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

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. GilbertPuragra Guhathakurta, Rodrigo Ibata, Michael J. Irwin, Myoungwon Jeon, Evan Kirby, Geraint F. Lewis, Dougal Mackey, Steven R. Majewski, Nicolas Martin, Alan McConnachie, Ekta Patel, R. Michael Rich, Joshua D. Simon, Sangmo Tony Sohn, Erik J. Tollerud, Roeland P. van der Marel

Research output: Contribution to journalArticlepeer-review


We measure homogeneous distances to M31 and 38 associated stellar systems (−16.8 ≤ MV ≤ −6.0), using time-series observations of RR Lyrae stars taken as part of the Hubble Space Telescope Treasury Survey of M31 Satellites. From >700 orbits of new/archival Advanced Camera for Surveys imaging, we identify >4700 RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 day and 0.04 mag. Based on period–Wesenheit–metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of ∼20 kpc (3%) and ∼10 kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that ∼80% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms 7–23 kpc) planar “arc” of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess the physical proximity of notable associations such as the NGC 147/185 pair and M33/AND XXII; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with MV > − 9.5, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system.

Original languageEnglish (US)
Article number101
JournalAstrophysical Journal
Issue number2
StatePublished - Oct 1 2022

Bibliographical note

Funding Information:
A.S. thanks Y. Zheng for useful discussions at various stages of this work. Support for this work was provided by NASA through grants GO-13768, GO-15746, GO-15902, AR-16159, and GO-16273 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. M.C.C. acknowledges support though NSF grant AST-1815475. M.B.K. acknowledges support from NSF CAREER award AST-1752913, NSF grants AST-1910346 and AST-2108962, NASA grant NNX17AG29G, and HST-AR-15006, HST-AR-15809, HST-GO-15658, HST-GO-15901, HST-GO-15902, HST-AR-16159, and HST-GO-16226 from STScI. A.S. wishes to thank M. Marconi and R. Molinaro for providing the PWZ scaling relations used in this paper. This research has made use of NASA's Astrophysics Data System Bibliographic Services. All of the HST data used in this paper can be found in MAST: 10.17909/jb41-ex86 .

Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.


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