The TREX Survey: Kinematical Complexity Throughout M33's Stellar Disk and Evidence for a Stellar Halo

Abstract

We present initial results from a large spectroscopic survey of stars throughout M33's stellar disk. We analyze a sample of 1667 red giant branch (RGB) stars extending to projected distances of $\sim 11$ kpc from M33's center ($\sim 18$ kpc, or $\sim 10$ scale lengths, in the plane of the disk). The line-of-sight velocities of RGB stars show the presence of two kinematical components. One component is consistent with rotation in the plane of M33's HI disk and has a velocity dispersion ($\sim 19$ km s$^{-1}$) consistent with that observed in a comparison sample of younger stars, while the second component has a significantly higher velocity dispersion. A two-component fit to the RGB velocity distribution finds that the high dispersion component has a velocity dispersion of $59.3^{+2.6}_{-2.5}$ km s$^{-1}$ and rotates very slowly in the plane of the disk (consistent with no rotation at the $<1.5σ$ level), which favors interpreting it as a stellar halo rather than a thick disk population. A spatial analysis indicates that the fraction of RGB stars in the high-velocity-dispersion component decreases with increasing radius over the range covered by the spectroscopic sample. Our spectroscopic sample establishes 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 HI disk begins to warp at 30$'$ ($\sim 7.5$ 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.