Abstract
ABSTRACT The vast majority of low-mass satellite galaxies around the Milky Way and M31 appear virtually devoid of cool gas and show no signs of recent or ongoing star formation. Cosmological simulations demonstrate that such quenching is expected and is due to the harsh environmental conditions that satellites face when joining the Local Group (LG). However, recent observations of Milky Way analogues in the SAGA survey present a very different picture, showing the majority of observed satellites to be actively forming stars, calling into question the realism of current simulations and the typicality of the LG. Here, we use the ARTEMIS suite of high-resolution cosmological hydrodynamical simulations to carry out a careful comparison with observations of dwarf satellites in the LG, SAGA, and the Local Volume (LV) survey. We show that differences between SAGA and the LG and LV surveys, as well as between SAGA and the ARTEMIS simulations, can be strongly reduced by considering differences in the host mass distributions and (more importantly) observational selection effects, specifically that low-mass satellites which have only recently been accreted are more likely to be star forming, have a higher optical surface brightness, and are therefore more likely to be included in the SAGA survey. This picture is confirmed using data from the deeper LV survey, which shows pronounced quenching at low masses, in accordance with the predictions of ΛCDM-based simulations.
Highlights
It is well established that dwarf galaxies present numerous challenges to, and opportunities for refining, theories of galaxy formation
Perhaps the most striking case of this is found in the Local Group (LG) pair of galaxies, the Milky Way (MW) and Andromeda (M31), around which the vast majority of dwarf galaxies are depleted of cool gas and have no significant ongoing star formation (Grcevich & Putman 2009; Spekkens et al 2014; Putman et al 2021)
Narrowing the comparison to the range −14 < MV < −12, this difference is increased, with the SAGA satellites tending to be significantly bluer than those in Local Volume (LV) and especially with respect to ARTEMIS, we caution that the number of satellites in the comparison is significantly reduced for this narrower magnitude range
Summary
It is well established that dwarf galaxies present numerous challenges to, and opportunities for refining, theories of galaxy formation (see Bullock & Boylan-Kolchin 2017 for a recent review). The emphasis given to different environmental processes may vary between the studies, a general consensus is that MW-like hosts should be highly efficient at quenching the star formation of satellite dwarf galaxies with stellar masses 107−8 M From this perspective, and taking into consideration that this picture appears to be confirmed observationally in the LG, the low quenched fractions derived from SAGA are puzzling. An interesting finding of Font et al (2021) is that systems with lower than typical surface brightness (at fixed magnitude) tended to be more centrally concentrated in the ARTEMIS simulations, suggesting an environmental origin to the scatter in surface brightness at fixed luminosity If this interpretation is correct, it raises the possibility that the identification/selection of satellites in the observations is itself potentially dependent on the past environmental history of the satellites and may have important implications for comparisons between the predicted and observed quenched fractions of satellites around MW-mass hosts.
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