A Minor-Axis Surface Brightness Profile for M31

We use data from the Isaac Newton Telescope Wide Field Camera survey of M31to determine the surface brightness profile of M31 along the south-east minoraxis. We combine surface photometry and faint red giant branch star counts totrace the profile from the innermost regions out to a projected radius of 4degrees (~55 kpc) where the V-band surface brightness is 32 mag per squarearcsec; this is the first time the M31 minor axis profile has been mapped oversuch a large radial distance using a single dataset. We confirm the finding byPritchet & van den Bergh (1994) that the minor axis profile can be described bya single de Vaucouleurs law out to a projected radius of 1.4 degrees or ~20kpc. Beyond this, the surface brightness profile flattens considerably and isconsistent with either a power-law of index -2.3 or an exponential ofscalelength 14 kpc. The fraction of the total M31 luminosity contained in thiscomponent is ~2.5%. While it is tempting to associate this outer component witha true Population II halo in M31, we find that the mean colour of the stellarpopulation remains approximately constant at V-i~1.6 from 0.5-3.5 degrees alongthe minor axis. This result suggests that the same metal-rich stellarpopulation dominates both structural components.