Rapid Environmental Quenching of Satellite Dwarf Galaxies in the Local Group


In the Local Group, nearly all of the dwarf galaxies (\(M_{\rm star}\lesssim 10^{9}~{}\mbox{M}_{\odot}\)) that are satellites within \(300~{}\mbox{kpc}\) (the virial radius) of the Milky Way (MW) and Andromeda (M31) have quiescent star formation and little-to-no cold gas. This contrasts strongly with comparatively isolated dwarf galaxies, which are almost all actively star-forming and gas-rich. This near dichotomy implies a rapid transformation after falling into the halos of the MW or M31. We combine the observed quiescent fractions for satellites of the MW and M31 with the infall times of satellites from the ELVIS suite of cosmological simulations to determine the typical timescales over which environmental processes within the MW/M31 halos remove gas and quench star formation in low-mass satellite galaxies. The quenching timescales for satellites with \(M_{\rm star}<10^{8}~{}\mbox{M}_{\odot}\) are short, \(\lesssim 2~{}\mbox{Gyr}\), and decrease at lower \(M_{\rm star}\). These quenching timescales can be \(1-2~{}\mbox{Gyr}\) longer if environmental preprocessing in lower-mass groups prior to MW/M31 infall is important. We compare with timescales for more massive satellites from previous works, exploring satellite quenching across the observable range of \(M_{\rm star}=10^{3-11}~{}\mbox{M}_{\odot}\). The environmental quenching timescale increases rapidly with satellite \(M_{\rm star}\), peaking at \(\approx 9.5~{}\mbox{Gyr}\) for \(M_{\rm star}\sim 10^{9}~{}\mbox{M}_{\odot}\), and rapidly decreases at higher \(M_{\rm star}\) to less than \(5~{}\mbox{Gyr}\) at \(M_{\rm star}>5\times 10^{9}~{}\mbox{M}_{\odot}\). Thus, satellites with \(M_{\rm star}\sim 10^{9}~{}\mbox{M}_{\odot}\), similar to the Magellanic Clouds, exhibit the longest environmental quenching timescales.


Galaxies in denser environments are more likely to have suppressed (quiescent) star formation and little-to-no cold gas than galaxies of similar stellar mass, \(M_{\rm star}\), in less dense environments. The observed environmental effects within the Local Group (LG) on the satellite galaxies within the halos of the Milky Way (MW) and Andromeda (M31) are particularly strong (e.g., Einasto et al., 1974; Grcevich et al., 2009; McConnachie, 2012; Phillips et al., 2014; Slater et al., 2014), even compared to the already strong effects on (more massive) satellites within massive groups/clusters. Specifically, the dwarf galaxies around the MW/M31 show a strikingly sharp and nearly complete transition in their properties within \(\approx 300~{}\mbox{kpc}\) (approximately the virial radius, \(R_{\rm vir}\), of the MW or M31), from irregular to spheroidal morphologies, from having significant cold atomic gas to having little-to-no measured cold gas, and from actively star-forming to quiescent. This trend has just a few exceptions: 4 gas-rich, star-forming galaxies persist within the halos of the MW (the LMC and SMC) and M31 (LGS 3 and IC 10), and 4 - 5 quiescent, gas-poor galaxies reside well beyond \(R_{\rm vir}\) of either the MW or M31: Cetus (Lewis et al., 2007), Tucana (Fraternali et al., 2009), KKR 25 (Makarov et al., 2012), KKs 3 (Karachentsev et al., 2015), and possibly Andromeda XVIII, though Cetus and Tucana may have orbited within the MW halo (Teyssier et al., 2012). This efficient satellite quenching is particularly striking because, other than KKR 25 and KKs 3, at \(M_{\rm star}<10^{9}~{}\mbox{M}_{\odot}\) all known galaxies that are sufficiently isolated (\(>1500~{}\mbox{kpc}\) from a more massive galaxy) are star-forming (Geha et al., 2012; Phillips et al., 2014). Thus, the MW and M31 halos show the strongest signal of environmental influence over their satellites of any known systems, and the LG is a compelling laboratory for studying environmental processes on galaxies.

Several such processes within a host halo can regulate the gas content, star formation, morphology, and eventual disruption of satellite galaxies, including gravitational tidal forces (e.g., Dekel et al., 2003), galaxy-galaxy tidal interactions (e.g., Farouki et al., 1981), galaxy-galaxy mergers (e.g., Deason et al., 2014), and ram-pressure stripping of extended gas (e.g., Larson et al., 1980; McCarthy et al., 2008) or cold inter-stellar medium (e.g., Gunn et al., 1972; Tonnesen et al., 2009). The key astrophysical challenge is understanding the relative importance of these processes, including which (if any) dominate, and how they vary across both satellite and host masses.

One strong constraint comes from determining the timescale over which environmental quenching occurs, as previous works have explored at higher masses (e.g., Balogh et al., 2000; Wetzel et al., 2013; Hirschmann et al., 2014; Wheeler et al., 2014). For the satellite dwarf galaxies in the LG, some works have shown that the environmental quenching efficiency is higher than than for more massive satellites in massive groups/clusters (Phillips et al., 2014; Slater et al., 2014). In this letter, we combine the observed quiescent fractions for satellites in the LG with the typical infall times of such satellites from cosmological simulations to infer the timescales over which environmental processes remove gas and quench star formation in the current satellite galaxies in the MW/M31 halos. Motivated by the results of Wetzel et al. (2015), we also consider the possible impact of group preprocessing on satellites before they fell into the MW/M31 halos. We also compare with the above works on more massive satellites, allowing us to examine satellite quenching timescales across the observable range of \(M_{\rm star}=10^{3-11}~{}\mbox{M}_{\odot}\).