# The Hubble Space Telescope Survey of M31 Satellite Galaxies I. RR Lyrae-based Distances and Refined 3D Geometric Structure

**Authors:** Alessandro Savino, Daniel R. Weisz, Evan D. Skillman, Andrew Dolphin, Nitya Kallivayalil, Andrew Wetzel, Jay Anderson, Gurtina Besla, Michael Boylan-Kolchin, James S. Bullock, Andrew A. Cole, Michelle L. M. Collins, M. C. Cooper, Alis J. Deason, Aaron L. Dotter, Mark Fardal, Annette M. N. Ferguson, Tobias K. Fritz, Marla C. Geha, Karoline M. Gilbert  
**arXiv:** [2206.02801](https://arxiv.org/abs/2206.02801)  
**Date:** 2022-06-06  
**Summary page:** https://www.summarizepaper.com/en/paper/2206.02801v2/

## Key points

- The study measures homogeneous distances to M31 and 38 associated stellar systems using time-series observations of RR Lyrae stars.
- Over 4700 RR Lyrae stars are identified and their periods and mean magnitudes are determined with high precision.
- Heliocentric and M31-centric distances are measured with a typical precision of approximately 20 kpc (3%) and 10 kpc (8%), respectively.
- The spatial distribution of M31's satellites is highly anisotropic, with approximately 80% residing on the near side of M31.
- A thin planar "arc" composed of satellites within a range of approximately 7-23 kpc from M31 is confirmed, consisting roughly half of all galaxies within this distance range.
- The study reassesses the physical proximity between notable associations such as NGC 147/185 pair and M33/AND XXII, providing new insights into their relationships.
- Challenges in accurately determining distances for faint galaxies using tip-of-the-red-giant branch methods are highlighted, with potential biases up to 35%.
- RR Lyrae stars are emphasized as important for accurate distance measurements to faint galaxies, particularly in light of upcoming facilities like the Rubin Observatory.
- Updated luminosities and sizes for the sample will serve as a basis for future investigations into star formation histories and orbital dynamics in the entire known M31 satellite system.

## Summary

In their study, the authors measure homogeneous distances to M31 (Andromeda galaxy) and 38 associated stellar systems using time-series observations of RR Lyrae stars. This research is part of the Hubble Space Telescope Treasury Survey of M31 Satellites. By analyzing over 700 orbits of new and archival ACS imaging data, they identify more than 4700 RR Lyrae stars and determine their periods and mean magnitudes with a typical precision of 0.01 days and 0.04 mag. Using Period-Wesenheit-Metallicity relationships consistent with the Gaia eDR3 distance scale, the authors uniformly measure heliocentric and M31-centric distances with a typical precision of approximately 20 kpc (3%) and 10 kpc (8%), respectively. These precise measurements allow them to revise the three-dimensional structure of the M31 galactic ecosystem. The findings reveal several interesting features about the distribution of satellites around M31. Firstly, they confirm a highly anisotropic spatial distribution where approximately 80% of M31's satellites reside on the near side of M31; this observation cannot be easily explained by observational effects alone. Additionally, they affirm the existence of a thin planar "arc" composed of satellites within a range of approximately 7-23 kpc from M31; this arc consists roughly half (15) all galaxies within this distance range from M31. The study also reassesses the physical proximity between notable associations such as NGC 147/185 pair and M33/AND XXII, providing new insights into their relationships. Furthermore, it highlights challenges in accurately determining distances for galaxies with absolute magnitudes fainter than -9.5 using tip-of-the-red-giant branch methods; these distances can be biased by up to 35%. The importance of RR Lyrae stars for accurate distance measurements to faint galaxies is emphasized, particularly in light upcoming facilities like the Rubin Observatory. The authors provide updated luminosities and sizes for their sample which will serve as a basis for future investigations into star formation histories and orbital dynamics in the entire known M31 satellite system. Overall, this study significantly contributes to our understanding structure and distribution satellites around M31; it sheds light on their spatial arrangement while providing valuable data for further research in this field.