The Large-Scale Structure of the Halo of the Andromeda Galaxy. I. Global Stellar Density, Morphology and Metallicity Properties

Abstract

We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely powerlaw profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations ([Fe/H] < −1.7) are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f stream = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f stream becoming as high as 86% for [Fe/H] > −0.6. The space density of the smooth metal-poor component has a global power-law slope of γ = −3.08 ± 0.07, and a nonparametric fit shows that the slope remains nearly constant from 30 kpc to ∼300 kpc. The total stellar mass in the halo at distances beyond 2• is ∼1.1 × 10 10 M , while that of the smooth component is ∼3 × 10 9 M . Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ∼8 × 10 9 M . We detect a substantial metallicity gradient, which declines from [Fe/H] = −0.7 at R = 30 kpc to [Fe/H] = −1.5 at R = 150 kpc for the full sample, with the smooth halo being ∼0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail.

The elusive stellar halo of the Triangulum galaxy

“…These holes are filled with fake stars by duplicating information from nearby regions (for details, see Ibata et al 2014). These entries make up only a few percent of the catalogue entries, and are included in the following analysis to best approximate homogeneous coverage of the M33 region.…”

Systematic Problems With Using Dark Matter Simulations to Model Stellar Halos

“…While it is now abundantly clear that much of the mass in the extended outer stellar envelopes of galaxies (stellar halos hereafter) is stripped from dwarf galaxies as they tidally interact with the central galaxy (Majewski et al 2003;Bullock & Johnston 2005, hereafter BJ05;Purcell et al 2007;Bell et al 2008;McConnachie et al 2009;Cooper et al 2010, hereafter C10;Xue et al 2011;Ibata et al 2014), a number of questions remain. Is all of the halo mass accreted (BJ05, C10; Rashkov et al 2012), or are substantial fractions kicked up from the stellar disk (called in situ; Kazantzidis et al 2008;Zolotov et al 2009;Font et al 2011) or formed within satellites after they have been accreted (Tissera et al 2013;Valluri et al, in preparation)?…”

“…Although stellar halos are potentially rich with signposts of the hierarchical galaxy formation process, they contain a very small fraction of the total stellar mass in a galaxy. For example, the stellar halos of the Milky Way and similar-mass galaxies account for only ≈1%-10% of their stellar content (BJ05; Bailin et al 2011;Ibata et al 2014). In order to resolve the tidal streams that constitute the halo and provide observational tests of the hierarchical merging paradigm, hundreds of thousands of particles must be used within the halo itself.…”

The kinematics of globular clusters systems in the outer halos of the Aquarius simulations

“…Similarly, wide-field observations of our closest neighbour, M 31, have revealed a highly complex halo dominated by various stellar streams and overdensities, extending at least out to 150 kpc in projection (e.g. McConnachie et al 2009;Ibata et al 2014). In addition, integrated photometric surveys have been able to uncover structure in the extended halos of other nearby galaxies outside the Local Group (e.g.…”