Rapid Environmental Quenching of Satellite Dwarf Galaxies in the Local Group

In the Local Group, nearly all of the dwarf galaxies (M_star < 10^9 M_sun) that are satellites within 300 kpc (the virial radius) of the Milky Way (MW) and Andromeda (M31) have quiescent star formation and little-to-no cold gas. This contrasts strongly with comparatively isolated dwarf galaxies, which are almost all actively star-forming and gas-rich. This near dichotomy implies a rapid transformation of satellite dwarf galaxies after falling into the halos of the MW or M31. We combine the observed quiescent fractions for satellites of the MW and M31 with the infall times of satellites from the Exploring the Local Volume in Simulations (ELVIS) suite of cosmological zoom-in simulations to determine the typical timescales over which environmental processes within the MW/M31 halos remove gas and quench star formation in low-mass satellite galaxies. The quenching timescales for satellites with M_star < 10^8 M_sun are short, < 2 Gyr, and quenching is more rapid at lower M_star. These satellite quenching timescales can be 1 - 2 Gyr longer if one includes the time that satellites were environmentally preprocessed by low-mass groups prior to MW/M31 infall. We compare with quenching timescales for more massive satellites from previous works to synthesize the nature of satellite galaxy quenching across the observable range of M_star = 10^{3-11} M_sun. The satellite quenching timescale increases rapidly with satellite M_star, peaking at ~9.5 Gyr for M_star ~ 10^9 M_sun, and the timescale rapidly decreases at higher M_star to < 5 Gyr at M_star > 5 x 10^9 M_sun. Overall, galaxies with M_star ~ 10^9 M_sun, similar to the Magellanic Clouds, exhibit the longest quenching timescales, regardless of environmental or internal mechanisms.Comment: 6 pages, 3 figures. Accepted in ApJ Letters. Matches published versio

The WHIQII Survey: Metallicities and Spectroscopic Properties of Luminous Compact Blue Galaxies

As part of the WIYN High Image Quality Indiana Irvine (WHIQII) survey, we present 123 spectra of emission-line galaxies, selected on intermediate redshift (.4<z<.8) galaxies with blue colors that appear physically compact. The sample includes 15 true Luminous Compact Blue Galaxies (LCBGs) and an additional 27 slightly less extreme emission-line systems. These galaxies represent a highly evolving class that may play an important role in the decline of star formation since z~1, but their exact nature and evolutionary pathways remain a mystery. Here, we use emission lines to determine metallicities and ionization parameters, constraining their intrinsic properties and state of star formation. Some LCBG metallicities are consistent with a "bursting dwarf" scenario, while a substantial fraction of others are not, further confirming that LCBGs are a highly heterogeneous population but are broadly consistent with the intermediate redshift field. In agreement with previous studies, we observe overall evolution in the luminosity-metallicity relation at intermediate redshift. Our sample, and particularly the LCBGs, occupy a region in the empirical R23-O32 plane that differs from luminous local galaxies and is more consistent with dwarf Irregulars at the present epoch, suggesting that cosmic "downsizing" is observable in even the most fundamental parameters that describe star formation. These properties for our sample are also generally consistent with lying between local galaxies and those at high redshift, as expected by this scenario. Surprisingly, our sample exhibits no detectable correlation between compactness and metallicity, strongly suggesting that at these epochs of rapid star formation, the morphology of compact star-forming galaxies is largely transient.Comment: ApJ accepted, 17 pages, 20 figures, 2 tables (complete tables in published version

The Mass Dependance of Satellite Quenching in Milky Way-like Halos

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 ($M_{*}$ = $10^{8.5}-10^{10.5} \, M_{\odot}$), with only $\sim~20\%$ 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 halos quench their satellites more efficiently. These results extend trends seen previously in more massive host halos and for higher satellite masses. Taken together, it appears that galaxies with stellar masses larger than about $10^{8}~M_{\odot}$ 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 stellar masses ($< 10^{8}~M_{\odot}$), 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.Comment: 14 pages, 8 figure

Dynamical evidence for a strong tidal interaction between the Milky Way and its satellite, Leo V

We present a chemodynamical analysis of the Leo~V dwarf galaxy, based on Keck II DEIMOS spectra of 8 member stars. We find a systemic velocity for the system of $\langle v_r\rangle = 170.9^{+ 2.1}_{-1.9}$kms$^{-1}$, and barely resolve a velocity dispersion for the system, with $\sigma_{vr} = 2.3^{+3.2}_{-1.6}$kms$^{-1}$, consistent with previous studies of Leo~V. The poorly resolved dispersion means we are unable to adequately constrain the dark matter content of Leo~V. We find an average metallicity for the dwarf of [Fe/H]$= -2.48\pm0.21$, and measure a significant spread in the iron abundance of its member stars, with $-3.1\le$[Fe/H]$\le-1.9$ dex, which cleanly identifies Leo~V as a dwarf galaxy that has been able to self-enrich its stellar population through extended star formation. Owing to the tentative photometric evidence for tidal substructure around Leo~V, we also investigate whether there is any evidence for tidal stripping or shocking of the system within its dynamics. We measure a significant velocity gradient across the system, of $\frac{{\rm d}v}{{\rm d}\chi} = -4.1^{+2.8}_{-2.6}$kms$^{-1}$ per arcmin (or $\frac{{\rm d}v}{{\rm d}\chi} = -71.9^{+50.8}_{-45.6}$kms$^{-1}$~kpc$^{-1}$), which points almost directly toward the Galactic centre. We argue that Leo~V is likely a dwarf on the brink of dissolution, having just barely survived a past encounter with the centre of the Milky Way.Comment: 14 pages, 12 figures, accepted for publication in MNRAS. Updated to include minor revisions from referee proces