Coral Wheeler

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.

On the morphologies, gas fractions, and star formation rates of small galaxies

We use a series of N-body/smoothed particle hydrodynamics simulations and analytic arguments to show that the presence of an effective temperature floor in the interstellar medium at T F ∼ 10 4 K naturally explains the tendency for low-mass galaxies to be more spheroidal, more gas rich, and less efficient in converting baryons into stars than larger galaxies. The trend arises because gas pressure support becomes important compared to angular momentum support in small dark matter haloes. We suggest that dwarf galaxies with rotational velocities ∼ 40 km s -1 do not originate as thin discs, but rather are born as thick, puffy systems. If accreted on to larger haloes, tenuous dwarfs of this kind will be more susceptible to gas loss or tidal transformation than scaled-down versions of larger spirals. For a constant temperature floor, pressure support becomes less important in large haloes, and this produces a tendency for massive isolated galaxies to have thinner discs and more efficient star formation than their less-massive counterparts, as observed.

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-

The no-spin zone: rotation vs dispersion support in observed and simulated dwarf galaxies

We perform a systematic Bayesian analysis of rotation vs. dispersion support (v_(rot)/σ) in 40 dwarf galaxies throughout the Local Volume (LV) over a stellar mass range 10^(3.5) M_⊙ < M⋆ < 10^8 M_⊙. We find that the stars in ∼90% of the LV dwarf galaxies studied -- both satellites and isolated systems -- are dispersion-supported. In particular, we show that 7/10 *isolated* dwarfs in our sample have stellar populations with v_(rot)/σ<0.6. All have v_(rot)/σ≲2. These results challenge the traditional view that the stars in gas-rich dwarf irregulars (dIrrs) are distributed in cold, rotationally-supported stellar disks, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion-supported stars. We see no clear trend between v_(rot)/σ and distance to the closest L⋆ galaxy, nor between v_(rot)/σ and M⋆ within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies (10^9M⊙<M_(vir)<10^(10)M⊙) and show that the simulated *isolated* dIrr galaxies have stellar ellipticities and stellar v_(rot)/σ ratios that are consistent with the observed population of dIrrs *and* dSphs without the need to subject these dwarfs to any external perturbations or tidal forces. We posit that most dwarf galaxies form as puffy, dispersion-supported systems, rather than cold, angular momentum-supported disks. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.

The mass dependence of satellite quenching in Milky Way-like haloes

© 2014 The Authors. Using the Sloan Digital Sky Survey, we examine the quenching of satellite galaxies around isolated Milky Way-like hosts in the local Universe. We find that the efficiency of satellite quenching around isolated galaxies is low and roughly constant over two orders of magnitude in satellite stellar mass (M* = 108.5-1010.5M), with only ~20 per cent of systems quenched as a result of environmental processes. While largely independent of satellite stellar mass, satellite quenching does exhibit clear dependence on the properties of the host. We show that satellites of passive hosts are substantially more likely to be quenched than those of star-forming hosts, and we present evidence that more massive haloes quench their satellites more efficiently. These results extend trends seen previously in more massive host haloes and for higher satellite masses. Taken together, it appears that galaxies with stellar masses larger than about 108M are uniformly resistant to environmental quenching, with the relative harshness of the host environment likely serving as the primary driver of satellite quenching. At lower stellarmasses (<108M), however, observations of the Local Group suggest that the vast majority of satellite galaxies are quenched, potentially pointing towards a characteristic satellite mass scale below which quenching efficiency increases dramatically.

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 M⊙. This suggest 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 H I 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* ≲ 10^(8 – 9) M⊙, reproducing the critical mass scale observed. However, for canonical values of the circumgalactic medium density (n_(halo) < 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 H I gas fractions (and thus the high quenched fraction and short quenching time-scale) 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.

An investigation of the temporal coherence length of light

An interference filter is constructed from a pair of front-surface two-way mirrors. These partially reflecting mirrors are clamped at both ends with a small spacer placed at one end, thus forming a thin wedge of air between their face-to-face mirrored surfaces. A series of thin film interference spectra are recorded by moving this air wedge apparatus in incremental steps across a beam of white light. The mirror separation as a function of position along the wedge is obtained from analysis of the transmitted spectra. As the separation increases, temporal coherence is lost for successively longer wavelengths until interference effects are no longer observed. We define coherence length measured in this way as twice the wedge thickness where the interference fringe pattern is reduced to the noise level of the transmitted spectrum. Using this definition and technique, a value of 224 ± 20 µm is obtained using a white light source. This value is much larger than one would expect for the coherence length of white light measured by more standard techniques, e.g. Michelson interferometry. The implication of our experiment is that coherence length is a property of the light reaching the detector, which in our case is the light transmitted by the interference filter.