THE LUMINOSITY PROFILE AND STRUCTURAL PARAMETERS OF THE ANDROMEDA GALAXY

Abstract

We have constructed an extended composite luminosity profile for the Andromeda galaxy, M31, and have decomposed it into three basic luminous structural components: a bulge, a disk and a halo. The dust-free Spitzer/IRAC imaging and extended spatial coverage of ground-based optical imaging and deep star counts allow us to map M31's structure from its center to 22 kpc along the major axis. We apply different decomposition methods for the 1D luminosity profiles and 2D images. These include non-linear least-squares and Bayesian Monte-Carlo Markov-chain analyses. The basic photometric model for M31 has a Sersic bulge with shape index n = 2.2 +/- 0.3 and effective radius R_e = 1.0 +/- 0.2 kpc, a dust-free exponential disk of scale length R_d = 5.3 +/- 0.5 kpc; the parameter errors reflect the range between various decomposition methods. The bulge parameter, n, is rather insensitive to bandpass effects and its value suggests a first rapid formation via mergers followed by secular growth from the disk. The M31 halo has a 2D power-law index ~ -2.5 +/- 0.2. We find that the M31 bulge light is mostly dominant over the range R_minor < 1.2 kpc. The disk takes over in the range 1.2 kpc < R_minor < 9 kpc, whereas the halo dominates at R_minor > 9 kpc. The stellar nucleus, bulge, disk, and halo components each contribute roughly 0.05%, 23%, 73% and 4% of the total light of M31 out to 200 kpc along the minor axis. Nominal errors for the structural parameters of the M31 bulge, disk and halo amount to 20%. If M31 and the Milky Way are at all typical, faint stellar halos should be routinely detected in galaxy surveys reaching below mu_i ~ 27 mag/arcsec^2. We stress that our results rely on this photometric analysis alone. Structural parameters may change when other fundamental constraints, such as those provided by abundance gradients and stellar kinematics, are considered simultaneously (Abridged).