The stellar halo and outer disk of M 33

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 σ ≃ 50 km s, a candidate disk component with a dispersion σ ≃ 16 km s, and a third component offset from the disk by ∼ 50 km s, but for which the dispersion is not well constrained. By comparing our data to a model of M33 based upon its HI rotation curve, we find that the stellar disk is offset in velocity by ∼ 25 km s from the HI disk, consistent with the warping which exists between these components. The spectroscopic metallicity of the halo component is [Fe/H] ≃ −1.5, significantly more metal-poor than the implied metallicity of the disk population ([Fe/H] ≃ −0.9), which also has a broader colour dispersion than the halo population. These data represent the first detections of individual stars in the halo of M33 and, despite being ∼ 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 to 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. Subject headings: galaxies: individual (M33/Triangulum) — Local Group — galaxies: stellar content — galaxies: structure