic Evolution Experiment, one of the sub-surveys of the Sloan Digital Sky Survey (SDSS). Between 2011-2020, SDSS/APOGEE has observed more than a million stars in the Milky Way Galaxy (MW), with roughly 10% of the targets being hot, blue stars that serve as telluric absorption standard stars (TSS). The TSS are selected mostly on the basis of having blue raw J-K color indices with no preference for any particular spectral type that might be known from optical spectroscopy. This targeting strategy has led to the TSS being a mixed bag, with those observed in the MW Halo typically being F-type stars that are only slightly more massive than the Sun, and with those observed in the MW Disk and Bulge being OBA-type stars of a few up to 20 times the mass of the Sun. While the vast majority of the TSS are superficially normal main sequence stars, the inclusion of large numbers of Be and CP stars has serendipitously resulted in the largest ever homogeneous spectroscopic surveys of these stellar classes, both of which present observational anomalies that remain very poorly understand despite more than a hundred years of research. Prior to SDSS/APOGEE, the H-band spectra of Be and CP stars had only been discussed in a handful of studies, all of which used small numbers of spectra of considerably lower resolution than the R=22,500 of the APOGEE instruments. The material presented in this thesis therefore represents the first ever detailed studies of Be and CP stars in the H-band, while also greatly expanding the known samples through discovery of many hundreds of new examples.