1,083 research outputs found
Measuring the Sources of the Intergalactic Ionizing Flux
We use a wide-field (0.9 square degree) X-ray sample with optical and GALEX
ultraviolet observations to measure the contribution of Active Galactic Nuclei
(AGNs) to the ionizing flux as a function of redshift. Our analysis shows that
the AGN contribution to the metagalactic ionizing background peaks around z=2.
The measured values of the ionizing background from the AGNs are lower than
previous estimates and confirm that ionization from AGNs is insufficient to
maintain the observed ionization of the intergalactic medium (IGM) at z>3. We
show that only sources with broad lines in their optical spectra have
detectable ionizing flux and that the ionizing flux seen in an AGN is not
correlated with its X-ray color. We also use the GALEX observations of the
GOODS-N region to place a 2-sigma upper limit of 0.008 on the average
ionization fraction fnu(700 A)/fnu(1500 A) for 626 UV selected galaxies in the
redshift range z=0.9-1.4. We then use this limit to estimate an upper bound to
the galaxy contribution in the redshift range z=0-5. If the z~1.15 ionization
fraction is appropriate for higher redshift galaxies, then contributions from
the galaxy population are also too low to account for the IGM ionization at the
highest redshifts (z>4).Comment: 15 pages, Accepted by The Astrophysical Journa
Arachidonic acid-induced release of calcium in permeabilized human neutrophils
AbstractThe addition of arachidonic acid to a suspension of digitonin-permeabilized human neutrophils was found to induce, in a dose-dependent manner (ED50 about 15 μM), the release of calcium from internal stores. Arachidic acid was without effect, while linoleic acid and linolenic acid were (on a concentration basis) at least 5-times less active than arachidonic acid. The activity of arachidonic acid appears to be due to the fatty acid itself and not to one of its metabolites. The pool of calcium mobilized by arachidonic acid includes that sensitive to inositol 1,4,5-trisphosphate. These results demonstrate a significant intracellular role for arachidonic acid at the level of the internal mobilization of calcium in human neutrophils
On The Nature of Variations in the Measured Star Formation Efficiency of Molecular Clouds
Measurements of the star formation efficiency (SFE) of giant molecular clouds
(GMCs) in the Milky Way generally show a large scatter, which could be
intrinsic or observational. We use magnetohydrodynamic simulations of GMCs
(including feedback) to forward-model the relationship between the true GMC SFE
and observational proxies. We show that individual GMCs trace broad ranges of
observed SFE throughout collapse, star formation, and disruption. Low measured
SFEs (<<1%) are "real" but correspond to early stages, the true "per-freefall"
SFE where most stars actually form can be much larger. Very high (>>10%) values
are often artificially enhanced by rapid gas dispersal. Simulations including
stellar feedback reproduce observed GMC-scale SFEs, but simulations without
feedback produce 20x larger SFEs. Radiative feedback dominates among mechanisms
simulated. An anticorrelation of SFE with cloud mass is shown to be an
observational artifact. We also explore individual dense "clumps" within GMCs
and show that (with feedback) their bulk properties agree well with
observations. Predicted SFEs within the dense clumps are ~2x larger than
observed, possibly indicating physics other than feedback from massive (main
sequence) stars is needed to regulate their collapse.Comment: Fixed typo in the arXiv abstrac
The Origin and Evolution of the Galaxy Mass-Metallicity Relation
We use high-resolution cosmological zoom-in simulations from the Feedback in
Realistic Environment (FIRE) project to study the galaxy mass-metallicity
relations (MZR) from z=0-6. These simulations include explicit models of the
multi-phase ISM, star formation, and stellar feedback. The simulations cover
halo masses Mhalo=10^9-10^13 Msun and stellar mass Mstar=10^4-10^11 Msun at z=0
and have been shown to produce many observed galaxy properties from z=0-6. For
the first time, our simulations agree reasonably well with the observed
mass-metallicity relations at z=0-3 for a broad range of galaxy masses. We
predict the evolution of the MZR from z=0-6 as
log(Zgas/Zsun)=12+log(O/H)-9.0=0.35[log(Mstar/Msun)-10]+0.93 exp(-0.43 z)-1.05
and log(Zstar/Zsun)=[Fe/H]-0.2=0.40[log(Mstar/Msun)-10]+0.67 exp(-0.50 z)-1.04,
for gas-phase and stellar metallicity, respectively. Our simulations suggest
that the evolution of MZR is associated with the evolution of stellar/gas mass
fractions at different redshifts, indicating the existence of a universal
metallicity relation between stellar mass, gas mass, and metallicities. In our
simulations, galaxies above Mstar=10^6 Msun are able to retain a large fraction
of their metals inside the halo, because metal-rich winds fail to escape
completely and are recycled into the galaxy. This resolves a long-standing
discrepancy between "sub-grid" wind models (and semi-analytic models) and
observations, where common sub-grid models cannot simultaneously reproduce the
MZR and the stellar mass functions.Comment: 17 pages, 14 figures, re-submitted to MNRAS after revisions on
referee comment
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