Beyond the Disk and Bulge of M31: Tracing the Transition to the Dark Halo

Although they have large masstolight ratios, galaxy halos host many stars. Thestars, which are typically old and optically dim, encode information about thehalos and the process of galaxy assembly. Some stars, however, are in brightXray binaries (XRBs). Those XRBs in M31.s halo can be detected over vastvolumes with XMMNewton. By using data from previous successful XMMNewtonprograms, we need only six 20ksec exposures to measure the gradient of XRBsbeyond the D25 isophote along both the major and minor galaxy axis. By makingcomplementary optical observations to identify counterparts, we will trace howthe stellar population diminishes across the transition region connectingrelatively bright areas to the outer halo, with its high ratio of mass to light. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]