Shea Garrison-Kimmel

Cosmological simulations with self-interacting dark matter – I. Constant-density cores and substructure

We use cosmological simulations to study the effects of self-interacting dark matter (SIDM) on the density profiles and substructure counts of dark matte r halos from the scales of spiral galaxies to galaxy clusters, focusing explicitly on mod els with cross sections over dark matter particle mass σ/m = 1 and 0.1 cm 2 /g. Our simulations rely on a new SIDM N-body algorithm that is derived self-consistently from the Boltz mann equation and that reproduces analytic expectations in controlled numerical experiments. We find that well-resolved SIDM halos have constant-density cores, with significantly lowe r central densities than their CDM counterparts. In contrast, the subhalo content of SIDM halos is only modestly reduced compared to CDM, with the suppression greatest for large hosts and small halo-centric distances. Moreover, the large-scale clustering and halo circular vel ocity functions in SIDM are effectively identical to CDM, meaning that all of the large-scale successes of CDM are equally well matched by SIDM. From our largest cross section runs we are able to extract scaling relations for core sizes and central densities over a range o f halo sizes and find a strong correlation between the core radius of an SIDM halo and the NFW scale radius of its CDM counterpart. We construct a simple analytic model, based on CDM scaling relations, that captures all aspects of the scaling relations for SIDM halos. Our results show that halo core densities in σ/m = 1 cm 2 /g models are too low to match observations of galaxy clusters, low surface brightness spirals (LSBs), and dwarf spheroidal galaxies. However, SIDM with σ/m ≃ 0.1 cm 2 /g appears capable of reproducing reported core sizes and central densities of dwarfs, LSBs, and galaxy clusters without the need for velocity dependence. Higher resolution simulations over a wider range of masses will be required to confirm this expectation. We discuss constraints arising from the Bullet cluster observ ations, measurements of dark matter density on small-scales and subhalo survival requirements, and show that SIDM models with σ/m ≃ 0.1 cm 2 /g ≃ 0.2 barn/GeV are consistent with all observational constraints.

Core formation in dwarf haloes with self-interacting dark matter: no fine-tuning necessary

Author(s): Elbert, OD; Bullock, JS; Garrison-Kimmel, S; Rocha, M; Onorbe, J; Peter, AHG | Abstract:

Sterile neutrino dark matter bounds from galaxies of the Local Group

We show that the canonical oscillation-based (nonresonant) production of sterile neutrino dark matter is inconsistent at >99% confidence with observations of galaxies in the Local Group. We set lower limits on the nonresonant sterile neutrino mass of 2.5 keV (equivalent to 0.7 keV thermal mass) using phase-space densities derived for dwarf satellite galaxies of the Milky Way as well as limits of 8.8 keV (equivalent to 1.8 keV thermal mass) based on subhalo counts of N-body simulations of M 31 analogs. Combined with improved upper mass limits derived from significantly deeper x-ray data of M 31 with full consideration for background variations, we show that there remains little room for nonresonant production if sterile neutrinos are to explain 100% of the dark matter abundance. Resonant and nonoscillation sterile neutrino production remain viable mechanisms for generating sufficient dark matter sterile neutrinos.

Sweating the small stuff: Simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites

We present Feedback in Realistic Environment (FIRE)/GIZMO hydrodynamic zoom-in simulations of isolated dark matter haloes, two each at the mass of classical dwarf galaxies (M_(vir) ≃ 10^(10)  M_⊙) and ultra-faint galaxies (M_(vir) ≃ 10^9  M_⊙), and with two feedback implementations. The resulting central galaxies lie on an extrapolated abundance matching relation from M_★ ≃ 10^6 to 10^4  M_⊙ without a break. Every host is filled with subhaloes, many of which form stars. Each of our dwarfs with M_★ ≃ 10^6  M_⊙ has 1–2 well-resolved satellites with M_★ = 3-200 × 10^3  M_⊙. Even our isolated ultra-faint galaxies have star-forming subhaloes. If this is representative, dwarf galaxies throughout the Universe should commonly host tiny satellite galaxies of their own. We combine our results with the Exploring the Local Volume in Simulations (ELVIS) simulations to show that targeting ∼ 50 kpc regions around nearby isolated dwarfs could increase the chances of discovering ultra-faint galaxies by ∼35 per cent compared to random pointings, and specifically identify the region around the Phoenix dwarf galaxy as a good potential target. The well-resolved ultra-faint galaxies in our simulations (M_★ ≃ 3-30 × 10^3  M_⊙) form within M_(peak) ≃ 0.5-3 × 10^9  M_⊙ haloes. Each has a uniformly ancient stellar population ( > 10 Gyr) owing to reionization-related quenching. More massive systems, in contrast, all have late-time star formation. Our results suggest that M_(halo) ≃ 5 × 10^9  M_⊙ is a probable dividing line between haloes hosting reionization ‘fossils’ and those hosting dwarfs that can continue to form stars in isolation after reionization.

FIRE-2 Simulations: Physics versus Numerics in Galaxy Formation

The Feedback In Realistic Environments (FIRE) project explores feedback in cosmological galaxy formation simulations. Previous FIRE simulations used an identical source code (“FIRE-1”) for consistency. Motivated by the development of more accurate numerics – including hydrodynamic solvers, gravitational softening, and supernova coupling algorithms – and exploration of new physics (e.g. magnetic fields), we introduce “FIRE-2”, an updated numerical implementation of FIRE physics for the GIZMO code. We run a suite of simulations and compare against FIRE-1: overall, FIRE-2 improvements do not qualitatively change galaxy-scale properties. We pursue an extensive study of numerics versus physics. Details of the star-formation algorithm, cooling physics, and chemistry have weak effects, provided that we include metal-line cooling and star formation occurs at higher-than-mean densities. We present new resolution criteria for high-resolution galaxy simulations. Most galaxy-scale properties are robust to numerics we test, provided: (1) Toomre masses are resolved; (2) feedback coupling ensures conservation, and (3) individual supernovae are time-resolved. Stellar masses and profiles are most robust to resolution, followed by metal abundances and morphologies, followed by properties of winds and circum-galactic media (CGM). Central (∼kpc) mass concentrations in massive (>L*) galaxies are sensitive to numerics (via trapping/recycling of winds in hot halos). Multiple feedback mechanisms play key roles: supernovae regulate stellar masses/winds; stellar mass-loss fuels late star formation; radiative feedback suppresses accretion onto dwarfs and instantaneous star formation in disks. We provide all initial conditions and numerical algorithms used.

SATELLITE DWARF GALAXIES IN A HIERARCHICAL UNIVERSE: THE PREVALENCE OF DWARF-DWARF MAJOR MERGERS

Mergers are a common phenomenon in hierarchical structure formation, especially for massive galaxies and clusters, but their importance for dwarf galaxies in the Local Group remains poorly understood. We investigate the frequency of major mergers between dwarf galaxies in the Local Group using the ELVIS suite of cosmological zoom-in dissipationless simulations of Milky Way- and M31-like host halos. We find that ~10% of satellite dwarf galaxies with Mstar > 10^6 M_☉ that are within the host virial radius experienced a major merger of stellar mass ratio closer than 0.1 since z = 1, with a lower fraction for lower mass dwarf galaxies. Recent merger remnants are biased toward larger radial distance and more recent virial infall times, because most recent mergers occurred shortly before crossing within the virial radius of the host halo. Satellite–satellite mergers also occur within the host halo after virial infall, catalyzed by the large fraction of dwarf galaxies that fell in as part of a group. The merger fraction doubles for dwarf galaxies outside of the host virial radius, so the most distant dwarf galaxies in the Local Group are the most likely to have experienced a recent major merger. We discuss the implications of these results on observable dwarf merger remnants, their star formation histories, the gas content of mergers, and massive black holes in dwarf galaxies.

How to zoom: bias, contamination and Lagrange volumes in multimass cosmological simulations

We perform a suite of multimass cosmological zoom simulations of individual dark matter halos and explore how to best select Lagrangian regions for resimulation without contaminating the halo of interest with low-resolution particles. Such contamination can lead to significant errors in the gas distribution of hydrodynamical simulations, as we show. For a fixed Lagrange volume, we find that the chance of contamination increases systematically with the level of zoom. In order to avoid contamination, the Lagrangian volume selected for resimulation must increase monotonically with the resolution difference between parent box and the zoom region. We provide a simple formula for selecting Lagrangian regions (in units of the halo virial volume) as a function of the level of zoom required. We also explore the degree to which a halos Lagrangian volume correlates with other halo properties (concentration, spin, formation time, shape, etc.) and find no significant correlation. There is a mild correlation between Lagrange volume and environment, such that halos living in the most clustered regions have larger Lagrangian volumes. Nevertheless, selecting halos to be isolated is not the best way to ensure inexpensive zoom simulations. We explain how one can safely choose halos with the smallest Lagrangian volumes, which are the least expensive to resimulate, without biasing ones sample.

Taking care of business in a flash : constraining the time-scale for low-mass satellite quenching with ELVIS

Mon. Not. R. Astron. Soc. 000, 1–12 (2015) Printed 27 August 2015 (MN L A TEX style file v2.2) Taking Care of Business in a Flash E: Constraining the Timescale for Low-Mass Satellite Quenching with ELVIS arXiv:1503.06803v2 [astro-ph.GA] 25 Aug 2015 Sean P. Fillingham, 1? Michael C. Cooper, 1 † Coral Wheeler, 1 Shea Garrison-Kimmel, 1 Michael Boylan-Kolchin, 2 James S. Bullock 1 Center for Cosmology, Department of Physics and Astronomy, 4129 Reines Hall, University of California, Irvine, CA 92697 of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742-2421 2 Department 27 August 2015 ABSTRACT 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 M ? ∼ 10 6 − 10 8 M . 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 M ? . 10 8 M . Combining our work with the results of complementary analyses in the literature, we conclude that the suppression of star formation in massive satellites (M ? ∼ 10 8 − 10 11 M ) 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 M , 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 M ? ∼ 10 8 M ) 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. Key words: Local Group – galaxies: formation – galaxies: evolution – galaxies: dwarf – galaxies: star formation INTRODUCTION Foremost among the results of galaxy surveys over the last decade has been the realization that the galaxy population at z . 2 is bimodal in nature (e.g. Strateva et al. 2001; Baldry et al. 2004; Bell et al. 2004; Cooper et al. 2006). That is, galaxies both locally and out to intermediate red- shift are effectively described as one of two distinct types: red, early-type galaxies lacking significant star formation and blue, late-type galaxies with active star formation. In color-magnitude space, the red galaxies populate a tight re- lation (often called the red sequence), while the distribu- tion of blue galaxies is more scattered (sometimes referred to as the blue cloud). While the red and blue populations ? e-mail: sfilling@uci.edu † e-mail: cooper@uci.edu c 2015 RAS comprise approximately equal portions of the cosmic stellar mass budget at z ∼ 1, galaxies on the red sequence domi- nate today, following a growth in stellar mass within the red population of roughly a factor of 2 over the past 7 Gyr (Bell et al. 2004; Bundy et al. 2006; Faber et al. 2007; Brown et al. 2007). Despite uncertainty regarding the particular physical process(es) at play, the suppression (or quenching) of star formation in blue galaxies, thereby making them red, is one of the principal drivers of this dramatic growth in the num- ber density of quiescent systems at late cosmic time. At both low and intermediate redshift, the local envi- ronment of a galaxy is known to be well-correlated with the suppression of star formation, such that passive or quies- cent galaxies preferentially live in higher-density environ- ments (Balogh et al. 2004; Kauffmann et al. 2004; Blanton et al. 2005; Cooper et al. 2006, 2007, 2010a). While the stel-

Not so lumpy after all: modelling the depletion of dark matter subhaloes by Milky Way-like galaxies

Among the most important goals in cosmology is detecting and quantifying small (M_(halo)≃10^(6−9) M⊙) dark matter (DM) subhaloes. Current probes around the Milky Way (MW) are most sensitive to such substructure within ∼20 kpc of the halo centre, where the galaxy contributes significantly to the potential. We explore the effects of baryons on subhalo populations in ΛCDM using cosmological zoom-in baryonic simulations of MW-mass haloes from the Latte simulation suite, part of the Feedback In Realistic Environments (FIRE) project. Specifically, we compare simulations of the same two haloes run using (1) DM-only (DMO), (2) full baryonic physics and (3) DM with an embedded disc potential grown to match the FIRE simulation. Relative to baryonic simulations, DMO simulations contain ∼2 × as many subhaloes within 100 kpc of the halo centre; this excess is ≳5 × within 25 kpc. At z = 0, the baryonic simulations are completely devoid of subhaloes down to 3×10^6M⊙ within 15 kpc of the MW-mass galaxy, and fewer than 20 surviving subhaloes have orbital pericentres <20 kpc. Despite the complexities of baryonic physics, the simple addition of an embedded central disc potential to DMO simulations reproduces this subhalo depletion, including trends with radius, remarkably well. Thus, the additional tidal field from the central galaxy is the primary cause of subhalo depletion. Subhaloes on radial orbits that pass close to the central galaxy are preferentially destroyed, causing the surviving population to have tangentially biased orbits compared to DMO predictions. Our method of embedding a potential in DMO simulations provides a fast and accurate alternative to full baryonic simulations, thus enabling suites of cosmological simulations that can provide accurate and statistical predictions of substructure populations.

Satellites of LMC-mass dwarfs: close friendships ruined by Milky Way mass haloes

Motivated by the recent discovery of several dwarfs near the Large Magellanic Cloud (LMC), we study the accretion of massive satellites onto Milky Way (MW)/M31-like haloes using the ELVIS suite of N-body simulations. We identify 25 surviving LMC-mass subhaloes, and investigate the lower-mass satellites that were associated with these subhaloes before they fell into the MW/M31 haloes. Typically, 7 per cent of the overall z = 0 satellite population of MW/M31 haloes were in a surviving LMC-group before falling into the MW/M31 halo. This fraction can vary between 1 and 25 per cent, being higher for groups with higher mass and/or more recent infall times. Groups of satellites disperse rapidly in phase space after infall, and their distances and velocities relative to the group centre become statistically similar to the overall satellite population after 4–8 Gyr. We quantify the likelihood that satellites were associated with an LMC-mass group as a function of both distance and velocity relative to the LMC at z = 0. The close proximity in distance of the nine Dark Energy Survey candidate dwarf galaxies to the LMC suggest that ∼2–4 are likely associated with the LMC. Furthermore, if several of these dwarfs are genuine members, then the LMC-group probably fell into the MW very recently, ≲2 Gyr ago. If the connection with the LMC is established with follow-up velocity measurements, these ‘satellites of satellites’ represent prime candidates to study the effects of group pre-processing on lower mass dwarfs.