The large-scale structure of the halo of the Andromeda Galaxy Part I: global stellar density, morphology and metallicity properties (1311.5888v1)

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 copious substructure, the global halo populations follow closely power law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component. 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 (42%) residing in discernible stream-like structures. 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 (86% for [Fe/H]>-0.6). The space density of the smooth metal-poor component has a global power-law slope of -3.08+/-0.07, and a non-parametric fit shows that the slope remains nearly constant from 30kpc to 300kpc. The total stellar mass in the halo at distances beyond 2 degrees is 1.1x10^10 Solar masses, while that of the smooth component is 3x10^9 Solar masses. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly 8x10^9 Solar masses. We detect a substantial metallicity gradient, which declines from [Fe/H]=-0.7 at R=30kpc to [Fe/H]=-1.5 at R=150kpc for the full sample, with the smooth halo being 0.2dex 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.

Analysis of the Large-Scale Structure of the Halo of the Andromeda Galaxy

The research paper, "The large-scale structure of the halo of the Andromeda Galaxy Part I," presents a comprehensive examination of the Andromeda Galaxy's stellar halo, utilizing data from the Pan-Andromeda Archeological Survey (PAndAS). This survey is notable for offering the most complete map of resolved stellar populations in any galactic halo. The authors aim to elucidate the global density, morphology, and metallicity properties of Andromeda's halo.

Key Findings

1. Stellar Density and Spatial Distribution:
   • Despite significant substructure, the global halo populations display a notable conformity to power-law density profiles that steepen with increased metallicity.
   • The survey categorizes the halo's stellar population into two components: stream-like populations identified through their distinct spatial configurations and a smooth halo component, which does not present such spatial signatures.
2. Three-Dimensional Halo Model:
   • Utilizing a three-dimensional model reveals that populations with low metallicity ([Fe/H] < -1.7) exhibit an approximately spherical distribution, with slight prolateness (ellipticity c/a=1.09±0.03). A minority (42%) of these stars reside in recognizable stream-like structures.
   • More metal-rich populations show a greater fraction of stars in streams, reaching 86% for [Fe/H] > -0.6.
   • The space density of the smooth, metal-poor component maintains a power-law slope of γ=-3.08±0.07 from 30 kpc to approximately 300 kpc.
3. Metallicity Gradient:
   • A significant metallicity gradient is observed, decreasing from ⟨[Fe/H]⟩=-0.7 at a radial distance of 30 kpc to ⟨[Fe/H]⟩=-1.5 at 150 kpc.
   • The smooth halo consistently appears ~0.2 dex more metal-poor than the overall sample at any given radius.
4. Total Stellar Mass and Theoretical Implications:
   • The total stellar mass in the halo at distances beyond 2° is estimated at ~1.1x10^10 solar masses, while the smooth component accounts for ~3x10^9 solar masses.
   • By extrapolating into the inner regions of Andromeda, the smooth halo's total stellar mass could plausibly be ~8x10^9 solar masses.

Implications and Future Work

• Theoretical Importance: The findings provide a crucial observational template for cosmological simulations that predict the spatial distribution, age, and metallicity of halo stars. The observed conformity with certain aspects of these simulations validates the assumptions about hierarchical galaxy formation processes.
• Dark Matter Halo Shape: The spherical nature of the ancient smooth component suggests that the underlying dark matter halo may also be approximately spherical, challenging current simulations predicting triaxial dark matter distributions.
• Satellite Galaxies and Substructures: Further exploration into the numerous low-mass accretion events revealed by the halo's components could enhance understanding of galaxy formation histories in a cosmological context.

This paper lays the groundwork for future research into the Andromeda halo, setting a formal precedent for detailed observational inquiry into galactic structures. Future studies might focus on incorporating more sophisticated kinematic models that integrate new spectroscopic data, to refine the understanding of the dynamics and evolution of such a significant and proximate cosmic structure. The results could have broad implications for theoretical models of galaxy formation and dark matter halo morphology in the universe.