A Rogues’ Gallery of Andromeda's Dwarf Galaxies. I. A Predominance of Red Horizontal Branches

Abstract

We present homogeneous, sub-horizontal branch photometry of 20 dwarf spheroidal satellite galaxies of M31 observed with the Hubble Space Telescope. Combining our new data for 16 systems with archival data in the same filters for another four, we show that Andromeda dwarf spheroidal galaxies favor strikingly red horizontal branches or red clumps down to ∼10 4.2 L e (M V ∼−5.8). The age-sensitivity of horizontal branch stars implies that a large fraction of the M31 dwarf galaxies have extended star formation histories (SFHs), and appear inconsistent with early star formation episodes that were rapidly shutdown. Systems fainter than ∼10 5.5 L e show the widest range in the ratios and morphologies of red and blue horizontal branches, indicative of both complex SFHs and a diversity in quenching timescales and/or mechanisms, which is qualitatively different from what is currently known for faint Milky Way (MW) satellites of comparable luminosities. Our findings bolster similar conclusions from recent deeper data for a handful of M31 dwarf galaxies. We discuss several sources for diversity of our data such as varying halo masses, patchy reionization, mergers/accretion, and the environmental influence of M31 and the Milky Way on the early evolution of their satellite populations. A detailed comparison between the histories of M31 and MW satellites would shed signifiant insight into the processes that drive the evolution of low-mass galaxies. Such a study will require imaging that reaches the oldest main-sequence turnoffs for a significant number of M31 companions.

Elemental Abundances in M31: [Fe/H] and [α/Fe] in M31 Dwarf Galaxies Using Coadded Spectra

“…We take the presence of a slope in our lower-mass galaxies as further evidence of an extended SFH in these galaxies, as also evidenced by the presence of red HB stars in the CMDs. In particular, And XIV is interesting because it hosts a more balanced (i.e., a more equal number of red HB and blue HB stars) HB (Martin et al 2017) in comparison to the other dSphs in the low-mass regime. From our spectroscopic abundance measurements, we find that it is more metal-poor than one would naively expect following the mass-metallicity relation (MZR, Section 7.3).…”

“…We also find tentative evidence for a slope in the [α/Fe]-[Fe/H] distribution for And IX (m = −0.12 ± 0.26). Using HST photometry, Martin et al (2017) found And IX was relatively metal-rich for its stellar mass, but they noted that their sample likely had some contamination from the M31 halo. Such contamination would flatten the slope.…”

Warm FIRE: simulating galaxy formation with resonant sterile neutrino dark matter

“…Counts of faint galaxies detected 7 This assumes that the truncation in the ultrafaint dwarf SFH is set by reionization feedback effects. Recent observations of M31 ultrafaint dwarf galaxies have found extended SFHs, implying that the Milky Way utlrafaint satellite SFHs could be a product of local environmental effects such as non-uniform patchy reionization or stripping by the host (Martin et al 2017). in future imaging and HI surveys are therefore likely to provide strong constraints on WDM models.…”

MORGOTH: incorporating horizontal branch modelling into star formation history determinations

“…Even so, the information that we can extract from these few nearby E-mail: A.savino@rug.nl dwarf galaxies is very valuable and complements what we can learn from the study of unresolved, more distant galaxies (e.g., Gallazzi et al 2008;Boylan-Kolchin et al 2016;Goddard et al 2017). Moving beyond the satellites of the Milky Way, detecting the main sequence of the old population becomes challenging, and photometric data is often only able to reach down to the upper red giant branch (RGB) and the horizontal branch (HB) of galaxies (e.g., Martin et al 2016Martin et al , 2017.…”