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Open Access Publications from the University of California

Scholarly Works (34 results)

  • Article
  • Peer Reviewed
UC Davis Previously Published Works (2014)
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 M star > 106 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.
    • Thesis
    • Peer Reviewed
    UC Irvine Electronic Theses and Dissertations (2015)

    Dwarf galaxies are among the most numerous objects in the Universe, and also appear to be the most dark matter (DM) dominated; consequently, they provide strong tests on the standard paradigm of hierarchical galaxy formation: cold dark matter with a cosmological constant (ΛCDM). Due to their low luminosities, however, observational studies of dwarfs have remained limited to the nearby Universe, with a primary focus on the satellites of the Milky Way (MW). Upcoming surveys will relax the observational constraints, allowing for studies of dwarf galaxies well beyond the virial radius (Rv) of the MW, where the presence of the Andromeda (M31) galaxy may have a measurable impact. In this thesis, I introduce the ELVIS (Exploring the Local Volume in Simulations) suite, which resolves the formation and evolution of Local Group (LG)-like pairs of galaxies, chosen to resemble the MW and M31 host halos, along with thousands of dwarf halos that surround them. Using ELVIS, I demonstrate that simulations of isolated MW-size hosts do not correctly predict dwarf counts and kinematics beyond Rv of the MW, and also explore the faint-end relationship between stellar mass (Mstar) and halo mass (Mhalo). I then use ELVIS to explore the “too-big- to-fail” (TBTF) problem, a challenge for ΛCDM that points out the overabundance of large (maximum circular velocity Vmax 􏰆~ 30 km/s) dwarf halos relative to observations, finding TBTF to be ubiquitous to MW-size hosts and the fields around them. Finally, I explore two possible explanations for TBTF in depth: fluctuations in the baryonic mass in the center of

    a dwarf halo, driven by supernovae feedback, which I find lacks the energy to solve TBTF, and a “running” initial power spectrum, as motivated by the BICEP-2 measurement of primordial gravitational waves, which significantly alleviates the problem and may enhance the impact of other processes, including baryonic feedback.

      Cover page: Galaxy Formation in the Local GroupCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2015)
      In the Local Group (LG), almost all satellite dwarf galaxies that are within the virial radius of the Milky Way (MW) and Andromeda (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 Mstar = 103-9 M⊙ using the Exploring the Local Volume in Simulations 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 Gyr ago at z = 0.5-1. However, they first fell into any host halo typically 7-10 Gyr ago at z = 0.7-1.5. This difference arises because many satellites experienced "group preprocessing" in another host halo, typically of Mvir ∼ 1010-12 M⊙, 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 Mstar < 106 M⊙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 LG occurred at distinct epochs, separated typically by 2-4 Gyr, so they are separable theoretically and, in principle, observationally.
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2014)
        The BICEP2 results, when interpreted as a gravitational wave signal and combined with other cosmic microwave background data, suggest a roll-off in power towards small scales in the primordial matter power spectrum. Among the simplest possibilities is a running of the spectral index. Here we show that the preferred level of running alleviates small-scale issues within the ΛCDM model, more so even than viable WDM models. We use cosmological zoom-in simulations of a MilkyWay-sized halo along with full-box simulations to compare predictions among four separate cosmologies: a BICEP2-inspired running index model (αs = -0.024), two fixed-tilt ΛCDM models motivated by Planck, and a 2.6 keV thermal WDM model. We find that the running BICEP2 model reduces the central densities of large dwarf-sized haloes (Vmax ~ 30-80 km s-1) and alleviates the too-big-to-fail problem significantly compared to our adopted Planck andWDMcases. Further, the BICEP2 model suppresses the count of small subhaloes by ~50 per cent relative to Planck models, and yields a significantly lower 'boost' factor for dark matter annihilation signals. Our findings highlight the need to understand the shape of the primordial power spectrum in order to correctly interpret small-scale data.
          Cover page: Running with BICEP2: implications for small-scale problems in CDM
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2018)
          We investigate the response of self-interacting dark matter (SIDM) halos to the growth of galaxy potentials using idealized simulations, with each run in tandem with collisionless cold dark matter (CDM). We find that if the stellar potential strongly dominates in the central parts of a galaxy, then SIDM halos can be as dense as CDM halos on observable scales. For extreme cases, core collapse can occur, leading to SIDM halos that are denser and cuspier than their CDM counterparts. If the stellar potential is not dominant, then SIDM halos retain isothermal cores with densities far below CDM predictions. When a disk is present, the inner SIDM halo becomes more flattened in the disk plane than the CDM halo. These results are in excellent quantitative agreement with the predictions of Kaplinghat et al. We also simulated a cluster halo with a central stellar distribution similar to the brightest central galaxy of the cluster A2667. An SIDM halo simulated with the cross-section over mass provides a good match to the measured dark matter (DM) density profile, while an adiabatically contracted CDM halo is denser and cuspier. The profile of the same halo simulated with is not dense enough. Our findings are in agreement with previous results that is disfavored for DM collision velocities above about 1500 km s-1. More generally, the interaction between baryonic potentials and SIDM densities offers new directions for constraining SIDM cross-sections in galaxies where baryons are dynamically important.
            Cover page: A Testable Conspiracy: Simulating Baryonic Effects on Self-interacting Dark Matter HalosCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2016)
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2014)
              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. © 2014 American Physical Society.
                Cover page: Sterile neutrino dark matter bounds from galaxies of the Local Group
                • Article
                • Peer Reviewed
                UC Davis Previously Published Works (2018)
                We identify subhalos in dark matter-only (DMO) zoom-in simulations that are likely to be disrupted due to baryonic effects by using a random forest classifier trained on two hydrodynamic simulations of Milky Way (MW)-mass host halos from the Latte suite of the Feedback in Realistic Environments (FIRE) project. We train our classifier using five properties of each disrupted and surviving subhalo: pericentric distance and scale factor at first pericentric passage after accretion and scale factor, virial mass, and maximum circular velocity at accretion. Our five-property classifier identifies disrupted subhalos in the FIRE simulations with an 85% out-of-bag classification score. We predict surviving subhalo populations in DMO simulations of the FIRE host halos, finding excellent agreement with the hydrodynamic results; in particular, our classifier outperforms DMO zoom-in simulations that include the gravitational potential of the central galactic disk in each hydrodynamic simulation, indicating that it captures both the dynamical effects of a central disk and additional baryonic physics. We also predict surviving subhalo populations for a suite of DMO zoom-in simulations of MW-mass host halos, finding that baryons impact each system consistently and that the predicted amount of subhalo disruption is larger than the host-to-host scatter among the subhalo populations. Although the small size and specific baryonic physics prescription of our training set limits the generality of our results, our work suggests that machine-learning classification algorithms trained on hydrodynamic zoom-in simulations can efficiently predict realistic subhalo populations.
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2015)
                  We investigate the effect of self-interacting dark matter (SIDM) on the density profiles of Vmax ≃ 40km s-1 isolated dwarf dark matter haloes-the scale of relevance for the too big to fail problem (TBTF)-using very high resolution cosmological zoom simulations. Each halo has millions of particles within its virial radius. We find that SIDM models with cross-sections per unitmass spanning the range s/m=0.5-50 cm2 g-1 alleviate TBTF and produce constantdensity cores of size 300-1000 pc, comparable to the half-light radii of M* ~ 105-7M⊙ dwarfs. The largest, lowest density cores develop for cross-sections in the middle of this range, σ/m ~ 5-10 cm2 g-1. Our largest SIDM cross-section run (σ/m = 50 cm2 g-1) develops a slightly denser core owing to mild core-collapse behaviour, but it remains less dense than the cold dark matter case and retains a constant-density core profile. Our work suggests that SIDM cross-sections as large or larger than 50 cm2 g-1 remain viable on velocity scales of dwarf galaxies (vrms ~ 40 km s-1). The range of SIDM cross-sections that alleviate TBTF and the cusp/core problem spans at least two orders of magnitude and therefore need not be particularly fine-tuned.
                    • Article
                    • Peer Reviewed
                    UC Irvine Previously Published Works (2016)
                    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 ~ 108M⊙. This suggest a physical scalewhere 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 turbulent viscous stripping. Across a broad range of host properties, we find that stripping becomes increasingly effective at M* ≲ 108-9M⊙, reproducing the critical mass scale observed. However, for canonical values of the circumgalactic medium density (nhalo < 10-3.5 cm-3), we find that stripping is not fully effective; infalling satellites are, on average, stripped of only ≲40-60 per cent 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 haloes.
                      Cover page: Under pressure: quenching star formation in low-mass satellite galaxies via strippingCreative Commons 'BY' version 4.0 license
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