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We present a deep photometric survey of M31, conducted with the CFHT and INT,covering the inner 50 kpc of the galaxy, the Southern quadrant out to 150 kpc,and extending to M33. This is the first systematic panoramic study of this veryoutermost region of galaxies. We detect several streams and other large-scalestructures, and two new dwarf galaxies: And XV and XVI. The discovery ofsubstructure on the minor axis, together with the fact that the light profilebetween 0.5 < R < 1.3 follows the exponential ``extended disk'', isparticularly important in shedding light on the mixed and sometimes conflictingresults reported in previous studies. Underlying the substructures lies afaint, metal-poor, smooth and extremely extended halo, reaching out to at least150 kpc. The smooth halo component in M31 has a profile that can be fit with aHernquist model of immense scale radius ~55 kpc, almost a factor of 4 largerthan theoretical predictions. Alternatively a power-law with exponent -1.91 +/-0.11 can be fit to the profile. The total luminosity of this structure issimilar to that of the halo of the Milky Way. This vast, smooth, underlyinghalo is reminiscent of a classical monolithic model and completely unexpectedfrom modern galaxy formation models. M33 is also found to have an extendedmetal-poor halo component, which can be fit with a Hernquist model also ofscale radius ~55 kpc. These extended slowly-decreasing halos will provide achallenge and strong constraints for further modeling. [Abridged]

The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action