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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2021)

Satellite dwarf galaxies are singularly useful for studies of galaxy formation because they are highly susceptible to environmental and stellar feedback effects, we can observe them in unparalleled detail in the Local Group (LG), and we can model them at high resolution in simulations. Of particular interest in galaxy formation are the Milky Way's (MW) satellites, which appear to be nearly uniformly quenched inside the MW's virial radius, likely because of both internal stellar feedback and environmental effects of the gaseous host halo. Many observational campaigns have been devoted to measuring LG satellite star formation histories and high precision proper motions from resolved stellar populations, critical to determining orbital effects on formation. However, LG surveys will revolutionize galaxy formation, only if we can provide sufficiently rigorous theoretical models to interpret them.

The spatial distribution, kinematics, and star formation histories of satellite dwarf galaxies in the LG present a unique opportunity to test galaxy formation and numerical resolution in cosmological hydrodynamic simulations. We find excellent agreement between LG observations and the inner radial distributions of ‘classical’ dwarf satellites around MW/M31-mass host galaxies from the FIRE simulations. These results also suggest potentially undiscovered classical dwarf-mass satellites in the outer halos of the MW and M31. Furthermore, we investigate the prevalence, longevity, and potential causes of so-called ‘satellite planes’ in the LG. Although satellites in the simulations are typically distributed almost isotropically around their hosts, we find that hosts with an LMC-like satellite are more likely to have MW-like satellite planes, and that M31's satellite distribution is much more common. Preliminary results on the gas content and star formation histories of simulated satellites indicate agreement with the high quenched fraction of satellites in the LG. However, the simulations and LG observations appear to be in tension with the small fraction of quenched satellites around MW analogs in the nearby universe. Our results suggest that environmental effects of the host halo may be the dominant mechanisms for quenching satellite dwarf galaxies.

    Cover page: Local Group Satellite Galaxies in Cosmological Simulations
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2021)
    In hierarchical structure formation, metal-poor stars in and around the Milky Way (MW) originate primarily from mergers of lower mass galaxies. A common expectation is therefore that metal-poor stars should have isotropic, dispersion-dominated orbits that do not correlate strongly with the MW disc. However, recent observations of stars in the MW show that metal-poor ([Fe/H] ≲ -2) stars are preferentially on prograde orbits with respect to the disc. Using the Feedback In Realistic Environments 2 (FIRE-2) suite of cosmological zoom-in simulations of MW/M31-mass galaxies, we investigate the prevalence and origin of prograde metal-poor stars. Almost all (11 of 12) of our simulations have metal-poor stars on preferentially prograde orbits today and throughout most of their history: we thus predict that this is a generic feature of MW/M31-mass galaxies. The typical prograde-to-retrograde ratio is ~2:1, which depends weakly on stellar metallicity at [Fe/H] ~ -1. These trends predicted by our simulations agree well with MW observations. Prograde metal-poor stars originate largely from a single Large/Small Magellanic Cloud (LMC/SMC)-mass gas-rich merger 7-12.5 Gyr ago, which deposited existing metal-poor stars and significant gas on an orbital vector that sparked the formation of and/or shaped the orientation of a long-lived stellar disc, giving rise to a prograde bias for all low-metallicity stars. We find subdominant contributions from in situ stars formed in the host galaxy before this merger, and in some cases, additional massive mergers. We find few clear correlations between any properties of our MW/M31-mass galaxies at z = 0 and the degree of this prograde bias as a result of diverse merger scenarios.
      Cover page: The origin of metal-poor stars on prograde disc orbits in FIRE simulations of Milky Way-mass galaxies
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2021)
      Characterizing the predicted environments of dwarf galaxies like the Large Magellanic Cloud (LMC) is becoming increasingly important as next generation surveys push sensitivity limits into this low-mass regime at cosmological distances. We study the environmental effects of LMC-mass halos ($M_{200m} \sim 10^{11}$ M$_\odot$) on their populations of satellites ($M_\star \geq 10^4$ M$_\odot$) using a suite of zoom-in simulations from the Feedback In Realistic Environments (FIRE) project. Our simulations predict significant hot coronas with $T\sim10^5$ K and $M_\text{gas}\sim10^{9.5}$ M$_\odot$. We identify signatures of environmental quenching in dwarf satellite galaxies, particularly for satellites with intermediate mass ($M_\star = 10^{6-7}$ M$_\odot$). The gas content of such objects indicates ram-pressure as the likely quenching mechanism, sometimes aided by star formation feedback. Satellites of LMC-mass hosts replicate the stellar mass dependence of the quiescent fraction found in satellites of MW mass hosts (i.e. that the quiescent fraction increases as stellar mass decreases). Satellites of LMC-mass hosts have a wider variety of quenching times when compared to the strongly bi-modal distribution of quenching times of nearby centrals. Finally, we identify significant tidal stellar structures around four of our six LMC-analogs, suggesting that stellar streams may be common. These tidal features originated from satellites on close orbits, extend to $\sim$80 kpc from the central galaxy, and contain $\sim10^{6-7}$ M$_\odot$~of stars.
        Cover page: The effects of LMC-mass environments on their dwarf satellite galaxies in the FIRE simulations
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2021)
        We examine the prevalence, longevity, and causes of planes of satellite dwarf galaxies, as observed in the Local Group. We use 14 Milky Way/Andromeda-(MW/M31) mass host galaxies from the Feedback In Realistic Environments-2 simulations. We select the 14 most massive satellites by stellar mass within ≤ 300, kpc of each host and correct for incompleteness from the foreground galactic disc when comparing to the MW. We find that MW-like planes as spatially thin and/or kinematically coherent as observed are uncommon, but they do exist in our simulations. Spatially thin planes occur in 1-2 per cent of snapshots during z = 0-0.2, and kinematically coherent planes occur in 5 per cent of snapshots. These planes are generally transient, surviving for <500 Myr. However, if we select hosts with a Large Magellanic Cloud-like satellite near first pericentre, the fraction of snapshots with MW-like planes increases dramatically to 7-16 per cent, with lifetimes of 0.7-1 Gyr, likely because of group accretion of satellites. We find that M31's satellite distribution is much more common: M31's satellites lie within ∼1σ of the simulation median for every plane metric we consider. We find no significant difference in average satellite planarity for isolated hosts versus hosts in LG-like pairs. Baryonic and dark matter-only simulations exhibit similar levels of planarity, even though baryonic subhaloes are less centrally concentrated within their host haloes. We conclude that planes of satellites are not a strong challenge to ΛCDM cosmology.
          Cover page: Planes of satellites around Milky Way/M31-mass galaxies in the FIRE simulations and comparisons with the Local Group
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2020)
          While many tensions between Local Group (LG) satellite galaxies and ⋀ cold dark matter cosmology have been alleviated through recent cosmological simulations, the spatial distribution of satellites remains an important test of physical models and physical versus numerical disruption in simulations. Using the FIRE-2 cosmological zoom-in baryonic simulations, we examine the radial distributions of satellites with M∗ > 105 M☉ around eight isolated Milky Way (MW) mass host galaxies and four hosts in LG-like pairs. We demonstrate that these simulations resolve the survival and physical destruction of satellites with M∗ ≲ 105 M☉. The simulations broadly agree with LG observations, spanning the radial profiles around the MW and M31. This agreement does not depend strongly on satellite mass, even at distances ≾100 kpc. Host-to-host variation dominates the scatter in satellite counts within 300 kpc of the hosts, while time variation dominates scatter within 50 kpc. More massive host galaxies within our sample have fewer satellites at small distances, likely because of enhanced tidal destruction of satellites via the baryonic discs of host galaxies. Furthermore, we quantify and provide fits to the tidal depletion of subhaloes in baryonic relative to dark matter-only simulations as a function of distance. Our simulated profiles imply observational incompleteness in the LG even at M∗ ≳ 105 M☉: we predict 2-10 such satellites to be discovered around the MW and possibly 6-9 around M31. To provide cosmological context, we compare our results with the radial profiles of satellites around MW analogues in the SAGA survey, finding that our simulations are broadly consistent with most SAGA systems.
            Cover page: A profile in FIRE: resolving the radial distributions of satellite galaxies in the Local Group with simulations
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2019)
            We study star formation histories (SFHs) of 500 dwarf galaxies (stellar mass M∗ = 105–109 M) from FIRE-2 cosmological zoom-in simulations. We compare dwarfs around individual Milky Way (MW)-mass galaxies, dwarfs in Local Group (LG)-like environments, and true field (i.e. isolated) dwarf galaxies. We reproduce observed trends wherein higher mass dwarfs quench later (if at all), regardless of environment. We also identify differences between the environments, both in terms of ‘satellite versus central’ and ‘LG versus individual MW versus isolated dwarf central.’ Around the individual MW-mass hosts, we recover the result expected from environmental quenching: central galaxies in the ‘near field’ have more extended SFHs than their satellite counterparts, with the former more closely resemble isolated (true field) dwarfs (though near-field centrals are still somewhat earlier forming). However, this difference is muted in the LG-like environments, where both near-field centrals and satellites have similar SFHs, which resemble satellites of single MW-mass hosts. This distinction is strongest for M∗ = 106–107 M but exists at other masses. Our results suggest that the paired halo nature of the LG may regulate star formation in dwarf galaxies even beyond the virial radii of the MW and Andromeda. Caution is needed when comparing zoom-in simulations targeting isolated dwarf galaxies against observed dwarf galaxies in the LG.
              Cover page: Star formation histories of dwarf galaxies in the FIRE simulations: dependence on mass and Local Group environment
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