Substructure and Tidal Streams in the Andromeda Galaxy and its Satellites

Abstract

Tidal streams from existing and destroyed satellite galaxies populate the outer regions of the Andromeda galaxy (M31). This inhomogeneous debris can be studied without many of the obstacles that plague Milky Way research. We review the history of tidal stream research in M31, and in its main satellite galaxies. We highlight the numerous tidal streams observed around M31, some of which reside at projected distances of up to ∼ 120 kpc from the center of this galaxy. Most notable is the Giant Stellar Stream, a signature of the most recent significant accretion event in the M31 system. This event involved an early-type progenitor of mass ∼ 10 9 M ⊙ that came within a few kpc of M31's center roughly a gigayear ago; almost all of the inner halo (R ≤ 50 kpc) debris in M31 can be tied either directly or indirectly to this event. We draw attention to the fact that most of M31's outer halo globular clusters lie preferentially on tidal streams and discuss the potential this offers to use these systems as probes of the accretion history. Tidal features observed around M33, M32, NGC 205 and NGC 147 are also reviewed. We conclude by discussing future prospects for this field.

GASP. I. Gas Stripping Phenomena in Galaxies with MUSE

“…The outer regions of galaxies hold a unique set of clues about the way in which galaxies are assembled (Ferguson et al 2016). With GASP it is possible to peer into galaxy outskirts, to study the stellar, gas and dust content out to large radii in galaxies, enabling to compare the star formation history and metallicity gradients with simulations of disk galaxy formation (Mayer et al 2008, Vogelsberger et al 2014, Kauffmann et al 2016, Christensen et al 2016.…”

The Large-scale Structure of the Halo of the Andromeda Galaxy. II. Hierarchical Structure in the Pan-Andromeda Archaeological Survey

“…• H -North-East Structure: Zucker et al (2004) identify this diffuse structure in photometry from an SDSS scan of M31, and it is also visible in the INT WFC footprint of M31 presented by Lewis et al (2004). Zucker et al (2004) called it "Andromeda North-East" and it is also occasionally referred to as the "NE Clump" (e.g., Richardson et al 2008;Lewis et al 2013;Ferguson & Mackey 2016), although this should not be confused with a seperate structure that was found by Davidge (2012) that this author also referred to as the NE Clump. This latter feature is only about 3.5kpc from the center of M31 in projection, and is possibly the remnant of a dwarf galaxy, or a fossil star formation region in the disk of M31.…”

“…globular clusters). Much of this is a review of existing literature, and we also refer the reader to the related discussion in Ferguson & Mackey (2016). We go on to discuss possible causal connections between these different individual substructures, and determine a "best guess" for the lower limit on the number of distinct accretion events in the recent history of M31.…”

Hi in the Outskirts of Nearby Galaxies

“…The arrow indicates the optical radius. Right panel: modern picture of the outskirts of M31, as determined from the PAndAS survey (reproduced with permission from Ferguson and Mackey 2016), where the outline of the optical image is in white. Note the change in scale data) and the Milky Way.…”