Is M31 at the center of its satellite system ?

TL;DR

The paper addresses whether M31 sits at the center of its dwarf-satellite distribution given the observed anisotropy. It uses forward modeling with completeness corrections in a Bayesian framework to compare two isotropic spatial profiles (a power-law and an NFW form) and to recover the center coordinates $(\alpha_c,\delta_c)$ and line-of-sight offset $\Delta D$. The results show the distribution center is generally compatible with M31, but a modest shift toward the Milky Way is favored, with $\Delta D$ ranging from about $-46^{+35}_{-30}$ to $-14^{+21}_{-24}$ kpc depending on model and priors; the strongest evidence is $\lesssim 1.9\sigma$, and a $3\sigma$ detection would require about four times as many satellites. The work highlights the critical need for a larger, more complete dwarf-galaxy census and, ideally, dynamical information, to robustly determine any offset and its implications for galaxy formation and cosmology.

Abstract

The arrangement of M31's dwarf galaxies exhibits anisotropy, with the majority located in the hemisphere between the Milky Way and the host galaxy. This study aims to investigate whether M31's present location is aligned with the center of its distribution of dwarf galaxies. We use forward modeling to infer the center of the M31 satellite 3D spatial distribution, folding in the completeness of dwarf galaxy searches. We observe a displacement of the center of the satellite distribution, relative to the center of M31, of approximately 10--50 kpc towards the Milky Way. Nonetheless, the center of M31 remains compatible with the center of the dwarf galaxy distribution given the broad constraints on its position, with the significance of the shift ranging from $\leq 1σ$ to $1.9σ$, depending on the assumed form of the volumetric spatial distribution of satellites. If M31 is truly offset from its satellite system, a quadrupling of the number of known satellites would be necessary to infer a significant ($3σ$) offset. Hence, expanding the number of known dwarf galaxies is crucial to deepen our understanding of the distribution of M31 satellites and further shed on its peculiar structure.

Figures (3)

• Figure 1: Left panels: Marginalized PDFs and correlation graphs for each parameter of the model composed by a power-law spatial distribution when enforcing a Gaussian prior on the central position parameters and when using the PAndAS sample.
• Figure 2: Marginalized PDFs of the distance between the center of M31 and the one of the satellites spatial distribution. While the favored distance of the center are shifted toward the MW, the center of the distribution is overall compatible with M31 center.
• Figure 3: PDFs for the center's position in the y-z plane in the case of a power law and NFW spatial distribution. M31's position is marked by a star, known dwarf galaxies are represented by purple dots, and the inferred satellite distribution center positions are denoted by blue/black crosses, along with their associated confidence contours (68%, 90%, 95%) . We infer that M31 center is compatible with the center of the distribution but the model favors a center shifted toward the MW.