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Exploring Ultra-Diffuse Galaxies and Milky Way Analogs in the Romulus Simulations


Jordan Van Nest (University of Oklahoma)

May 18 @ 12:00 PM - 1:00 PM

I will present results from two studies utilizing the Romulus cosmological simulations. In the first we explore populations of ultra-diffuse galaxies (UDGs) in isolated, satellite, and cluster environments using Romulus25 and RomulusC, including how the populations vary with UDG definition and viewing orientation. Using a fiducial definition of UDGs, we find that isolated UDGs have notably larger semi-major (b/a) and smaller semi-minor (c/a) axis ratios than their non-UDG counterparts, i.e., they are more oblate, or diskier.  This is in line with previous results that adopted the same UDG definition and showed that isolated UDGs form via early, high spin mergers. However, the choice of UDG definition can drastically affect what subset of a dwarf population are classified as UDGs, changing the number of UDGs by up to ~45% of the dwarf population.  We also find that a galaxy’s classification as a UDG is dependent on its viewing orientation, and this dependence decreases as environmental density increases. Overall, we conclude that some definitions for UDG used in the literature manage to isolate a specific formation mechanism for isolated dwarfs, while less restrictive definitions erase a link to formation mechanism. In the second study we select Milky-Way like galaxies from the Romulus25 simulation to study their satellites as a function of the host galaxy’s mass and environment. We primarily focus on Milky Way analogs that are at least 2 virial radii from another equal or greater mass galaxy, and our sample naturally includes Milky-Way/Andromeda analogs. We create various samples of analogs by selecting according to different properties: virial mass, stellar mass, and K-band magnitude. While we find that these analog samples largely overlap, none are simple subsets of the others; rather, they tend to diverge at the boundaries of the criteria. In analyzing the satellite populations of our samples, we find that the number of satellites hosted by the analog is primarily driven by host mass while host environment has no significant effect. We perform an analysis of satellite specific frequencies normalized to host mass and environment to confirm these results.

This talk will be in hybrid format. Please contact the department office for Zoom information.