Andrew Emerick - Graduate Student, Columbia University

2013 - 2019
Graduate Student / NSF Graduate Fellow
Department of Astronomy
Columbia University

Physics, B.S
Astrophysics, B.S. Summa Cum Ladue
School of Physics and Astronomy
University of Minnesota

I am driven by a desire to better understand the universe around us, from what can be seen with the naked eye, to phenomena seen only recently with the advent of powerful telescopes. My tools are hydrodynamics simulations run on national, high performance computers. My current interests focus on using hydrodynamics simulations to study galaxy evolution. Working with Greg Bryan and Mordecai-Mark Mac Low, my thesis focuses on understanding the formation and evolution of the smallest galaxies in the Universe, dwarf galaxies. We intend to use these tiny galaxies as clean tests of the physics of galaxy evolution, focusing specifically on a wide range of feedback physics. In order to take advantage of recent and upcoming observations that can place tight constraints on these physics, we are working on implementing new methods into the cosmological hydrodynamics code Enzo to develop chemodynamical simulations of dwarf galaxies.

In addition to the work above, I've probed the presence of warm gas in galaxy clusters through synthetic absorption line studies of simulated galaxy clusters, and produced simulations examining gas stripping of the lowest mass Milky Way dwarf satellites. During my time as an undergraduate at the University of Minnesota I've studied the presence of radio halos in galaxy clusters and the evolution of magnetic fields in the cluster ICM.

When I am not doing Astronomy I enjoy rock climbing, a surprisingly easy hobby to maintain even when living in the middle of NYC. Particularly when places like this are just a short drive away.


Star Formation, Feedback, and Chemical Evolution in Low Mass Dwarf Galaxies

My thesis project uses a new method for star formation and feedback in the hydrodynamics code Enzo. We are running a set of low mass, isolated dwarf galaxies to parsec scale resolution, following star formation to individual stars. This allows us to accurately track stellar feedback physics in detail, as we study the effects of stellar winds, supernovae, ionizing radiation (through radiative transfer), photoelectric heating, and Lyman-Werner dissociation radiation on dwarf galaxy dynamical and chemical evolution. In addition, we track 12 individual metal abundances ejected from our star particles, using stellar yields from the NuGrid collaboration, and chemically tag each star with these abundances.

Gas Stripping of the Lowest Mass Dwarf Galaxies

Emerick et. al. in prep: The Milky Way galaxy is host to on the order of 30-40 observed dwarf galaxy satellites. With the exception of the most massive of these satellites, all within the virial radius of the Milky Way are found to be devoid of both gas and star formation. Outside of the virial radius, however, we do still find gassy dwarf galaxies. This points to an environmental process to strip and remove gas from these galaxies. Recent work synthesizing understanding of satellite accretion from large dark matter only simulations and the observed properties of these dwarfs suggests a stripping timescale on the order of 2 Gyr, assuming that ram pressure stripping is the dominant environmental gas loss process in these dwarfs. We conduct a series of hydrodynamical wind tunnel simulations of the lowest mass dwarf galaxies (galaxies that are similar to Leo T in size and mass) to examine this expected timescale in detail, investating whether or not gas stripping via ram pressure is sufficient, or if other physics is at play. In addition, we investigate the role supernovae play in aiding stripping in these tiny dwarf galaxies.

Gas Stripping of a Small Dwarf Galaxy from Andrew Emerick on Vimeo.

Four panels showing density slices through four simulations of gas stripping of a small dwarf galaxy. The dwarf galaxy is modeled to be roughly the same physical size and mass as Leo T (though not meant to reproduce any other properties of this galaxy), and is the same in all four slices. The top row shows simulations without supernovae feedback (but includes radiative cooling), and the bottom row shows what happens when we turn on supernovae feedback (surprisingly little difference!). In each case, there is a wind coming in through the left hand side of the box, at a velocity of either 200 km/s (left column) or 400 km/s (right column). The simulation lasts for 2 Gyr with a frame every 10 Myr.

Warm Gas in Galaxy Clusters

Emerick et. al. 2015: Though galaxy clusters in both the local universe and at high redshift have been well studied through a host of observations at all wavelengths, there is much about clusters we do not yet know, and much still that we can find from observations. Although warm gas is expected to exist in and around galaxy clusters from, for example, ram pressure stripping of infalling galaxies or substructures, to date only one systematic set of observations have been made looking for this gas. In this work, we produced two simulated galaxy clusters with Enzo and examined the warm gas content from an observational standpoint using synthetic absorption sightlines. We find that the resulting Lyman Alpha absorption in our simulations predominanlty traces low column, low metallicity HI that is likely associated with infalling material along filaments from the IGM.
This image shows the integrated column density of neutral hydrogen in a 10 Mpc cubical region around a Virgo-like galaxy cluster (similar in size and mass). This cluster was obtained from a cosmological simulation made using Enzo. The black circles denote one and two virial radii.

Magnetic Fields in Galaxy Clusters

Galaxy clusters are the largest virial structures in the universe, and contain a diffuse gas that makes up most of the cluster's baryonic mass, the intracluster medium (ICM). The ICM is host to many astrophysical phenomena, and is subject to weather driven, for example, by AGN, supernovae, and shocks; this weather leads to turbulence. This can lead to an amplification and feeding of the microgauss magenetic field in the ICM. During my last two years at the Univeristy of Minnesota, I worked under Dr. Thomas W. Jones in computational astrophysics, studying the evolution of weak magnetic fields in turbulent plasmas in the context of galaxy cluster ICM. We were concerned primarily with the detailed process of how energy is fed from the kinetic, turbulent motions to the magentic field at all length scales within a system.

Radio Halos in Galaxy Clusters

Brown, S. Emerick, A. et. al. 2011 For or two years at the University of Minnesota, I worked under Dr. Lawrence Rudnick (UMN) and Dr. Shea Brown (CSIRO). We focused on studying the properties of galaxy clusterse. More specifically, we focused on the diffuse non-thermal radio component some cluster's emission, that originates within the intracluster medium (ICM). These radio halos probe the charge particle population of the ICM, as well as the presence of ICM magnetic fields; they ultimately help constrain many cluster properties.



As of Nov. 2013, I joined the writing team of the graduate student run website Astrobites. This is a daily updated blog centered around connecting current undergraduate physics or astronomy majors to current research advances in astronomy. As a writer, I will be contributing primarily to the daily paper summaries, where we write a concise, approachable summary of a recently submitted journal article. Check out my posts here.


Star Formation, Feedback, and Chemical Evolution in Low Mass Dwarf Galaxies

The following movies show the evolution of an isolated, low mass dwarf galaxy as part of my thesis project. These below movies are low resolution tests of the underlying physics and star formation algorithm. We follow star formatino to individual stars and include stellar winds, supernovae, radiative transfer tracking ionizing radiation, stellar photoelectric heating, and Lyman-Werner radiation. In addition, we follow 12 individual metal abundances. We are investigating the roles each of these feedback processes play in the dynamical and chemical evolution of low mass dwarf galaxies. In particular, we are examining how feedback affects metal mixing and metal ejection / retention in these dwarf galaxies. Parsec scale resolution production simulations are currently running on the Stampede supercomputer.
Nota bene: These simulations are low resolution, proof of concept tests ONLY. The star formation threshold in these simulations has been lowered to an unphysical value, and the star formation efficiency artificially increased. This was done to quickly form feedback producing stars to more easily stress test the feedback physics methods. Therefore, the star formation rate is artificially high in these simulations.


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