Cosmic ecosystems

Galaxies, their gaseous halos, and interactions among them

Welcome! Here you should get an overview of why I get up in the morning (and sometimes stay up all night). I am currently a Postdoctoral Fellow working in extragalactic astrophysics at the University of California – Santa Cruz. Using techniques to measure light across a wide range of the electromagnetic spectrum, including radio, optical, ultraviolet, and X-rays, all of my scientific work revolves around a central theme of studying the life cycle and evolution of galaxies. Many answers lie in the vast gaseous envelopes surrounding the brilliant spirals and spheroids composed of stars, and the composition and physical state of these gaseous envelopes (or circumgalactic medium, CGM) are the focus of my work.

Selected Science Results

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Environmental density and the CGM

Burchett et al. 2016
Burchett et al. 2017

Galaxies, in general, do not live in isolation. It has long been observed that galaxies living in environments densely populated with other galaxies form fewer stars, have different shapes, and contain less cold gas than those more isolated. Using triply ionized carbon (C IV), a tracer of ionized halo gas, we find that galaxies in dense environments also show lower incidences of C IV in their CGM -- and in less dense environments than where the deficiency of cold gas and star formation are typically observed. This may indicate that the same processes that eventually shut off star formation and render galaxies devoid of cold gas in their central regions are first evident in the CGM and set in at relatively low densities, much lower than that of galaxy clusters, the densest regions in the Universe. In clusters, I have shown that even neutral hydrogen (H I), which is ubiquitous in more isolated galaxies, is greatly suppressed. In fact, the H I contents of galaxies show an apparent steady decline from the field to groups to clusters, indicating progressively greater environmental influence on galaxies' gas reservoirs.

The metal-enriched CGM and host galaxy mass

Burchett et al. 2016

By number, the Universe is dominated by very low-mass galaxies, even though these quickly become difficult to detect at even modest distances. While absorption line spectroscopy, the technique we use to study the CGM and intergalactic medium (IGM), is sensitive to diffuse gas, the galaxy observations necessary to connect the gas to nearby galaxies can easily miss these faint dwarfs. By focusing on the very nearby Universe, we find that the CGM of isolated dwarf galaxies are missing the metal-enriched ionized gas often detected in the halos of more massive galaxies. Because the gas expelled by stars and supernovae can more freely escape dwarf galaxies, this outflowing material may have left the CGM altogether while it would remain trapped by the gravity of massive galaxies.

Chemical evolution of the Universe

Burchett et al. 2015

Elements such as carbon, oxygen, and iron ('metals' to astronomers) are forged during the lives and deaths of stars. Their cosmic abundances are built-up over many generations of star formation throughout the history of the Universe, and these metals are distributed within galaxies and into the IGM as galaxies evolve. Therefore, models of galaxy formation and evolution must reproduce the abundance, distribution, and evolution of metals in the CGM/IGM to maintain self-consistency while reproducing galaxy masses, shapes, etc. We have characterized the cosmic mass density, number density, and distribution of absorber strengths for triply ionized carbon (C IV) over the last 2 billion years, confirming that the mass density (ΩC IV) has only evolved modestly over the last 9.5 billion years (70% of the age of the Universe). However, the weakest and strongest systems may be suppressed at current times.

Gas transport into and out of galaxies

Burchett et al. 2013

To maintain star formation, galaxies must be fueled by fresh gas from the IGM and recycled gas from the CGM. Conversely, galaxies expel gas through supernova explosions and the activity of their central supermassive black holes. As a case study, we examined the galaxy environment of a C IV absorber located an unusually large distance from any possible host galaxy. Deep follow-up observations revealed a star-forming, low-surface brightness galaxy very close to the absorber on the sky. However, the C IV gas cloud is moving at very high velocity relative to the galaxy. We tested whether the absorber is tracing a high-velocity outflow from the dwarf galaxy but conclude that the dwarf galaxy was likely unable to drive an outflow at such a high velocity. We instead favor a scenario where the absorber traces an extended gaseous disk possibly fueling a more massive galaxy located further away on the sky but at a similar velocity.

Personal Info

Music has played an important role in my life since even before my parents put a guitar in my hands as a child and was the focus of my career before beginning to study astrophysics in 2009. As a musician, I have been fortunate to perform and record with many amazing artists whom I've been honored to call friends. Having grown up in the foothills of the Appalachian mountains in eastern Kentucky, bluegrass and traditional acoustic music is in my blood and has crept into my playing and composition at every step. While based in Louisville, KY, I played mandolin and fiddle in the acoustic alt-country outfit Fire the Saddle. We recorded 3 studio albums from 2002-2006: Sometimes It Ain't the Horse, Four Feet Off the Ground, and Roll Back the Rug. While we played our last official show in 2006, Fire the Saddle recently convened to record Joni Mitchell's 'River' for an upcoming compilation. In 2009, my country-influenced progressive rock band The Mandelbrots released You Are All Perceivers, featuring the instrumentation of pedal steel, fretless electric bass, acoustic guitar, and drums. These days, I play more casually, mostly sitting in on bluegrass jams and picking with the talented musicians I've befriended here in western MA.

I am also proud to have been a founding board member of Star Duck Charities, a 501(c)(3) nonprofit organization dedicated to enriching the lives of children in need. We have instituted several recurring programs for the kids at St. Joseph Children's Home in Louisville, KY. These are things many kids take for granted such as birthday parties and receiving new backpacks for school. I still help out when I can, but Star Duck continues to make a huge difference in the lives of needy kids through the efforts of the current board and dozens of volunteers and donors.

Contact me

email: burchett at