The stellar halo and outer disk of M33

We present first results from a Keck DEIMOS spectroscopic survey of red giant branch (RGB) stars in M33. The radial velocity distributions of the stars in our fields are well described by three Gaussian components, corresponding to a candidate halo component with an uncorrected radial velocity dispersion of sigma similar or equal to 50 km s(-1), a candidate disk component with a dispersion sigma similar or equal to km s(-1), and a third component offset from the disk by similar to 50 km s(-1), but for which the dispersion is not well constrained. By comparing our data to a model of M33 based on its H I rotation curve, we find that the stellar disk is offset in velocity by similar to 25 km s(-1) from the H I disk, consistent with the warping that exists between these components. The spectroscopic metallicity of the halo component is [Fe/H] similar or equal to -1.5, significantly more metal-poor than the implied metallicity of the disk population ([Fe/H] similar to -0.9), which also has a broader color dispersion than the halo population. These data represent the first detections of individual stars in the halo of M33 and, despite being similar to 10 times less massive than M31 or the Milky Way, all three of these disk galaxies have stellar halo components with a similar metallicity. The color distribution of the third component is different from the disk and the halo but is similar to that expected for a single, coeval, stellar population, and could represent a stellar stream. More observations are required to determine the true nature of this intriguing third kinematic component in M33.