Comparing the Quenching Times of Faint M31 and Milky Way Satellite Galaxies

TL;DR

This study compares the star formation histories of faint M31 satellites with Milky Way satellites, revealing differences in quenching times likely due to their distinct accretion histories, and identifies two distinct SFH groups among M31 satellites.

Contribution

It provides new measurements of quenching times for 20 faint M31 satellites and highlights differences with MW satellites, suggesting varied accretion histories influence satellite evolution.

Findings

Most M31 satellites quenched between 3-9 Gyr ago.

MW satellites show different quenching patterns based on luminosity.

Two distinct SFH groups identified among M31 satellites.

Abstract

We present the star formation histories (SFHs) of 20 faint M31 satellites ($-12 \lesssim M_V \lesssim -6$) that were measured by modeling sub-horizontal branch (HB) depth color-magnitude diagrams constructed from Hubble Space Telescope (HST) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration $3-6$ Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent ($<3$ Gyr) quenching times. We suggest that systematic differences in the quenching times of M31 and MW satellites may be a reflection of the varying accretion histories of M31 and the MW. This result implies that the formation histories of low-mass satellites may not be broadly representative of low-mass galaxies in general. Among the M31 satellite population we identify two distinct groups based on their SFHs: one with exponentially declining SFHs ($\tau \sim 2$ Gyr) and one with rising SFHs with abrupt quenching. We speculate how these two groups could be related to scenarios for a recent major merger involving M31. The Cycle 27 HST Treasury survey of M31 satellites will provide well-constrained ancient SFHs to go along with the quenching times we measure here. The discovery and characterization of M31 satellites with $M_V \gtrsim -6$ would help quantify the relative contributions of reionization and environment to quenching of the lowest-mass satellites.