FAINT SURFACE PHOTOMETRY OF THE HALO OF M31

Counts of stars near the tip of the red giant branch have been used to estimate the surface brightness of the halo of M31 down to a level of mu(V) congruent-to 30 mag arcsec-2 (R almost-equal-to 20 kpc). Over the range 0.2 less-than-or-equal-to R (kpc) less-than-or-equal-to 20 the surface brightness distribution in the halo of the Andromeda galaxy is well-represented by a single de Vaucouleurs law. Alternatively, the outer halo of M31 can also be modelled by a power-law density distribution of the form rho(R) is-proportional-to R-5. This result suggests that the globular cluster component of the halo of M31 (for which rho is-proportional-to R-3) is more extended than the stellar halo of this galaxy. At mu(V) almost-equal-to 28 mag arcsec-2 (R almost-equal-to 10 kpc), the axial ratio of the halo of M31 is found to be b/a = 0.55 +/- 0.05.