593 research outputs found
Anaerobic Metazoans: No longer an oxymoron
The sediments of a deep-sea hypersaline and sulfidic Mediterranean basin have yielded an unexpected discovery, the first multicellular animals living entirely without oxygen. Reported by Danovaro et al. in BMC Biology, these three new species of Loricifera add a new and remarkable dimension to anoxic ecosystems previously thought to support only unicellular life
Intentional and unintentional contributions to nonspecific preparation: Electrophysiological evidence.
The authors hypothesized that there are distinct intentional and unintentional influences on nonspecific preparation for a future event. In 2 experiments, participants responded to an imperative stimulus (
Surface photometry and structure of high redshift disk galaxies in the HDF-S NICMOS field
A photometric study of 22 disk galaxies at redshifs z=0.5-2.6 is conducted,
using deep NICMOS J and H band and STIS open mode observations of the HDF-S
NICMOS parallel field. Rest-frame B-profiles and (U-V) color profiles are
constructed. A number of disks show steeper decrease of luminosity than
exponential, referring to disk truncation. Shape of the luminosity profiles
does not vary with redshift, but galactic sizes decrease significantly. (U-V)
colors and color gradients suggest more intense and centrally concentrated star
formation at earlier epochs. On the basis of (U-V) color and chemical evolution
models, the disks at z~2.5 have formed between z=3.5-7. The studied parameters
are idependent of absolute B luminosity within the sample.Comment: 13 pages, 8 figures, Astron. Astrophys. accepte
On the Origin of Exponential Galaxy Disks
We use a disk galaxy evolution model to investigate whether galaxies with
exponential surface brightness profiles can be produced in a cosmologically
motivated framework for disk galaxy formation. Our model follows the accretion,
cooling, and ejection of baryonic mass, as a function of radius, inside growing
dark matter haloes. The surface density profile of the disk is determined by
detailed angular momentum conservation, starting from the distribution of
specific angular momentum as found in cosmological simulations. Exponential and
quasi-exponential disks can be produced by our model through a combination of
supernova driven galactic outflows (which preferentially remove low angular
momentum material), intrinsic variation in the angular momentum distribution of
the halo gas, and the inefficiency of star formation at large radii. We use
observations from the SDSS NYU-VAGC to show that the median Sersic index of
blue galaxies is a strong function of stellar mass. For blue galaxies, low mass
galaxies have a median n=1.3, while high mass galaxies have a median n=4. Our
model with energy driven outflows correctly reproduces this trend, whereas our
models with momentum driven outflows and no outflows over predict the Sersic
indices in low mass galaxies. We show that the observed fraction of
"bulge-less" exponential galaxies is a strong function of stellar mass. For
Milky-Way mass galaxies less than 0.1% of blue galaxies are bulge-less, whereas
for M33 mass galaxies bulge-less and quasi-bulgeless galaxies are typical.
These results suggest that the difficulty of hierarchical formation models to
produce bulge-less Milky-Way mass galaxies is in fact not a problem. However,
the problem of producing M33 like galaxies remains, and will provide a key test
for hierarchical galaxy formation models. [Abridged]Comment: 23 pages, 13 figures, accepted to MNRAS, two new figure
A Field Training Guide for Human Subjects Research Ethics
Maria Merritt and colleagues report on a Field Training Guide for Human Subjects Research Ethics that they have developed to help train field workers in ethics for research
Building up the Stellar Halo of the Galaxy
We study numerical simulations of satellite galaxy disruption in a potential
resembling that of the Milky Way. Our goal is to assess whether a merger origin
for the stellar halo would leave observable fossil structure in the phase-space
distribution of nearby stars. We show how mixing of disrupted satellites can be
quantified using a coarse-grained entropy. Although after 10 Gyr few obvious
asymmetries remain in the distribution of particles in configuration space,
strong correlations are still present in velocity space. We give a simple
analytic description of these effects, based on a linearised treatment in
action-angle variables, which shows how the kinematic and density structure of
the debris stream changes with time. By applying this description we find that
a single satellite of current luminosity 10^8 L_\sun disrupted 10 Gyr ago
from an orbit circulating in the inner halo (mean apocentre kpc)
would contribute about kinematically cold streams with internal
velocity dispersions below 5 km/s to the local stellar halo. If the whole
stellar halo were built by disrupted satellites, it should consist locally of
300 - 500 such streams. Clear detection of all these structures would require a
sample of a few thousand stars with 3-D velocities accurate to better than 5
km/s. Even with velocity errors several times worse than this, the expected
clumpiness should be quite evident. We apply our formalism to a group of stars
detected near the North Galactic Pole, and derive an order of magnitude
estimate for the initial properties of the progenitor system.Comment: 28 pages, 10 figures, minor changes, matches the version to appear in
MNRAS, Vol. 307, p.495-517 (August 1999
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
Density profiles of dark matter haloes on Galactic and Cluster scales
In the present paper, we improve the "Extended Secondary Infall Model" (ESIM)
of Williams et al. (2004) to obtain further insights on the cusp/core problem.
The model takes into account the effect of ordered and random angular momentum,
dynamical friction and baryon adiabatic contraction in order to obtain a
secondary infall model more close to the collapse reality. The model is applied
to structures on galactic scales (normal and dwarf spiral galaxies) and on
cluster of galaxies scales. The results obtained suggest that angular momentum
and dynamical friction are able, on galactic scales, to overcome the competing
effect of adiabatic contraction eliminating the cusp. The NFW profile can be
reobtained, in our model only if the system is constituted just by dark matter
and the magnitude of angular momentum and dynamical friction are reduced with
respect to the values predicted by the model itself. The rotation curves of
four LSB galaxies from de Blok & Bosma (2002) are compared to the rotation
curves obtained by the model in the present paper obtaining a good fit to the
observational data. On scales smaller than
the slope and on cluster scales we observe a similar
evolution of the dark matter density profile but in this case the density
profile slope flattens to for a cluster of . The total mass profile, differently from that of dark
matter, shows a central cusp well fitted by a NFW model.Comment: 26 pages; 4 figures A&A Accepte
Pseudomonas aeruginosa Overrides the Virulence Inducing Effect of Opioids When It Senses an Abundance of Phosphate
The gut during critical illness represents a complex ecology dominated by the presence of healthcare associated pathogens, nutrient scarce conditions, and compensatory host stress signals. We have previously identified key environmental cues, opioids and phosphate depletion that independently activate the virulence of Pseudomonas aeruginosa. Opioids induce quinolone signal production (PQS), whereas phosphate depletion leads to a triangulated response between MvfR-PQS, pyoverdin, and phosphosensory/phosphoregulatory systems (PstS-PhoB). Yet how P. aeruginosa manages its response to opioids during nutrient scarce conditions when growth is limited and a quorum is unlikely to be achieved is important in the context of pathogenesis in gut during stress. To mimic this environment, we created nutrient poor conditions and exposed P. aeruginosa PAO1 to the specific k-opioid receptor agonist U-50,488. Bacterial cells exposed to the k-opioid expressed a striking increase in virulence- and multi-drug resistance-related genes that correlated to a lethal phenotype in C. elegans killing assays. Under these conditions, HHQ, a precursor of PQS, rather than PQS itself, became the main inducer for pqsABCDE operon expression. P. aeruginosa virulence expression in response to k-opioids required PqsE since ΔPqsE was attenuated in its ability to activate virulence- and efflux pumps-related genes. Extracellular inorganic phosphate completely changed the transcriptional response of PAO1 to the k- opioid preventing pqsABCDE expression, the activation of multiple virulence- and efflux pumps-related genes, and the ability of P. aeruginosa to kill C. elegans. These results indicate that when P. aeruginosa senses resource abundance in the form of phosphate, it overrides its response to compensatory host signals such as opioids to express a virulent and lethal phenotype. These studies confirm a central role for phosphate in P. aeruginosa virulence that might be exploited to design novel anti- virulence strategies
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