Satellite Dwarf Galaxies in a Hierarchical Universe: Infall Histories, Group Preprocessing, and Reionization


In the Local Group, almost all satellite dwarf galaxies that are within the virial radius of the Milky Way (MW) and M31 exhibit strong environmental influence. The orbital histories of these satellites provide the key to understanding the role of the MW/M31 halo, lower-mass groups, and cosmic reionization on the evolution of dwarf galaxies. We examine the virial-infall histories of satellites with \({M_{\rm star}}=10^{3-9} {~\mbox{M}_\odot}\) using the ELVIS suite of cosmological zoom-in dissipationless simulations of 48 MW/M31-like halos. Satellites at \(z=0\) fell into the MW/M31 halos typically \(5-8 {~\mbox{Gyr}}\) ago at \(z=0.5-1\). However, they first fell into any host halo typically \(7-10 {~\mbox{Gyr}}\) ago at \(z=0.7-1.5\). This difference arises because many satellites experienced “group preprocessing” in another host halo, typically of \({M_{\rm vir}}\sim 10^{10-12} {~\mbox{M}_\odot}\), before falling into the MW/M31 halos. Satellites with lower-mass and/or those closer to the MW/M31 fell in earlier and are more likely to have experienced group preprocessing; half of all satellites with \({M_{\rm star}}< 10^6 {~\mbox{M}_\odot}\) were preprocessed in a group. Infalling groups also drive most satellite-satellite mergers within the MW/M31 halos. Finally, none of the surviving satellites at \(z=0\) were within the virial radius of their MW/M31 halo during reionization (\(z > 6\)), and only \(< 4\%\) were satellites of any other host halo during reionization. Thus, effects of cosmic reionization versus host-halo environment on the formation histories of surviving dwarf galaxies in the Local Group occurred at distinct epochs and are separable in time.


Galaxies in dense environments are more likely to have suppressed (quiescent) star-formation rates (SFR), more elliptical/spheroidal/bulge-dominated morphologies, and less cold gas in/around them than galaxies of similar stellar mass, \({M_{\rm star}}\), in less dense environments. While such environmental effects long have been studied in massive galaxy groups and clusters (for example, Oemler, 1974; Dressler, 1980; Dressler et al., 1983; Balogh et al., 1997; Blanton et al., 2009, for review), the observed effects on the dwarf galaxies of the Local Group (LG), in particular, the satellites within the host halos of the Milky Way (MW) and M31, are even stronger (Mateo, 1998; McConnachie, 2012; Phillips et al., 2014; Slater et al., 2014; Spekkens et al., 2014).

Specifically, the galaxies around the Milky Way (MW) and Andromeda (M31) show a strikingly sharp transition in their properties within \(\approx 300 {~\mbox{kpc}}\), corresponding to the virial radii, \({R_{\rm vir}}\), of the halos of the MW and M31 for \({M_{\rm vir}}\approx 10 ^ {12} {~\mbox{M}_\odot}\) (e.g., Deason et al., 2012; van der Marel et al., 2012; Boylan-Kolchin et al., 2013). Within this distance, galaxies transition from (1) having irregular to elliptical/spheroidal morphologies, (2) having most of their baryonic mass in cold atomic/molecuar gas to having little-to-no detectible cold gas, and (3) being actively star-forming to quiescent (McConnachie, 2012, and references therein). This environmental transition of the population is nearly a complete one, with just a few exceptions. Four gas-rich, star-forming, irregular galaxies persist within the halos of the MW (the LMC and SMC) and M31 (LGS 3 and IC 10). However, the LMC and SMC are likely on their first infall (Besla et al., 2007; Kallivayalil et al., 2013), and given their distances to M31, LGS 3 and IC 10 may be as well. Furthermore, 3 - 4 gas-poor, quiescent, spheroidal galaxies exist just beyond the halos of the MW (Cetus and Tucana) and M31 (KKR 25 and possibly Andromeda XVIII), though the radial velocities of Cetus and Tucana imply that they likely orbited within the MW halo (Teyssier et al., 2012). The fact that almost all of the satellite galaxies within the MW/M31 halos show such strong environmental effects is particularly striking given that, other than KKR 25, all known galaxies at \({M_{\rm star}}< 10 ^ 9 {~\mbox{M}_\odot}\) that are isolated (not within \(1500 {~\mbox{kpc}}\) of a more massive galaxy, and thus not strongly influenced by environmental effects) are actively star-forming (Geha et al., 2012) and gas-rich. Thus, the MW and M31 halos exert the strongest observed environmental influence on their galaxy populations of any known systems, making the LG one of the most compelling laboratories to study environmental effects on galaxy evolution.

Several environmental processes within a host halo can play a role in regulating the gas content, star formation, morphology, and eventual tidal distruption of satellite galaxies. Gravitationally, the strong tidal forces of the host halo will strip mass from the satellite (subhalo) from the outside-in (Dekel et al., 2003; Diemand et al., 2007; Wetzel et al., 2010). In addition, the dense collection of satellites within a host halo can drive impulsive gravitational interactions with each other (Farouki et al., 1981; Moore et al., 1998), and satellites can merge with one another (Angulo et al., 2009; Wetzel et al., 2009; Wetzel et al., 2009a; Deason et al., 2014). Moreover, tidal shocking and resonant interactions with the host’s galactic disk can lead to particularly efficient morphological evolution, coring, stripping, and disruption (Mayer et al., 2001; D’Onghia et al., 2010; Zolotov et al., 2012). Hydrodynamically, if the host halo contains thermalized hot gas, this can strip and heat the extended gas from the orbiting satellite subhalo (Balogh et al., 2000; McCarthy et al., 2008), leading to reduced gas cooling/accretion into the satellite’s disk (Larson et al., 1980). More drastically, given a sufficiently high density of hot gas and high orbital velocity, ram-pressure can strip cold gas directly from the satellite’s disk (Gunn et al., 1972; Abadi et al., 1999; Mayer et al., 2006; Chung et al., 2009; Tonnesen et al., 2009). Furthermore, galactic winds driven by feedback within satellite galaxies can allow these environmentel process to operate even more efficiently (for example, Bahé et al., 2015).

Understanding the relative efficiency of the above environmental processes, including the timescales over which they have operated, requires understanding in detail the orbital and virial-infall histories of the current satellite population in the context of the hierarchical structure formation of \(\Lambda\)CDM. While some authors examined the viria