The remnants of galaxy formation from a panoramic survey of the region around M31

Abstract

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10-100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than M(v) = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

The elusive stellar halo of the Triangulum galaxy

“…M33 has also been found to possess extensive stellar substructure, in the form of a highly distorted outer disk, thought to have been formed in an interaction with the larger M31 (McConnachie et al 2009;McConnachie et al 2010); this substructure is roughly aligned with the previously detected distorted HI disk (Putman et al 2009;Lewis et al 2013). Being about a tenth the size of the two other large galaxies within the Local Group, the properties of any stellar halo of M33 would provide clues to galaxy evolution on a different mass scale than for the Milky Way and M31.…”

The extended disc and halo of the Andromeda galaxy observed withSpitzer-IRAC

“…Examples include stellar population studies via comparison with visible-light datasets such as PAndAS (McConnachie et al 2009) and PHAT (Dalcanton et al 2012), searches for dusty extremeasymptotic-giant-branch stars as in Boyer et al (2015), comparing a larger sample of M31 globular clusters' mid-infrared colours to population synthesis models as in Barmby & Jalilian (2012), and determining the mid-infrared properties of a uniquely-selected population of quasars in the M31-background (Huo et al 2015). Data-mining of the catalogue for unusual objects which could be followed-up with higher-resolution infrared imaging or spectroscopy is another possible direction, as is exploration of stellar variability.…”

Computational Intelligence in Astronomy – A Win-Win Situation

“…involving smaller dwarf galaxies or satellites). When a satellite merges with a giant galaxy, it gets tidally disrupted in the process and eventually is completely assimilated in the giant galaxy [20]. Deep observations of the nearby Universe have revealed many examples of such tidal streams that resulting from tidal disruptions.…”

“…As an illustration, we present in figure 2 two sets of 3-dimensional projections of the simulation data for the triplet (f, g, h) = (20,21,22). The first projection is onto the spatial coordinates, the second is onto the leading 3 eigenvectors found by the Principal Component Analysis of the merged f, g and h sets.…”

“…We run a rolling window of size 3 over the series f, g, h), starting with simulation sets (f, g, h) at stages (0,1,2) and ending with the triplet (f, g, h) containing simulation set for stages (20,21,22). For each triplet of consecutive simulation sets (f, g, h), we estimated 3 models, one on 90% of data from f , one on 90% of data from g, and one on 90% of data from h. We also compared with Gaussian Mixture Model initialized randomly from the data (GMM) or initialized with K-means (GMM k ).…”