The Luminosity Profile and Structural Parameters of the Andromeda Galaxy
  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).
