We demonstrate that dwarf galaxies (10 super(7) < M sub(stellar) < 10 super(9) M sub(middot in circle), -12 < M sub(r) > -18) with no active star formation are extremely rare (<0.06%) in the field. ...We examine the relative number of quenched versus star-forming dwarf galaxies, defining quenched galaxies as having no Halpha emission (EW sub(H)alpha < 2 A) and a strong 4000 A break. The fraction of quenched dwarf galaxies decreases rapidly with increasing distance from a massive host, leveling off for distances beyond 1.5 Mpc. We define galaxies beyond 1.5 Mpc of a massive host galaxy to be in the field. We demonstrate that there is a stellar mass threshold of M sub(stellar) < 1.0 x 10 super(9) M sub(middot in circle) below which quenched galaxies do not exist in the field. The majority of quenched dwarf galaxies (often classified as dwarf elliptical galaxies) are within 2 virial radii of a massive galaxy, and only a few percent of quenched dwarf galaxies exist beyond 4 virial radii.
Atopic dermatitis (AD) lesional skin is often colonized with S. aureus, and the load of S. aureus correlates with disease severity. However, a causative and mechanistic link between S. aureus skin ...colonization and severity of AD is not well established. We made use of well-established mouse model of AD elicited by epicutaneous sensitization of tape stripped skin with ovalbumin to investigate the relationship between allergic skin inflammation and cutaneous S. aureus colonization. Topical application of S aureus exacerbated allergic skin inflammation induced by epicutaneous sensitization with ovalbumin, whereas allergic skin inflammation generated a permissive environment for S. aureus persistence. Our results establish a mutually reinforcing role of allergic skin inflammation and S. aureus skin colonization.
•Topical application of S aureus exacerbated allergic skin inflammation.•Allergic skin inflammation generated a permissive environment for S. aureus persistence.•IL-4 and IL-13 impair S. aureus clearance from sites of allergic skin inflammation.
We investigate the gas content and baryonic Tully-Fisher relationship for extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 > M sub(r) - 5 log h sub(70) > -16. The sample ...is selected from the Sloan Digital Sky Survey and consists of 101 galaxies for which we have obtained follow-up H I observations using the Arecibo Observatory and Green Bank Telescope. This represents the largest homogeneous sample of dwarf galaxies at low luminosities with well-measured H I and optical properties. The sample spans a range of environments, from dense groups to truly isolated galaxies. The average neutral gas fraction is (f sub(gas)) = 0.6, significantly exceeding that of typical gas-rich galaxies at higher luminosities. Dwarf galaxies are therefore less efficient at turning gas into stars over their lifetimes. The strong environmental dependence of the gas fraction distribution demonstrates that while internal processes can reduce the gas fractions to roughly f sub(gas) = 0.4, external processes are required to fully remove gas from a dwarf galaxy. The average rotational velocity of our sample is (v sub(rot)) = (W20 sub(i,t)/2) = 50 km s super(-1), based on H I line widths. In this luminosity range, the optical Tully-Fisher relationship has significantly more scatter compared to the baryonic relationship. By including more massive galaxies from the literature, we fit a baryonic Tully-Fisher slope of M sub(bary) 8 v super(3) sub(r) super(.) sub(o) super(7) sub(t) super(0c0.15). This slope compares well with CDM models that assume an equal baryon-to-dark matter ratio at all masses. While gas stripping or other processes may modify the baryon-to-dark matter ratio for dwarfs in the densest environments, the majority of dwarf galaxies in our sample have not preferentially lost significant baryonic mass relative to more massive galaxies.
ABSTRACT
We introduce the southern stellar stream spectroscopy survey (S5), an on-going program to map the kinematics and chemistry of stellar streams in the southern hemisphere. The initial focus of ...S5 has been spectroscopic observations of recently identified streams within the footprint of the dark energy survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9 m Anglo-Australian Telescope’s 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S5 has mapped nine DES streams and three streams outside of DES; the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S5 program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction, and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S5, including future public data releases, can be found at http://s5collab.github.io.
We present Magellan/M2FS, Very Large Telescope/GIRAFFE, and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II. Based on the spectra of 25 Ret II member stars ...selected from Dark Energy Survey imaging, we measure a mean heliocentric velocity of 62.8 + or - 0.5 km s super(-1) and a velocity dispersion of 3.3 + or - 0.7 km s super(-1). The mass-to-light ratio of Ret II within its half-light radius is 470 + or - 210 M sub(middot in circle)L sub(middot in circle), demonstrating that it is a strongly dark matter-dominated system. Despite its spatial proximity to the Magellanic Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 km s super(-1), respectively, suggesting that it is not gravitationally bound to the Magellanic system. The likely member stars of Ret II span 1.3 dex in metallicity, with a dispersion of 0.28 + or - 0.09 dex, and we identify several extremely metal-poor stars with Fe/H < -3. In combination with its luminosity, size, and ellipticity, these results confirm that Ret II is an ultra-faint dwarf galaxy. With a mean metallicity of Fe/H = -2.65 + or - 0.07, Ret II matches Segue 1 as the most metal-poor galaxy known. Although Ret II is the third-closest dwarf galaxy to the Milky Way, the line-of-sight integral of the dark matter density squared is log sub(10) (J) = 18.8 0.6 GeV cm = + or - 2 -5 within 0degrees.2, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies.
We present kinematic and metallicity profiles for the M 31 dwarf elliptical (dE) satellite galaxies NGC 147 and NGC 185. The profiles represent the most extensive spectroscopic radial coverage for ...any dE galaxy, extending to a projected distance of 8 half-light radii (8r{sub eff} {approx} 14'). We achieve this coverage via Keck/DEIMOS multislit spectroscopic observations of 520 and 442 member red giant branch stars in NGC 147 and NGC 185, respectively. In contrast to previous studies, we find that both dEs have significant internal rotation. We measure a maximum rotational velocity of 17 +- 2 km s{sup -1} for NGC 147 and 15 +- 5 km s{sup -1} for NGC 185. While both rotation profiles suggest a flattening in the outer regions, there is no indication that we have reached the radius of maximum rotation velocity. The velocity dispersions decrease gently with radius with average dispersions of 16 +- 1 km s{sup -1} and 24 +- 1 km s{sup -1} for NGC 147 and NGC 185, respectively. The average metallicities for NGC 147 and NGC 185 are Fe/H = -1.1 +- 0.1 and Fe/H = -1.3 +- 0.1, respectively; both dEs have internal metallicity dispersions of 0.5 dex, but show no evidence for a radial metallicity gradient. We construct two-{integral} axisymmetric dynamical models and find that the observed kinematical profiles cannot be explained without modest amounts of non-baryonic dark matter. We measure central mass-to-light ratios of M/L{sub V} = 4.2 +- 0.6 and M/L{sub V} = 4.6 +- 0.6 for NGC 147 and NGC 185, respectively. Both dE galaxies are consistent with being primarily flattened by their rotational motions, although some anisotropic velocity dispersion is needed to fully explain their observed shapes. The velocity profiles of all three Local Group dEs (NGC 147, NGC 185, and NGC 205) suggest that rotation is more prevalent in the dE galaxy class than previously assumed, but often manifests only at several times the effective radius. Since all dEs outside the Local Group have been probed to only inside the effective radius, this opens the door for formation mechanisms in which dEs are transformed or stripped versions of gas-rich rotating progenitor galaxies.
Abstract
We report the discovery of six ultra-faint Milky Way satellites identified through matched-filter searches conducted using Dark Energy Camera (DECam) data processed as part of the second ...data release of the DECam Local Volume Exploration (DELVE) survey. Leveraging deep Gemini/GMOS-N imaging (for four candidates) as well as follow-up DECam imaging (for two candidates), we characterize the morphologies and stellar populations of these systems. We find that these candidates all share faint absolute magnitudes (
M
V
≥ −3.2 mag) and old, metal-poor stellar populations (
τ
> 10 Gyr, Fe/H < −1.4 dex). Three of these systems are more extended (
r
1/2
> 15 pc), while the other three are compact (
r
1/2
< 10 pc). From these properties, we infer that the former three systems (Boötes V, Leo Minor I, and Virgo II) are consistent with ultra-faint dwarf galaxy classifications, whereas the latter three (DELVE 3, DELVE 4, and DELVE 5) are likely ultra-faint star clusters. Using data from the Gaia satellite, we confidently measure the proper motion of Boötes V, Leo Minor I, and DELVE 4, and tentatively detect a proper-motion signal from DELVE 3 and DELVE 5; no signal is detected for Virgo II. We use these measurements to explore possible associations between the newly discovered systems and the Sagittarius dwarf spheroidal, the Magellanic Clouds, and the Vast Polar Structure, finding several plausible associations. Our results offer a preview of the numerous ultra-faint stellar systems that will soon be discovered by the Vera C. Rubin Observatory and highlight the challenges of classifying the faintest stellar systems.
Abstract
We present the discovery of DELVE 6, an ultra-faint stellar system identified in the second data release of the DECam Local Volume Exploration (DELVE) survey. Based on a maximum-likelihood ...fit to its structure and stellar population, we find that DELVE 6 is an old (
τ
> 9.8 Gyr at 95% confidence) and metal-poor (Fe/H < −1.17 dex at 95% confidence) stellar system with an absolute magnitude of
M
V
=
−
1.5
−
0.6
+
0.4
mag and an azimuthally averaged half-light radius of
r
1
/
2
=
10
−
3
+
4
pc. These properties are consistent with the population of ultra-faint star clusters uncovered by recent surveys. Interestingly, DELVE 6 is located at an angular separation of ∼10° from the center of the Small Magellanic Cloud (SMC), corresponding to a 3D physical separation of ∼20 kpc given the system’s observed distance (
D
⊙
= 80 kpc). This also places the system ∼35 kpc from the center of the Large Magellanic Cloud (LMC), lying within recent constraints on the size of the LMC’s dark matter halo. We tentatively measure the proper motion of DELVE 6 using data from Gaia, which we find supports a potential association between the system and the LMC/SMC. Although future kinematic measurements will be necessary to determine its origins, we highlight that DELVE 6 may represent only the second or third ancient (
τ
> 9 Gyr) star cluster associated with the SMC, or one of fewer than two dozen ancient clusters associated with the LMC. Nonetheless, we cannot currently rule out the possibility that the system is a distant Milky Way halo star cluster.
We present kinematical profiles and metallicity for the M31 dwarf spheroidal (dSph) satellite galaxy Andromeda II (And II) based on Keck DEIMOS spectroscopy of 531 red giant branch stars. Our ...kinematical sample is among the largest for any M31 satellite and extends out to two effective radii (r sub(eff) = 5'.3 = 1.1 kpc). We find a mean systemic velocity of -192.4 + or - 0.5 km s super(-1) and an average velocity dispersion of sigma sub(upsilon) = 7.8 + or - 1.1 km s super(-1). While the rotation velocity along the major axis of And II is nearly zero (<1 km s super(-1)), the rotation along the minor axis is significant with a maximum rotational velocity of upsilon sub(max) = 8.6 + or - 1.8 km s super(-1). We find a kinematical major axis, with a maximum rotational velocity of upsilon sub(max) = 10.9 + or - 2.4 km s super(-1), misaligned by 67degrees to the isophotal major axis. And II is thus the first dwarf galaxy with evidence for nearly prolate rotation with a upsilon sub(max)/sigma sub(upsilon) = 1.1, although given its ellipticity of epsilon = 0.10, this object may be triaxial. We measured metallicities for a subsample of our data, finding a mean metallicity of Fe/H = -1.39 + or - 0.03 dex and an internal metallicity dispersion of 0.72 + or - 0.03 dex. We find a radial metallicity gradient with metal-rich stars more centrally concentrated, but do not observe a significant difference in the dynamics of the two metallicity populations. And II is the only known dwarf galaxy to show minor axis rotation, making it a unique system whose existence offers important clues on the processes responsible for the formation of dSphs.
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