Architecture of the Andromeda: a quantitative analysis of clustering in the inner stellar halo

TL;DR

This paper quantitatively analyzes the clustering of stars in the inner 30 kpc of M31's stellar halo, revealing significant substructure consistent with predictions from ΛCDM cosmology.

Contribution

It provides a detailed measurement of stellar clustering in M31's halo and compares it with ΛCDM simulations, confirming the presence of expected substructures.

Findings

M31's stellar halo has 20-40% more close pairs than a smooth distribution.

Clustering in M31's halo matches typical ΛCDM halo substructure levels.

The analysis supports ΛCDM predictions for galaxy halo substructure.

Abstract

We present a quantitative measurement of the amount of clustering present in the inner $\sim30$ kpc of the stellar halo of the Andromeda galaxy (M31). For this we analyse the angular positions and radial velocities of the carefully selected Planetary Nebulae (PNe) in the M31 stellar halo. We study the cumulative distribution of pair-wise distances in angular position and line of sight velocity space, and find that the M31 stellar halo contains substantially more stars in the form of close pairs as compared to that of a featureless smooth halo. In comparison to a smoothed/scrambled distribution we estimate that the clustering excess in the M31 inner halo is roughly $40\%$ at maximum and on average $\sim 20\%$. Importantly, comparing against the 11 stellar halo models of \cite{2005ApJ...635..931B}, which were simulated within the context of the $\Lambda{\rm CDM}$ cosmological paradigm, we find that the amount of substructures in the M31 stellar halo closely resembles that of a typical $\Lambda{\rm CDM}$ halo.