We present a deep photometric survey of the Andromeda galaxy, conducted with the wide-field cameras of CFHT and INT, that covers the inner 50 kpc of the galaxy and the southern quadrant out to ~150 kpc and includes an extension to M33 at >200 kpc. This is the first systematic panoramic study of this very outermost region of galaxies. We detect a multitude of large-scale structures of low surface brightness, including several streams, and two new relatively luminous (MV ∼ − 9) dwarf galaxies: And XV and And XVI. Significant variations in stellar populations due to intervening stream-like structures are detected in the inner halo, which is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Underlying the many substructures lies a faint, smooth, and extremely extended halo component, reaching out to 150 kpc, whose stellar populations are predominantly metal-poor. We find that the smooth halo component in M31 has a radially decreasing profile that can be fitted with a Hernquist model of immense scale radius ~55 kpc, almost 4 times larger than theoretical predictions. Alternatively a power law with ΣV ∝ R−1.91 ± 0.11 can be fitted to the projected profile, similar to the density profile in the Milky Way. If it is symmetric, the total luminosity of this structure is ~109 L☉, again similar to the stellar halo of the Milky Way. This vast, smooth, underlying halo is reminiscent of a classical "monolithic" model and completely unexpected from modern galaxy formation models. M33 is also found to have an extended metal-poor halo component, which can be fitted with a Hernquist model also of scale radius ~55 kpc. These extended slowly decreasing halos will provide a challenge and strong constraints for further modeling.

The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action