Structure and population of the Andromeda stellar halo from a Subaru/Suprime-Cam survey

We present a photometric survey of the stellar halo of the nearest giant spiral galaxy, Andromeda (M31), using Suprime-Cam on the Subaru Telescope. A detailed analysis of VI color-magnitude diagrams of the resolved stellar population is used to measure properties such as line-of-sight distance, surface brightness, metallicity, and age. These are used to isolate and characterize different components of the M31 halo: (1) the giant southern stream (GSS); (2) several other substructures; and (3) the smooth halo. First, the GSS is characterized by a broad red giant branch (RGB) and a metal-rich/intermediate- age red clump (RC). The I magnitude of the well-defined tip of the RGB suggests that the distance to the observed GSS field is (m - M)₀ = 24.73 ± 0.11 (883 ± 45 kpc) at a projected radius of R ∼ 30 kpc from M31's center. The GSS shows a high metallicity peaked at [Fe/H]≳-0.5 with a mean (median) of -0.7 (-0.6), estimated via comparison with theoretical isochrones. Combined with the luminosity of the RC, we estimate the mean age of its stellar population to be ∼ 8 Gyr. The mass of its progenitor galaxy is likely in the range of 10⁷-10⁹ M _⊙. Second, we study M31's halo substructure along the northwest/southeast minor axis out to R 100 kpc and the southwest major-axis region at R 60 kpc. We confirm two substructures in the southeast halo reported by Ibata etal. and discover two overdense substructures in the northwest halo. We investigate the properties of these four substructures as well as other structures including the western shelf and find that differences in stellar populations among these systems, thereby suggesting each has a different origin. Our statistical analysis implies that the M31 halo as a whole may contain at least 16 substructures, each with a different origin, so its outer halo has experienced at least this many accretion events involving dwarf satellites with mass 10 ⁷-10⁹ M _⊙ since a redshift of z ∼ 1. Third, we investigate the properties of an underlying, smooth, and extended halo component out to R>100 kpc. We find that the surface density of this smooth halo can be fitted to a Hernquist model of scale radius ∼ 17 kpc or a power-law profile with Σ(R) R ^{-2.17 ± 0.15}. In contrast to the relative smoothness of the halo density profile, its metallicity distribution appears to be spatially non-uniform with non-monotonic variations with radius, suggesting that the halo population has not had sufficient time to dynamically homogenize the accreted populations. Further implications for the formation of the M31 halo are discussed.