IAUS379: Dynamical Masses of Local Group Galaxies

Triangulum (M33), a satellite of the Andromeda (M31) galaxy, is the only dwarf Spiral in the Local Group. With a mass ten times lower than M31’s and a star formation rate 10 times higher, M33 is the best local analog for high z galaxies. The Triangulum Extended Survey (TREX) is a large resolved stellar spectroscopic survey of M33 and its extended structures. With contiguous spectroscopic fields covering M33's inner disk, out to M33's disk break, and beyond, we are investigating the evidence for both internal and external heating mechanisms affecting M33. Using a sample of over 4500 M33 stars with line of sight velocity measurements, spanning from young, massive main sequence stars to old red giant branch stars, we established that a significant high-velocity-dispersion component is present in M33's RGB population from near M33's center to at least the radius where M33's H I disk begins to warp at 30' (~7.5 kpc) in the plane of the disk. This is the first detection and spatial characterization of a kinematically hot stellar component throughout M33's inner regions. However, beyond the break in M33's disk, we find the stellar population is dominated by stars with disk-like kinematics, with only marginal evidence for a kinematically hot halo component, casting doubt onto whether this component is likely to have been formed from accretion of smaller systems. We have also measured the velocity dispersion and asymmetric drift of stars in the disk as a function of stellar age, finding neither increases with stellar age and the youngest disk stars are dynamically hotter than predicted by simulated M33 analogs in Illustris. This indicates an additional, currently unknown source of dynamical heating of the young stars in the disk of M33. I will discuss these TREX results, as well as future prospects, in the context of our understanding of the orbital history of M33, in particular, the question of whether M33 is on first infall or has interacted substantially with M31 in the past.