A New Approach to Detailed Structural Decomposition: Kicked-up Disk Stars in Andromeda's Halo? [GA]


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.

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Date added: Fri, 18 Oct 13