7,163 research outputs found
The Distribution of High Redshift Galaxy Colors: Line of Sight Variations in Neutral Hydrogen Absorption
We model, via Monte Carlo simulations, the distribution of observed U-B, B-V,
V-I galaxy colors in the range 1.75<z<5 caused by variations in the
line-of-sight opacity due to neutral hydrogen (HI). We also include HI internal
to the source galaxies. Even without internal HI absorption, comparison of the
distribution of simulated colors to the analytic approximations of Madau (1995)
and Madau et al (1996) reveals systematically different mean colors and
scatter. Differences arise in part because we use more realistic distributions
of column densities and Doppler parameters. However, there are also
mathematical problems of applying mean and standard deviation opacities, and
such application yields unphysical results. These problems are corrected using
our Monte Carlo approach. Including HI absorption internal to the galaxies
generaly diminishes the scatter in the observed colors at a given redshift, but
for redshifts of interest this diminution only occurs in the colors using the
bluest band-pass. Internal column densities < 10^17 cm^2 do not effect the
observed colors, while column densities > 10^18 cm^2 yield a limiting
distribution of high redshift galaxy colors. As one application of our
analysis, we consider the sample completeness as a function of redshift for a
single spectral energy distribution (SED) given the multi-color selection
boundaries for the Hubble Deep Field proposed by Madau et al (1996). We argue
that the only correct procedure for estimating the z>3 galaxy luminosity
function from color-selected samples is to measure the (observed) distribution
of redshifts and intrinsic SED types, and then consider the variation in color
for each SED and redshift. A similar argument applies to the estimation of the
luminosity function of color-selected, high redshift QSOs.Comment: accepted for publication in ApJ; 25 pages text, 14 embedded figure
Low Mass Group Environments have no Substantial Impact on the Circumgalactic Medium Metallicity
We explore how environment affects the metallicity of the circumgalactic
medium (CGM) using 13 low mass galaxy groups (2-5 galaxies) at identified near background quasars. Using quasar spectra
from HST/COS and from Keck/HIRES or VLT/UVES we measure column densities of, or
determine limits on, CGM absorption lines. We use a Markov chain Monte Carlo
approach with Cloudy to estimate metallicities of cool (K) CGM gas
within groups and compare them to CGM metallicities of 47 isolated galaxies.
Both group and isolated CGM metallicities span a wide range ([Si/H]),
where the mean group () and isolated () CGM
metallicities are similar. Group and isolated environments have similar
distributions of {\HI} column densities as a function of impact parameter.
However, contrary to isolated galaxies, we do not find an anti-correlation
between {\HI} column density and the nearest group galaxy impact parameter. We
additionally divided the groups by member luminosity ratios (i.e.,
galaxy-galaxy and galaxy-dwarf groups). While there was no significant
difference in their mean metallicities, a modest increase in sample size should
allow one to statistically identify a higher CGM metallicity in galaxy-dwarf
groups compared to galaxy-galaxy groups. We conclude that either environmental
effects have not played an important role in the metallicity of the CGM at this
stage and expect that this may only occur when galaxies are strongly
interacting or merging, or that some isolated galaxies have higher CGM
metallicities due to past interactions. Thus, environment does not seem to be
the cause of the CGM metallicity bimodality.Comment: 14 pages, 5 figures, 2 figure sets, 1 machine-readable tabl
A View of Point Sources in Hickson Compact Groups: High AGN fraction but a dearth of strong AGNs
We present X-ray point source catalogs for 9 Hickson Compact Groups
(HCGs, 37 galaxies) at distances Mpc. We perform detailed X-ray point
source detection and photometry, and interpret the point source population by
means of simulated hardness ratios. We thus estimate X-ray luminosities ()
for all sources, most of which are too weak for reliable spectral fitting. For
all sources, we provide catalogs with counts, count rates, power-law indices
(), hardness ratios, and , in the full ( keV), soft
( keV) and hard ( keV) bands. We use optical emission-line
ratios from the literature to re-classify 24 galaxies as star-forming,
accreting onto a supermassive black hole (AGNs), transition objects, or
low-ionization nuclear emission regions (LINERs). Two-thirds of our galaxies
have nuclear X-ray sources with /UVOT counterparts. Two nuclei have
~ erg s, are strong
multi-wavelength AGNs and follow the known correlation for strong AGNs. Otherwise, most nuclei are X-ray faint,
consistent with either a low-luminosity AGN or a nuclear X-ray binary
population, and fall in the "non-AGN locus" in space, which also hosts other, normal, galaxies. Our results suggest
that HCG X-ray nuclei in high specific star formation rate spiral galaxies are
likely dominated by star formation, while those with low specific star
formation rates in earlier types likely harbor a weak AGN. The AGN fraction in
HCG galaxies with and erg
s is , somewhat higher than the fraction
in galaxy clusters.Comment: 77 pages (emulateapj), 28 tables, 11 figures. Accepted by ApJS on
March 5, 201
Neutron reflection from the liquid helium surface.
The reflection of neutrons from a helium surface has been observed for the first time. The 4He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state we also observe a surface layer ~200 Å thick which has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface. The application of neutron reflectometry described in this paper creates new and exciting opportunities for the surface and interfacial study of quantum fluids
Do red deer stags (Cervus elaphus) use roar fundamental frequency (F0) to assess rivals?
It is well established that in humans, male voices are disproportionately lower pitched than female voices, and recent studies suggest that this dimorphism in fundamental frequency (F0) results from both intrasexual (male competition) and intersexual (female mate choice) selection for lower pitched voices in men. However, comparative investigations indicate that sexual dimorphism in F0 is not universal in terrestrial mammals. In the highly polygynous and sexually dimorphic Scottish red deer Cervus elaphus scoticus, more successful males give sexually-selected calls (roars) with higher minimum F0s, suggesting that high, rather than low F0s advertise quality in this subspecies. While playback experiments demonstrated that oestrous females prefer higher pitched roars, the potential role of roar F0 in male competition remains untested. Here we examined the response of rutting red deer stags to playbacks of re-synthesized male roars with different median F0s. Our results show that stags’ responses (latencies and durations of attention, vocal and approach responses) were not affected by the F0 of the roar. This suggests that intrasexual selection is unlikely to strongly influence the evolution of roar F0 in Scottish red deer stags, and illustrates how the F0 of terrestrial mammal vocal sexual signals may be subject to different selection pressures across species. Further investigations on species characterized by different F0 profiles are needed to provide a comparative background for evolutionary interpretations of sex differences in mammalian vocalizations
Stellar Populations in Compact Galaxy Groups: a Multi-Wavelength Study of HCGs 16, 22, and 42, their Star Clusters and Dwarf Galaxies
We present a multi-wavelength analysis of three compact galaxy groups, HCGs
16, 22, and 42, which describe a sequence in terms of gas richness, from space-
(Swift, HST, Spitzer) and ground-based (LCO, CTIO) imaging and spectroscopy. We
study various signs of past interactions including a faint, dusty tidal feature
about HCG 16A, which we tentatively age-date at <1 Gyr. This represents the
possible detection of a tidal feature at the end of its phase of optical
observability. Our HST images also resolve what were thought to be double
nuclei in HCG 16C and D into multiple, distinct sources, likely to be star
clusters. Beyond our phenomenological treatment, we focus primarily on
contrasting the stellar populations across these three groups. The star
clusters show a remarkable intermediate-age population in HCG 22, and identify
the time at which star formation was quenched in HCG 42. We also search for
dwarf galaxies at accordant redshifts. The inclusion of 33 members and 27
'associates' (possible members) radically changes group dynamical masses, which
in turn may affect previous evolutionary classifications. The extended
membership paints a picture of relative isolation in HCGs 16 and 22, but shows
HCG 42 to be part of a larger structure, following a dichotomy expected from
recent studies. We conclude that (a) star cluster populations provide an
excellent metric of evolutionary state, as they can age-date the past epochs of
star formation; and (b) the extended dwarf galaxy population must be considered
in assessing the dynamical state of a compact group.Comment: 29 pages, 13 figures. Accepted for publication in The Astrophysical
Journa
Pickoff and spin-conversion quenchings of ortho-positronium in oxygen
The quenching processes of the thermalized ortho-positronium(o-Ps) on an
oxygen molecule have been studied by the positron annihilation age-momentum
correlation techinique(AMOC). The Doppler broadening spectrum of the 511 keV
gamma-rays from the 2gamma annihilation of o-Ps in O_2 has been measured as a
function of the o-Ps age. The rate of the quenching, consisting of the pickoff
and the spin-conversion, is estimated from the positron lifetime spectrum. The
ratio of the pickoff quenching rate to the spin-conversion rate is deduced from
the Doppler broadening of the 511 keV gamma-rays from the annihilation of the
o-Ps. The pickoff parameter ^1Z_eff, the effective number of the electrons per
molecule which contribute to the pickoff quenching, for O_2 is determined to be
0.6 +- 0.4. The cross-section for the elastic spin-conversion quenching is
determined to be (1.16 +- 0.01) * 10^{-19} cm^2.Comment: 4 pages with 5 eps figures, LaTeX2e(revtex4
The RNA Helicase DDX6 Controls Cellular Plasticity by Modulating P-Body Homeostasis
Post-transcriptional mechanisms have the potential to influence complex changes in gene expression, yet their role in cell fate transitions remains largely unexplored. Here, we show that suppression of the RNA helicase DDX6 endows human and mouse primed embryonic stem cells (ESCs) with a differentiation-resistant, “hyper-pluripotent” state, which readily reprograms to a naive state resembling the preimplantation embryo. We further demonstrate that DDX6 plays a key role in adult progenitors where it controls the balance between self-renewal and differentiation in a context-dependent manner. Mechanistically, DDX6 mediates the translational suppression of target mRNAs in P-bodies. Upon loss of DDX6 activity, P-bodies dissolve and release mRNAs encoding fate-instructive transcription and chromatin factors that re-enter the ribosome pool. Increased translation of these targets impacts cell fate by rewiring the enhancer, heterochromatin, and DNA methylation landscapes of undifferentiated cell types. Collectively, our data establish a link between P-body homeostasis, chromatin organization, and stem cell potency
New imaging tools to measure nephron number in vivo: Opportunities for developmental nephrology
The mammalian kidney is a complex organ, requiring the concerted function of up to millions of nephrons. The number of nephrons is constant after nephrogenesis during development, and nephron loss over a life span can lead to susceptibility to acute or chronic kidney disease. New technologies are under development to count individual nephrons in the kidney in vivo. This review outlines these technologies and highlights their relevance to studies of human renal development and disease
The Relation Between Galaxy ISM and Circumgalactic OVI Gas Kinematics Derived from Observations and CDM Simulations
We present the first galaxy-OVI absorption kinematic study for 20 absorption
systems (EW>0.1~{\AA}) associated with isolated galaxies (0.150.55) that
have accurate redshifts and rotation curves obtained using Keck/ESI. Our sample
is split into two azimuthal angle bins: major axis () and
minor axis (). OVI absorption along the galaxy major axis is
not correlated with galaxy rotation kinematics, with only 1/10 systems that
could be explained with rotation/accretion models. This is in contrast to
co-rotation commonly observed for MgII absorption. OVI along the minor axis
could be modeled by accelerating outflows but only for small opening angles,
while the majority of the OVI is decelerating. Along both axes, stacked OVI
profiles reside at the galaxy systemic velocity with the absorption kinematics
spanning the entire dynamical range of their galaxies. The OVI found in AMR
cosmological simulations exists within filaments and in halos of ~50 kpc
surrounding galaxies. Simulations show that major axis OVI gas inflows along
filaments and decelerates as it approaches the galaxy while increasing in its
level of co-rotation. Minor axis outflows in the simulations are effective
within 50-75 kpc beyond that they decelerate and fall back onto the galaxy.
Although the simulations show clear OVI kinematic signatures they are not
directly comparable to observations. When we compare kinematic signatures
integrated through the entire simulated galaxy halo we find that these
signatures are washed out due to full velocity distribution of OVI throughout
the halo. We conclude that OVI alone does not serve as a useful kinematic
indicator of gas accretion, outflows or star-formation and likely best probes
the halo virial temperature.Comment: 24 pages, 21 figures, 4 tables. Accepted to ApJ on November 14, 201
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