180 research outputs found
Dependence of Galaxy Quenching on Halo Mass and Distance from its Centre
We study the dependence of star-formation quenching on galaxy mass and
environment, in the SDSS (z~0.1) and the AEGIS (z~1). It is crucial that we
define quenching by low star-formation rate rather than by red colour, given
that one third of the red galaxies are star forming. We address stellar mass
M*, halo mass Mh, density over the nearest N neighbours deltaN, and distance to
the halo centre D. The fraction of quenched galaxies appears more strongly
correlated with Mh at fixed M* than with M* at fixed Mh, while for satellites
quenching also depends on D. We present the M*-Mh relation for centrals at z~1.
At z~1, the dependence of quenching on M* at fixed Mh is somewhat more
pronounced than at z~0, but the quenched fraction is low (10%) and the haloes
are less massive. For satellites, M*-dependent quenching is noticeable at high
D, suggesting a quenching dependence on sub-halo mass for recently captured
satellites. At small D, where satellites likely fell in more than a few Gyr
ago, quenching strongly depends on Mh, and not on M*. The Mh-dependence of
quenching is consistent with theoretical wisdom where virial shock heating in
massive haloes shuts down accretion and triggers ram-pressure stripping,
causing quenching. The interpretation of deltaN is complicated by the fact that
it depends on the number of observed group members compared to N, motivating
the use of D as a better measure of local environment.Comment: 23 pages, 13 figures, accepted by MNRA
Feedback and Recycled Wind Accretion: Assembling the z=0 Galaxy Mass Function
We analyse cosmological hydrodynamic simulations that include
observationally-constrained prescriptions for galactic outflows. If these
simulated winds accurately represent winds in the real Universe, then material
previously ejected in winds provides the dominant source of gas infall for new
star formation at redshifts z<1. This recycled wind accretion, or wind mode,
provides a third physically distinct accretion channel in addition to the "hot"
and "cold" modes emphasised in recent theoretical studies. Because of the
interaction between outflows and gas in and around halos, the recycling
timescale of wind material (t_rec) is shorter in higher-mass systems, which
reside in denser gaseous environments. In these simulations, this differential
recycling plays a central role in shaping the present-day galaxy stellar mass
function (GSMF). If we remove all particles that were ever ejected in a wind,
then the predicted GSMFs are much steeper than observed; galaxy masses are
suppressed both by the direct removal of gas and by the hydrodynamic heating of
their surroundings, which reduces subsequent infall. With wind recycling
included, the simulation that incorporates our favoured momentum-driven wind
scalings reproduces the observed GSMF for stellar masses 10^9 < M < 5x10^10
Msolar. At higher masses, wind recycling leads to excessive galaxy masses and
excessive star formation rates relative to observations. In these massive
systems, some quenching mechanism must suppress the re-accretion of gas ejected
from star-forming galaxies. In short, as has long been anticipated, the form of
the GSMF is governed by outflows; the unexpected twist here for our simulated
winds is that it is not primarily the ejection of material but how the ejected
material is re-accreted that governs the GSMF.Comment: 16 pages, 7 figures, accepted by MNRA
An Analytic Model for the Evolution of the Stellar, Gas, and Metal Content of Galaxies
We present an analytic formalism that describes the evolution of the stellar,
gas, and metal content of galaxies. It is based on the idea, inspired by
hydrodynamic simulations, that galaxies live in a slowly-evolving equilibrium
between inflow, outflow, and star formation. We argue that this formalism
broadly captures the behavior of galaxy properties evolving in simulations. The
resulting equilibrium equations for the star formation rate, gas fraction, and
metallicity depend on three key free parameters that represent ejective
feedback, preventive feedback, and re-accretion of ejected material. We
schematically describe how these parameters are constrained by models and
observations. Galaxies perturbed off the equilibrium relations owing to inflow
stochasticity tend to be driven back towards equilibrium, such that deviations
in star formation rate at a given mass are correlated with gas fraction and
anti-correlated with metallicity. After an early gas accumulation epoch,
quiescently star-forming galaxies are expected to be in equilibrium over most
of cosmic time. The equilibrium model provides a simple intuitive framework for
understanding the cosmic evolution of galaxy properties, and centrally features
the cycle of baryons between galaxies and surrounding gas as the driver of
galaxy growth.Comment: 11 pages, MNRAS, accepte
Expression of Stem Cell Markers in the Human Fetal Kidney
In the human fetal kidney (HFK) self-renewing stem cells residing in the metanephric mesenchyme (MM)/blastema are induced to form all cell types of the nephron till 34th week of gestation. Definition of useful markers is crucial for the identification of HFK stem cells. Because wilms' tumor, a pediatric renal cancer, initiates from retention of renal stem cells, we hypothesized that surface antigens previously up-regulated in microarrays of both HFK and blastema-enriched stem-like wilms' tumor xenografts (NCAM, ACVRIIB, DLK1/PREF, GPR39, FZD7, FZD2, NTRK2) are likely to be relevant markers. Comprehensive profiling of these putative and of additional stem cell markers (CD34, CD133, c-Kit, CD90, CD105, CD24) in mid-gestation HFK was performed using immunostaining and FACS in conjunction with EpCAM, an epithelial surface marker that is absent from the MM and increases along nephron differentiation and hence can be separated into negative, dim or bright fractions. No marker was specifically localized to the MM. Nevertheless, FZD7 and NTRK2 were preferentially localized to the MM and emerging tubules (<10% of HFK cells) and were mostly present within the EpCAMneg and EpCAMdim fractions, indicating putative stem/progenitor markers. In contrast, single markers such as CD24 and CD133 as well as double-positive CD24+CD133+ cells comprise >50% of HFK cells and predominantly co-express EpCAMbright, indicating they are mostly markers of differentiation. Furthermore, localization of NCAM exclusively in the MM and in its nephron progenitor derivatives but also in stroma and the expression pattern of significantly elevated renal stem/progenitor genes Six2, Wt1, Cited1, and Sall1 in NCAM+EpCAM- and to a lesser extent in NCAM+EpCAM+ fractions confirmed regional identity of cells and assisted us in pinpointing the presence of subpopulations that are putative MM-derived progenitor cells (NCAM+EpCAM+FZD7+), MM stem cells (NCAM+EpCAM-FZD7+) or both (NCAM+FZD7+). These results and concepts provide a framework for developing cell selection strategies for human renal cell-based therapies
Vertically Extended Neutral Gas in the Massive Edge-on Spiral NGC 5746
We present Very Large Array 21-cm observations of the massive edge-on spiral
galaxy NGC 5746. This galaxy has recently been reported to have a luminous
X-ray halo, which has been taken as evidence of residual hot gas as predicted
in galaxy formation scenarios. Such models also predict that some of this gas
should undergo thermal instabilities, leading to a population of warm clouds
falling onto the disk. If so, then one might expect to find a vertically
extended neutral layer. We detect a substantial high-latitude component, but
conclude that almost all of its mass of 1.2-1.6 billion solar masses most
likely resides in a warp. Four features far from the plane containing about 100
million solar masses are found at velocities distinct from this warp. These
clouds may be associated with the expected infall, although an origin in a
disk-halo flow cannot be ruled out, except for one feature which is
counter-rotating. The warp itself may be a result of infall according to recent
models. But clearly this galaxy lacks a massive, lagging neutral halo as found
in NGC 891. The disk HI is concentrated into two rings of radii 1.5 and 3
arcminutes. Radial inflow is found in the disk, probably due to the bar in this
galaxy. A nearby member of this galaxy group, NGC 5740, is also detected. It
shows a prominent one-sided extension which may be the result of ram pressure
stripping.Comment: 55 pages, 20 figure
The interstellar medium and feedback in the progenitors of the compact passive galaxies at z~2
Quenched galaxies at z>2 are nearly all very compact relative to z~0,
suggesting a physical connection between high stellar density and efficient,
rapid cessation of star-formation. We present restframe UV spectra of
Lyman-break galaxies (LBGs) at z~3 selected to be candidate progenitors of
quenched galaxies at z~2 based on their compact restframe optical sizes and
high surface density of star-formation. We compare their UV properties to those
of more extended LBGs of similar mass and star formation rate (non-candidates).
We find that candidate progenitors have faster ISM gas velocities and higher
equivalent widths of interstellar absorption lines, implying larger velocity
spread among absorbing clouds. Candidates deviate from the relationship between
equivalent widths of Lyman-alpha and interstellar absorption lines in that
their Lyman-alpha emission remains strong despite high interstellar absorption,
possibly indicating that the neutral HI fraction is patchy such that
Lyman-alpha photons can escape. We detect stronger CIV P-Cygni features
(emission and absorption) and HeII emission in candidates, indicative of larger
populations of metal rich Wolf-Rayet stars compared to non-candidates. The
faster bulk motions, broader spread of gas velocity, and Lyman-alpha properties
of candidates are consistent with their ISM being subject to more energetic
feedback than non-candidates. Together with their larger metallicity (implying
more evolved star-formation activity) this leads us to propose, if
speculatively, that they are likely to quench sooner than non-candidates,
supporting the validity of selection criteria used to identify them as
progenitors of z~2 passive galaxies. We propose that massive, compact galaxies
undergo more rapid growth of stellar mass content, perhaps because the gas
accretion mechanisms are different, and quench sooner than normally-sized LBGs
at these early epochs.Comment: Accepted for publication in the Astrophysical Journa
Intergalactic Dust Extinction in Hydrodynamic Cosmological Simulations
Recently Menard et al. detected a subtle but systematic change in the mean
color of quasars as a function of their projected separation from foreground
galaxies, extending to comoving separations of ~10Mpc/h, which they interpret
as a signature of reddening by intergalactic dust. We present theoretical
models of this remarkable observation, using SPH cosmological simulations of a
(50Mpc/h)^3 volume. Our primary model uses a simulation with galactic winds and
assumes that dust traces the intergalactic metals. The predicted galaxy-dust
correlation function is similar in form to the galaxy-mass correlation
function, and reproducing the MSFR data requires a dust-to-metal mass ratio of
0.24, about half the value in the Galactic ISM. Roughly half of the reddening
arises in dust that is more than 100Kpc/h from the nearest massive galaxy. We
also examine a simulation with no galactic winds, which predicts a much smaller
fraction of intergalactic metals (3% vs. 35%) and therefore requires an
unphysical dust-to-metal ratio of 2.18 to reproduce the MSFR data. In both
models, the signal is dominated by sightlines with E(g-i)=0.001-0.1. The
no-wind simulation can be reconciled with the data if we also allow reddening
to arise in galaxies up to several x 10^10 Msun. The wind model predicts a mean
visual extinction of A_V ~0.0133 mag out to z=0.5, with a
sightline-to-sightline dispersion similar to the mean, which could be
significant for future supernova cosmology studies. Reproducing the MSFR
results in these simulations requires that a large fraction of ISM dust survive
its expulsion from galaxies and its residence in the intergalactic medium.
Future observational studies that provide higher precision and measure the
dependence on galaxy type and environment will allow detailed tests for models
of enriched galactic outflows and the survival of IG dust.Comment: Matches version accepted by MNRA
No More Active Galactic Nuclei in Clumpy Disks Than in Smooth Galaxies at z~2 in CANDELS / 3D-HST
We use CANDELS imaging, 3D-HST spectroscopy, and Chandra X-ray data to
investigate if active galactic nuclei (AGNs) are preferentially fueled by
violent disk instabilities funneling gas into galaxy centers at 1.3<z<2.4. We
select galaxies undergoing gravitational instabilities using the number of
clumps and degree of patchiness as proxies. The CANDELS visual classification
system is used to identify 44 clumpy disk galaxies, along with mass-matched
comparison samples of smooth and intermediate morphology galaxies. We note
that, despite being being mass-matched and having similar star formation rates,
the smoother galaxies tend to be smaller disks with more prominent bulges
compared to the clumpy galaxies. The lack of smooth extended disks is probably
a general feature of the z~2 galaxy population, and means we cannot directly
compare with the clumpy and smooth extended disks observed at lower redshift.
We find that z~2 clumpy galaxies have slightly enhanced AGN fractions selected
by integrated line ratios (in the mass-excitation method), but the spatially
resolved line ratios indicate this is likely due to extended phenomena rather
than nuclear AGNs. Meanwhile the X-ray data show that clumpy, smooth, and
intermediate galaxies have nearly indistinguishable AGN fractions derived from
both individual detections and stacked non-detections. The data demonstrate
that AGN fueling modes at z~1.85 - whether violent disk instabilities or
secular processes - are as efficient in smooth galaxies as they are in clumpy
galaxies.Comment: ApJ accepted. 17 pages, 17 figure
Smooth(er) Stellar Mass Maps in CANDELS: Constraints on the Longevity of Clumps in High-redshift Star-forming Galaxies
We perform a detailed analysis of the resolved colors and stellar populations
of a complete sample of 323 star-forming galaxies at 0.5 < z < 1.5, and 326
star-forming galaxies at 1.5 < z < 2.5 in the ERS and CANDELS-Deep region of
GOODS-South. Galaxies were selected to be more massive than 10^10 Msun and have
specific star formation rates above 1/t_H. We model the 7-band optical ACS +
near-IR WFC3 spectral energy distributions of individual bins of pixels,
accounting simultaneously for the galaxy-integrated photometric constraints
available over a longer wavelength range. We analyze variations in rest-frame
color, stellar surface mass density, age, and extinction as a function of
galactocentric radius and local surface brightness/density, and measure
structural parameters on luminosity and stellar mass maps. We find evidence for
redder colors, older stellar ages, and increased dust extinction in the nuclei
of galaxies. Big star-forming clumps seen in star formation tracers are less
prominent or even invisible on the inferred stellar mass distributions.
Off-center clumps contribute up to ~20% to the integrated SFR, but only 7% or
less to the integrated mass of all massive star-forming galaxies at z ~ 1 and z
~ 2, with the fractional contributions being a decreasing function of
wavelength used to select the clumps. The stellar mass profiles tend to have
smaller sizes and M20 coefficients, and higher concentration and Gini
coefficients than the light distribution. Our results are consistent with an
inside-out disk growth scenario with brief (100 - 200 Myr) episodic local
enhancements in star formation superposed on the underlying disk.
Alternatively, the young ages of off-center clumps may signal inward clump
migration, provided this happens efficiently on the order of an orbital
timescale.Comment: Accepted by The Astrophysical Journal, 27 pages, 1 table, 16 figure
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