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 southeast 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° (~55 kpc), where μV~32 mag arcsec-2 this is the first time the M31 minor-axis profile has been mapped over such a large radial distance using a single data set. We confirm the finding by Pritchet & van den Bergh that the minor-axis profile can be described by a single de Vaucouleurs law out to a projected radius of 1.4d or ~20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power law of index about -2.3 or an exponential of scale length 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 color of the stellar population remains roughly constant at V-i~1.6 from 0.5d to 3.5d along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components. Based on observations made with the Isaac Newton Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.