Near-Field Limits on the Role of Faint Galaxies in Cosmic Reionization

Reionizing the Universe with galaxies appears to require significant star formation in low-mass halos at early times, while local dwarf galaxy counts tell us that star formation has been minimal in small halos around us today. Using simple models and the ELVIS simulation suite, we show that reionization scenarios requiring appreciable star formation in halos with $M_{\rm vir} \approx 10^{8}\,M_{\odot}$ at $z=8$ are in serious tension with galaxy counts in the Local Group. This tension originates from the seemingly inescapable conclusion that 30 - 60 halos with $M_{\rm vir} > 10^{8}\,M_{\odot}$ at $z=8$ will survive to be distinct bound satellites of the Milky Way at $z = 0$. Reionization models requiring star formation in such halos will produce dozens of bound galaxies in the Milky Way's virial volume today (and 100 - 200 throughout the Local Group), each with $\gtrsim 10^{5}\,M_{\odot}$ of old stars ($\gtrsim 13$ Gyr). This exceeds the stellar mass function of classical Milky Way satellites today, even without allowing for the (significant) post-reionization star formation observed in these galaxies. One possible implication of these findings is that star formation became sharply inefficient in halos smaller than $\sim 10^9 \,M_{\odot}$ at early times, implying that the high-$z$ luminosity function must break at magnitudes brighter than is often assumed (at ${\rm M_{UV}} \approx -14$). Our results suggest that JWST (and possibly even HST with the Frontier Fields) may realistically detect the faintest galaxies that drive reionization. It remains to be seen how these results can be reconciled with the most sophisticated simulations of early galaxy formation at present, which predict substantial star formation in $M_{\rm vir} \sim 10^8 \, M_{\odot}$ halos during the epoch of reionization.Comment: 6 pages, 4 figures; minor updates. Published in MNRAS Letter

Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxies

We use Local Group galaxy counts together with the ELVIS N-body simulations to explore the relationship between the scatter and slope in the stellar mass vs. halo mass relation at low masses, $M_\star \simeq 10^5 - 10^8 M_\odot$. Assuming models with log-normal scatter about a median relation of the form $M_\star \propto M_\mathrm{halo}^\alpha$, the preferred log-slope steepens from $\alpha \simeq 1.8$ in the limit of zero scatter to $\alpha \simeq 2.6$ in the case of $2$ dex of scatter in $M_\star$ at fixed halo mass. We provide fitting functions for the best-fit relations as a function of scatter, including cases where the relation becomes increasingly stochastic with decreasing mass. We show that if the scatter at fixed halo mass is large enough ($\gtrsim 1$ dex) and if the median relation is steep enough ($\alpha \gtrsim 2$), then the "too-big-to-fail" problem seen in the Local Group can be self-consistently eliminated in about $\sim 5-10\%$ of realizations. This scenario requires that the most massive subhalos host unobservable ultra-faint dwarfs fairly often; we discuss potentially observable signatures of these systems. Finally, we compare our derived constraints to recent high-resolution simulations of dwarf galaxy formation in the literature. Though simulation-to-simulation scatter in $M_\star$ at fixed $M_\mathrm{halo}$ is large among separate authors ($\sim 2$ dex), individual codes produce relations with much less scatter and usually give relations that would over-produce local galaxy counts.Comment: 15 pages, 6 figures, 1 table. Accepted for publication into MNRA

On the stark difference in satellite distributions around the Milky Way and Andromeda

We compare spherically-averaged radial number counts of bright (> 10^5 Lsun) dwarf satellite galaxies within 400 kpc of the Milky Way (MW) and M31 and find that the MW satellites are much more centrally concentrated. Remarkably, the two satellite systems are almost identical within the central 100 kpc, while M31 satellites outnumber MW satellites by about a factor of four at deprojected distances spanning 100 - 400 kpc. We compare the observed distributions to those predicted for LCDM suhbalos using a suite of 44 high-resolution ~10^12 halo zoom simulations, 22 of which are in pairs like the MW and M31. We find that the radial distribution of satellites around M31 is fairly typical of those predicted for subhalos, while the Milky Way's distribution is more centrally concentrated that any of our simulated LCDM halos. One possible explanation is that our census is bright (> 10^5 Lsun) MW dwarf galaxies is significantly incomplete beyond ~ 100 kpc of the Sun. If there were ~8 - 20 more bright dwarfs orbiting undetected at 100 - 400 kpc, then the Milky Way's radial distribution would fall within the range expected from subhalo distributions and alos look very much like the known M31 system. We use our simulations to demonstrate that there is enough area left unexplored by the Sloan Digital Sky Survey and its extensions that the discovery of ~10 new bright dwarfs is not implausible given the expected range of angular anisotropy of subhalos in the sky.Comment: 10 pages, 8 figures, submitted to MNRA

Too Big to Fail in the Local Group

We compare the dynamical masses of dwarf galaxies in the Local Group (LG) to the predicted masses of halos in the ELVIS suite of $\Lambda$CDM simulations, a sample of 48 Galaxy-size hosts, 24 of which are in paired configuration similar to the LG. We enumerate unaccounted-for dense halos ($V_\mathrm{max} \gtrsim 25$ km s$^{-1}$) in these volumes that at some point in their histories were massive enough to have formed stars in the presence of an ionizing background ($V_\mathrm{peak} > 30$ km s$^{-1}$). Within 300 kpc of the Milky Way, the number of unaccounted-for massive halos ranges from 2 - 25 over our full sample. Moreover, this "too big to fail" count grows as we extend our comparison to the outer regions of the Local Group: within 1.2 Mpc of either giant we find that there are 12-40 unaccounted-for massive halos. This count excludes volumes within 300 kpc of both the MW and M31, and thus should be largely unaffected by any baryonically-induced environmental processes. According to abundance matching -- specifically abundance matching that reproduces the Local Group stellar mass function -- all of these missing massive systems should have been quite bright, with $M_\star > 10^6M_\odot$. Finally, we use the predicted density structure of outer LG dark matter halos together with observed dwarf galaxy masses to derive an $M_\star-V_\mathrm{max}$ relation for LG galaxies that are outside the virial regions of either giant. We find that there is no obvious trend in the relation over three orders of magnitude in stellar mass (a "common mass" relation), from $M_\star \sim 10^8 - 10^5 M_\odot$, in drastic conflict with the tight relation expected for halos that are unaffected by reionization. Solutions to the too big to fail problem that rely on ram pressure stripping, tidal effects, or statistical flukes appear less likely in the face of these results.Comment: 16 pages, 14 figures, 2 tables, submitted to MNRA

Environmental Quenching of Low-Mass Field Galaxies

In the local Universe, there is a strong division in the star-forming properties of low-mass galaxies, with star formation largely ubiquitous amongst the field population while satellite systems are predominantly quenched. This dichotomy implies that environmental processes play the dominant role in suppressing star formation within this low-mass regime (${M}_{\star} \sim 10^{5.5-8}~{\rm M}_{\odot}$). As shown by observations of the Local Volume, however, there is a non-negligible population of passive systems in the field, which challenges our understanding of quenching at low masses. By applying the satellite quenching models of Fillingham et al. (2015) to subhalo populations in the Exploring the Local Volume In Simulations (ELVIS) suite, we investigate the role of environmental processes in quenching star formation within the nearby field. Using model parameters that reproduce the satellite quenched fraction in the Local Group, we predict a quenched fraction -- due solely to environmental effects -- of $\sim 0.52 \pm 0.26$ within $1< R/R_{\rm vir} < 2$ of the Milky Way and M31. This is in good agreement with current observations of the Local Volume and suggests that the majority of the passive field systems observed at these distances are quenched via environmental mechanisms. Beyond $2~R_{\rm vir}$, however, dwarf galaxy quenching becomes difficult to explain through an interaction with either the Milky Way or M31, such that more isolated, field dwarfs may be self-quenched as a result of star-formation feedback.Comment: 9 pages, 4 figures, MNRAS accepted version, comments welcome - RIP Ducky...gone but never forgotte

Taking Care of Business in a Flash: Constraining the Timescale for Low-Mass Satellite Quenching with ELVIS

The vast majority of dwarf satellites orbiting the Milky Way and M31 are quenched, while comparable galaxies in the field are gas-rich and star-forming. Assuming that this dichotomy is driven by environmental quenching, we use the ELVIS suite of N-body simulations to constrain the characteristic timescale upon which satellites must quench following infall into the virial volumes of their hosts. The high satellite quenched fraction observed in the Local Group demands an extremely short quenching timescale (~ 2 Gyr) for dwarf satellites in the mass range Mstar ~ 10^6-10^8 Msun. This quenching timescale is significantly shorter than that required to explain the quenched fraction of more massive satellites (~ 8 Gyr), both in the Local Group and in more massive host halos, suggesting a dramatic change in the dominant satellite quenching mechanism at Mstar < 10^8 Msun. Combining our work with the results of complementary analyses in the literature, we conclude that the suppression of star formation in massive satellites (Mstar ~ 10^8 - 10^11 Msun) is broadly consistent with being driven by starvation, such that the satellite quenching timescale corresponds to the cold gas depletion time. Below a critical stellar mass scale of ~ 10^8 Msun, however, the required quenching times are much shorter than the expected cold gas depletion times. Instead, quenching must act on a timescale comparable to the dynamical time of the host halo. We posit that ram-pressure stripping can naturally explain this behavior, with the critical mass (of Mstar ~ 10^8 Msun) corresponding to halos with gravitational restoring forces that are too weak to overcome the drag force encountered when moving through an extended, hot circumgalactic medium.Comment: 12 pages, 6 figures; resubmitted to MNRAS after referee report (August 25, 2015

Under Pressure: Quenching Star Formation in Low-Mass Satellite Galaxies via Stripping

Recent studies of galaxies in the local Universe, including those in the Local Group, find that the efficiency of environmental (or satellite) quenching increases dramatically at satellite stellar masses below ~ $10^8\ {\rm M}_{\odot}$. This suggests a physical scale where quenching transitions from a slow "starvation" mode to a rapid "stripping" mode at low masses. We investigate the plausibility of this scenario using observed HI surface density profiles for a sample of 66 nearby galaxies as inputs to analytic calculations of ram-pressure and viscous stripping. Across a broad range of host properties, we find that stripping becomes increasingly effective at $M_{*} < 10^{8-9}\ {\rm M}_{\odot}$, reproducing the critical mass scale observed. However, for canonical values of the circumgalactic medium density ($n_{\rm halo} < 10^{-3.5}$${\rm cm}^{-3}$), we find that stripping is not fully effective; infalling satellites are, on average, stripped of < 40 - 70% of their cold gas reservoir, which is insufficient to match observations. By including a host halo gas distribution that is clumpy and therefore contains regions of higher density, we are able to reproduce the observed HI gas fractions (and thus the high quenched fraction and short quenching timescale) of Local Group satellites, suggesting that a host halo with clumpy gas may be crucial for quenching low-mass systems in Local Group-like (and more massive) host halos.Comment: updated version after review, now accepted to MNRAS; Accepted 2016 August 22. Received 2016 August 18; in original form 2016 June 2

in Violence Against Women, (2002)

This substantial article examines the issues of men who are victimized by domestic violence in heterosexual relationships. Over the past several years, there has increasing attention to the issues of men who are victimized by heterosexual domestic violence, most of which is based on research done that is based on the Conflict Tactics Scale (CTS) developed by Murray Straus and Richard Gelles. In this current paper, Kimmel addresses the research that suggests men are victimized as often as women from both substantive and methodological perspectives. Through the process, Kimmel also addresses the CTS and raises substantive issues with the continued use of this tool to examine domestic violence. Kimmel notes that the language (both media and in much of the specialized literature and theory) describing domestic violence has increasingly come to be that of gender symmetry. Review of the research (Fierbert, 1997, Archer, 2000) found that between 79 and 82 empirical and 16 review articles that demonstrated gender symmetry. As Kimmel notes, these studies “raise troubling questions ” about what has come to be accepted as relatively common knowledge about domestic violence – that it is something men do to women, that it is one of the leading causes of serious injury to women, and that it is one of the world’s most widespread public health issues. Beyond this, however, the research suggesting gender symmetry raises far more questions than it supposed answers. These questions largely revolv