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

Abstract

We measure homogeneous distances to M31 and 38 associated stellar systems (−16.8 ≤ M V ≤ −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 M V > − 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 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.

Pegasus W: An Ultrafaint Dwarf Galaxy Outside the Halo of M31 Not Quenched by Reionization

“…For reference, the MW sits at the SG origin. Note that the axis ranges on each plot are fixed at 600 kpc, but the plot includes only galaxies found to be within 500 kpc of Pegasus W in the updated compilation from McConnachie (2012) with revised distances from Savino et al (2022) where available. We also include M31 with an adopted distance of 776.2 21 22 -+ kpc determined from RR Lyrae stars (Savino et al 2022).…”

On the Reionization-era Globular Cluster in the Low-mass Galaxy Eridanus II

“…For example, Cusano et al (2016) report a putative faint GC (M V = −4.6) in And XXV (M V = −9.1) and Caldwell et al (2017) re-discovered a faint GC (M V = −3.5; comparable to the GC in Eri II) in And I (M V = −11.7). The M31 galaxy and GC luminosities have been updated with the RR Lyrae distances from Savino et al (2022).…”

CHAOS. VII. A Large-scale Direct Abundance Study in M33

“…There are many distances to M33 in the literature to choose from (see discussion in de Grijs et al 2017;Lee et al 2022). We adopt the distance modulus of 24.67 ± 0.06 (corresponding to -+ 859 23 24 kpc) based on HST observations of RR Lyrae (Savino et al 2022) anchored to the GAIA eDR3 reference frame (Nagarajan et al 2022), which is consistent with the values favored by de Grijs et al (2017) and Lee et al (2022). The effective radius of M33, R e , is determined from the z0MGS WISE 3.4 μm maps (Leroy et al 2019) with the same fitting method that is described in Leroy et al (2021) and has been utilized in previous CHAOS studies (e.g., Berg et al 2020).…”

“…Local spiral galaxies present the best opportunity to study gas-phase abundances in a large quantity of H II regions with sampling on small spatial scales. A quintessential example of such a spiral galaxy is M33; the third largest member of the Local Group, M33 is very close (adopted distance of 0.86 Mpc from Savino et al 2022), relatively face-on (inclination ∼55°), and hosts a number of bright H II regions across the disk of the galaxy. Given its angular size and proximity, M33 has been the focus of many pioneering chemical abundances studies (Smith 1975;Kwitter & Aller 1981;Diaz & Tosi 1984;Vilchez et al 1988).…”