4 M ay 2 00 5 A Minor Axis Surface Brightness Profile for M

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 (≈ 55 kpc) where μV ∼ 32 mag 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 along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components. Subject headings: galaxies: individual (M31)– galaxies: evolution – galaxies: halos – Local Group – galaxies: stellar content – galaxies: structure Based on observations made with the Isaac NewtonTelescope 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 Astrofisica de Canarias Institute of Astronomy, Madingley Road, Cambridge UK CB3 0HA; [email protected] Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh UK EH9 3HJ Observatoire de Strasbourg, 11, rue de l’Université, F-67000, Strasbourg, France Institute of Astronomy, School of Physics, A29, University of Sydney, NSW 2006, Australia Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield UK AL10 9AB