Paul Zivick holds a Ph.D. in galactic and extragalactic astronomy from the University of Virginia. His research, under the supervision of Dr. Nitya Kallivayalil, focuses on measuring and understanding the stellar dynamics of dwarf galaxy satellites around the Milky Way, in particular the Magellanic Clouds. This work is highlighted in two first-author publications in the Astrophysical Journal and has been presented at conferences in Potsdam, Germany and Noordwijk, Netherlands in addition to the Space Telescope Science Institute in Baltimore, Maryland. Beyond his academic work, Paul is actively involved with the broader University community. He has served as a vice president (2017-2018) and president (2018-2019) for the Graduate School of Arts and Sciences council before serving as Vice-Chair of the Graduate Professional Council (2019-2020). Since 2016, he has led the collaboration between the Central Virginia Governor’s School and the UVA Astronomy Department, where three to four high school students work on a supervised astronomy research project. Past projects have included use of data from the APO Galactic Evolution Experiment and the Gaia Space Telescope. Paul has also mentored undergraduate students as well, both at UVA, through the Astronomy Mentoring Program, and summer research students from Spelman College in Atlanta, GA. Paul completed his B.S. in physics and astronomy at The Ohio State University and his M.S. in astronomy from the University of Virginia. He will begin as a postdoctoral research associate in the Milky Way research group at Texas A&M University in College Station, Texas in Fall 2020.
Understanding Satellite Galaxies Through the Kinematics of the Small Magellanic Cloud in the Age of HST and Gaia
With the advent of modern all-sky surveys, the number of known Milky Way (MW) satellite galaxies has expanded well beyond the original eleven classical satellites. One key use of this catalog of objects has been to test predictions of cosmological cold dark matter simulations on the smallest scales in the universe. However, these tests rely on modeling of the density profiles of the satellite galaxies. To build the most accurate model, three dimensional motions of the stars in the satellite are re- quired, a measurement only now becoming possible with the new era of precision astrometry, led by work with the Hubble Space Telescope (HST) and the Gaia Space Telescope. We use data from both of these observatories to study one of the brightest nearby satellites, the Small Magellanic Cloud (SMC), to both understand its own history with the MW and its companion the Large Magellanic Cloud (LMC) and develop a novel kinematic modeling technique for application to the broader MW satellite population.
Given the SMC’s irregular nature and large spatial extent across the sky, we require broad spatial sampling to thoroughly study its kinematics. To provide this, we observe 30 new fields in the SMC using HST, producing a new proper motion catalog for analysis. Using this catalog, we improve the known systemic motion of the SMC and constrain the minimum separation distance in its last interaction with the LMC to roughly 7 kpc. This places the center of the SMC passing directly through the disk of the LMC, indicating a highly turbulent interaction. Internal kinematics reveal coherent outward motion in the southeastern side of the SMC in the direction of the Magellanic Bridge, consistent with the scenario of ongoing tidal disruption.
The Gaia Data Release 2 (DR2) expanded our proper motion catalogs to well over a billion stars across the MW with proper motions, including many thousands in the direction of the Magellanic Clouds. We use this database to present the first kinematic characterization of the stellar component of the Magellinc Bridge. This analysis reveals in the young stars a roughly linearly increasing relative motion from the SMC towards the LMC, at velocities above 100 km/s, suggesting an active outflow of stars (and gas) from the SMC towards the LMC. We compare these kinematics against numerical simulations of the interactions between the SMC and LMC and find good agreement for a recent direct collision scenario, consistent with our original HST work.
However, significant uncertainties remain in our understanding of the full internal kinematics of the SMC. We present a new analysis of this system using the large DR2 catalog, attempting to account for both possible coherent rotation within the SMC and a tidal expansion component due to the LMC, as suggested by both our Bridge and earlier HST results. To capture the full 3D information present in the observations (as the proper motions are measured for stars at varying depths along the line of sight), we generate a toy 3D model of the SMC and create mock data for comparisons to the DR2 catalog. We find a need for an updated center of mass location and systemic motion for the older stellar population compared to earlier averaged measurements for the SMC, with the older stars located further from the LMC and moving away from the LMC faster than the younger stars. Taken together, we can understand this as a Bullet Cluster-like scenario where existent old stellar populations in the SMC and LMC passed through relatively unscathed while the gas violently collided, imparting different kinematics on the stars formed post-interaction. Intriguingly, we find a need for a non-zero rotation throughout much of the SMC, at a relatively high inclination angle, in addition to accounting for the tidal expansion in the SMC RGs caused by recent interactions with the LMC.
We present a similar analysis for the red supergiant (RSG) population in the SMC, whose age closely coincides with the time of last interaction between the SMC and LMC. We find compelling evidence for the existence of coherent rotation in a subpop- ulation of the RSGs, potentially mapping onto a previously measured age bimodality in the SMC Classical Cepheid population. The rotation signal appears when the internal kinematics are studied assuming the systemic properties for the SMC RG population, suggesting that the gas within the SMC may have been relatively un- perturbed before the most recent interaction. The small area of rotation may also provide new leverage on improving the constraints on the mass ratios of the Magel- lanic system as it could place a hard boundary on where the gas remained fully bound to the SMC.
Taken as a whole, the analysis of the SMC has led to the creation of a model- ing framework capable of accounting for compounding kinematic mechanisms (like rotation and tidal expansion) and developing intuition for understanding the broader MW satellite population in a truly 3D manner.