A rogues gallery of Andromeda's dwarf galaxies II. Precise Distances to 17 Faint Satellites

TL;DR

This paper provides precise horizontal branch distance measurements for 17 faint M31 satellite galaxies using HST data, improving distance accuracy and updating their properties without significantly altering the known satellite system structure.

Contribution

It introduces a method for accurate distance measurement of faint satellites using deep HST imaging and well-defined HBs, enhancing previous ground-based estimates.

Findings

Average distance precision of 4% (~30 kpc at 800 kpc)

Most distances agree with ground-based TRGB, with some discrepancies

Updated luminosities and structural parameters for the satellites

Abstract

We present new horizontal branch (HB) distance measurements to 17 of the faintest known M31 satellites ($-6 \lesssim M_{V} \lesssim -13$) based on deep Hubble Space Telescope (HST) imaging. The color-magnitude diagrams extend $\sim$1-2 magnitudes below the HB, which provides for well-defined HBs, even for faint galaxies in which the tip of the red giant branch (TRGB) is sparsely populated. We determine distances across the sample to an average precision of 4% ($\sim 30$~kpc at $800$~kpc). We find that the majority of these galaxies are in good agreement, though slightly farther (0.1-0.2 mag) when compared to recent ground-based TRGB distances. Two galaxies (And~IX and And~XVII) have discrepant HST and ground-based distances by $\sim 0.3$ mag ($\sim 150$~kpc), which may be due to contamination from Milky Way foreground stars and/or M31 halo stars in sparsely populated TRGB regions. We use the new distances to update the luminosities and structural parameters for these 17 M31 satellites. The new distances do not substantially change the spatial configuration of the M31 satellite system. We comment on future prospects for precise and accurate HB distances for faint galaxies in the Local Group and beyond.