A New Approach to Detailed Structural Decomposition: Kicked-up Disk
  Stars in Andromeda's Halo?
  We characterize the bulge, disk, and halo subcomponents in the Andromeda
galaxy (M31) over the radial range 0.4 < R < 225 kpc. The cospatial nature of
these subcomponents renders them difficult to disentangle using surface
brightness (SB) information alone, especially interior to ~20 kpc. Our new
decomposition technique combines information from the luminosity function (LF)
of over 1.5 million bright (20 < I < 22) stars from the Panchromatic Hubble
Andromeda Treasury (PHAT) survey, radial velocities of over 5000 red giant
branch stars in the same magnitude range from the Spectroscopic and Photometric
Landscape of Andromeda's Stellar Halo (SPLASH) survey, and integrated I-band SB
profiles from various sources. We use an affine-invariant Markov chain Monte
Carlo algorithm to fit an appropriate toy model to these three data sets. The
bulge, disk, and halo SB profiles are modeled as a Sersic, exponential, and
cored power-law, respectively, and the LFs are modeled as broken power-laws. We
find that the number of stars with a disk-like LF is ~5% larger than the number
in the dynamically cold component, suggesting that some stars born in the disk
have been dynamically heated to the point that they are kinematically
indistinguishable from halo members. This is the first kinematical evidence for
a "kicked-up disk" population in the stellar halo of M31. The fraction of
kicked-up disk stars is consistent with that found in simulations. See Dorman
et al. (2013, submitted) for more information.
