Photometry and the Metallicity Distribution of the Outer Halo of M31. II. The 30 Kiloparsec Field

We present the results of a wide-field (VI) photometric study of the red giant branch (RGB) stars in the outer halo of M31, in a field located 30–35 kpc from the center of the galaxy along the southeast minor axis. At this remote location, we find that RGB stars belonging to M31 are sparsely but definitely present, after statistical subtraction of field contamination. We derive the metallicity distribution function (MDF) for the halo stars using interpolation within a standard (I, V-I) grid of RGB evolutionary tracks. The halo MDF is quite broad but dominated by a moderately high-metallicity population peaking at [M/H] ∼ -0.5, strikingly different from the [M/H] ∼ -1.3 level that characterizes the outer halo of the Milky Way. However, the shape and peak metallicity for this region are entirely similar to those found in other studies for the inner regions of the M31 halo, particularly our previous study of a 20 kpc region employing similar data. In summary, we find no evidence for a metallicity gradient or systematic change in the MDF out to quite large distances in the M31 halo: it appears to be a homogeneous and moderately metal-rich subsystem of the galaxy at all locations. The star counts in the 30 kpc field are also consistent with the r1/4 law that fits the interior regions of the M31 spheroid surface brightness profile. The metal-rich MDF and the r1/4 spheroid suggests M31 more strongly resembles a giant elliptical galaxy than other, Milky Way–like, spirals.