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

## TL;DR

This study provides a comprehensive panoramic survey of M31 and M33, revealing extensive halos, substructures, and new dwarf galaxies, challenging existing galaxy formation theories with unexpectedly large halo profiles.

## Contribution

First systematic panoramic deep photometric survey of M31's outer regions, discovering new dwarf galaxies and detailed halo structures that challenge current galaxy formation models.

## Key findings

- Detection of several streams and large-scale structures.
- Identification of two new dwarf galaxies, And XV and XVI.
- Halo profiles with scale radii ~55 kpc, larger than predicted.

## Abstract

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 very outermost region of galaxies. We detect several streams and other large-scale structures, and two new dwarf galaxies: And XV and XVI. The discovery of substructure on the minor axis, together with the fact that the light profile between 0.5 < R < 1.3 follows the exponential ``extended disk'', is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Underlying the substructures lies a faint, metal-poor, smooth and extremely extended halo, reaching out to at least 150 kpc. The smooth halo component in M31 has a profile that can be fit with a Hernquist model of immense scale radius ~55 kpc, almost a factor of 4 larger than theoretical predictions. Alternatively a power-law with exponent -1.91 +/- 0.11 can be fit to the profile. The total luminosity of this structure is similar to that of the 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 fit 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. [Abridged]