A Minor Axis Surface Brightness Profile for M31

Abstract

We use data from the Isaac Newton Telescope Wide Field Camera survey of M31 to determine the surface brightness profile of M31 along the south-east minor axis. We combine surface photometry and faint red giant branch star counts to trace the profile from the innermost regions out to a projected radius of 4 degrees (~55 kpc) where the V-band surface brightness is 32 mag per square arcsec; this is the first time the M31 minor axis profile has been mapped over such a large radial distance using a single dataset. We confirm the finding by Pritchet & van den Bergh (1994) that the minor axis profile can be described by a single de Vaucouleurs law out to a projected radius of 1.4 degrees or ~20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power-law of index -2.3 or an exponential of scalelength 14 kpc. The fraction of the total M31 luminosity contained in this component is ~2.5%. While it is tempting to associate this outer component with a true Population II halo in M31, we find that the mean colour of the stellar population remains approximately constant at V-i~1.6 from 0.5-3.5 degrees along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components.Comment: 11 pages, 3 figures, ApJ Letters in press, extremely minor modification