217 research outputs found
The Molecular Interstellar Medium of the Local Group Dwarf NGC6822
Do molecular clouds collapse to form stars at the same rate in all
environments? In large spiral galaxies, the rate of transformation of H2 into
stars (hereafter SFE) varies little. However, the SFE in distant objects (z~1)
is much higher than in the large spiral disks that dominate the local universe.
Some small local group galaxies share at least some of the characteristics of
intermediate-redshift objects, such as size or color. Recent work has suggested
that the Star Formation Efficiency (SFE, defined as the SFRate per unit H2) in
local Dwarf galaxies may be as high as in the distant objects. A fundamental
difficulty in these studies is the independent measure of the H2 mass in
metal-deficient environments. At 490 kpc, NGC6822 is an excellent choice for
this study; it has been mapped in the CO(2-1) line using the multibeam receiver
HERA on the 30 meter IRAM telescope, yielding the largest sample of giant
molecular clouds (GMCs) in this galaxy. Despite the much lower metallicity, we
find no clear difference in the properties of the GMCs in NGC 6822 and those in
the Milky Way except lower CO luminosities for a given mass. Several
independent methods indicate that the total H2 mass in NGC 6822 is about 5 x
10^6 Msun in the area we mapped and less than 10^7 Msun in the whole galaxy.
This corresponds to a NH2/ICO ~ 4 x 10^{21} cm^-2 /(Kkm/s) over large scales,
such as would be observed in distant objects, and half that in individual GMCs.
No evidence was found for H2 without CO emission. Our simulations of the
radiative transfer in clouds are entirely compatible with these NH2/ICO values.
The SFE implied is a factor 5 - 10 higher than what is observed in large local
universe spirals.Comment: 16 pages, 13 figures. Accepted for publication in Astronomy and
Astrophysic
Particularly Efficient Star Formation in M33
The Star Formation (SF) rate in galaxies is an important parameter at all
redshifts and evolutionary stages of galaxies. In order to understand the
increased SF rates in intermediate redshift galaxies one possibility is to
study star formation in local galaxies with properties frequently found at this
earlier epoch like low metallicity and small size. We present sensitive
observations of the molecular gas in M 33, a small Local Group spiral at a
distance of 840 kpc which shares many of the characteristics of the
intermediate redshift galaxies. The observations were carried out in the
CO(2--1) line with the HERA heterodyne array on the IRAM 30 m telescope. A
11\arcmin22\arcmin region in the northern part of M 33 was observed,
reaching a detection threshold of a few 10 \msol. The correlation in this
field between the CO emission and tracers of SF (8\mum, 24\mum, \Ha, FUV) is
excellent and CO is detected very far North, showing that molecular gas forms
far out in the disk even in a small spiral with a subsolar metallicity. One
major molecular cloud was discovered in an interarm region with no HI peak and
little if any signs of SF -- without a complete survey this cloud would never
have been found. The radial dependence of the CO emission has a scale length
similar to the dust emission, less extended than the \Ha or FUV. If, however,
the \ratioo ratio varies inversely with metallicity, then the scale length of
the H becomes similar to that of the \Ha or FUV. Comparing the SF rate to
the H mass shows that M 33, like the intermediate redshift galaxies it
resembles, has a significantly higher SF efficiency than large local universe
spirals.Comment: 16 pages, 15 figure
HI and CO in the circumstellar environment of the oxygen-rich AGB star RX Lep
Circumstellar shells around AGB stars are built over long periods of time
that may reach several million years. They may therefore be extended over large
sizes (~1 pc, possibly more), and different complementary tracers are needed to
describe their global properties. In the present work, we combined 21-cm HI and
CO rotational line data obtained on an oxygen-rich semi-regular variable, RX
Lep, to describe the global properties of its circumstellar environment. With
the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile
is parabolic and implies an expansion velocity of ~4.2 km/s and a mass-loss
rate ~1.7 10^-7 Msun/yr (d = 137 pc). The HI line at 21 cm was detected with
the Nancay Radiotelescope on the star position and at several offset positions.
The linear shell size is relatively small, ~0.1 pc, but we detect a trail
extending southward to ~0.5 pc. The line profiles are approximately Gaussian
with an FWHM ~3.8 km/s and interpreted with a model developed for the detached
shell around the carbon-rich AGB star Y CVn. Our HI spectra are well-reproduced
by assuming a constant outflow (Mloss = 1.65 10^-7 Msun/yr) of ~4 10^4 years
duration, which has been slowed down by the external medium. The spatial offset
of the HI source is consistent with the northward direction of the proper
motion, lending support to the presence of a trail resulting from the motion of
the source through the ISM, as already suggested for Mira, RS Cnc, and other
sources detected in HI. The source was also observed in SiO (3 mm) and OH (18
cm), but not detected. The properties of the external parts of circumstellar
shells around AGB stars should be dominated by the interaction between stellar
outflows and external matter for oxygen-rich, as well as for carbon-rich,
sources, and the 21-cm HI line provides a very useful tracer of these regions.Comment: 15 pages, 9 figures, accepted for publication in A&
Norspermidine is not a self-produced trigger for biofilm disassembly
SummaryFormation of Bacillus subtilis biofilms, consisting of cells encapsulated within an extracellular matrix of exopolysaccharide and protein, requires the polyamine spermidine. A recent study reported that (1) related polyamine norspermidine is synthesized by B. subtilis using the equivalent of the Vibrio cholerae biosynthetic pathway, (2) exogenous norspermidine at 25 μM prevents B. subtilis biofilm formation, (3) endogenous norspermidine is present in biofilms at 50–80 μM, and (4) norspermidine prevents biofilm formation by condensing biofilm exopolysaccharide. In contrast, we find that, at concentrations up to 200 μM, exogenous norspermidine promotes biofilm formation. We find that norspermidine is absent in wild-type B. subtilis biofilms at all stages, and higher concentrations of exogenous norspermidine eventually inhibit planktonic growth and biofilm formation in an exopolysaccharide-independent manner. Moreover, orthologs of the V. cholerae norspermidine biosynthetic pathway are absent from B. subtilis, confirming that norspermidine is not physiologically relevant to biofilm function in this species
Cool gas and dust in M33: Results from the Herschel M33 extended survey (HERM33ES)
We present an analysis of the first space-based far-IR-submm observations of
M 33, which measure the emission from the cool dust and resolve the giant
molecular cloud complexes. With roughly half-solar abundances, M33 is a first
step towards young low-metallicity galaxies where the submm may be able to
provide an alternative to CO mapping to measure their H content. In this
Letter, we measure the dust emission cross-section using SPIRE and
recent CO and \HI\ observations; a variation in is present from a
near-solar neighborhood cross-section to about half-solar with the maximum
being south of the nucleus. Calculating the total H column density from the
measured dust temperature and cross-section, and then subtracting the \HI\
column, yields a morphology similar to that observed in CO. The H/\HI\ mass
ratio decreases from about unity to well below 10% and is about 15% averaged
over the optical disk. The single most important observation to reduce the
potentially large systematic errors is to complete the CO mapping of M 33.Comment: 5 pages, 5 figures Accepted for publication in Astronomy and
Astrophysic
X Her and TX Psc: Two cases of ISM interaction with stellar winds observed by Herschel
The asymptotic giant branch (AGB) stars X Her and TX Psc have been imaged at
70 and 160 microns with the PACS instrument onboard the Herschel satellite, as
part of the large MESS (Mass loss of Evolved StarS) Guaranteed Time Key
Program. The images reveal an axisymmetric extended structure with its axis
oriented along the space motion of the stars. This extended structure is very
likely to be shaped by the interaction of the wind ejected by the AGB star with
the surrounding interstellar medium (ISM). As predicted by numerical
simulations, the detailed structure of the wind-ISM interface depends upon the
relative velocity between star+wind and the ISM, which is large for these two
stars (108 and 55 km/s for X Her and TX Psc, respectively). In both cases,
there is a compact blob upstream whose origin is not fully elucidated, but that
could be the signature of some instability in the wind-ISM shock. Deconvolved
images of X Her and TX Psc reveal several discrete structures along the
outermost filaments, which could be Kelvin-Helmholtz vortices. Finally, TX Psc
is surrounded by an almost circular ring (the signature of the termination
shock?) that contrasts with the outer, more structured filaments. A similar
inner circular structure seems to be present in X Her as well, albeit less
clearly.Comment: 11 pages, Astronomy & Astrophysics, in pres
Identification of discriminative characteristics for clusters from biologic data with InforBIO software
Rescue of Dystrophic Skeletal Muscle by PGC-1α Involves a Fast to Slow Fiber Type Shift in the mdx Mouse
Increased utrophin expression is known to reduce pathology in dystrophin-deficient skeletal muscles. Transgenic over-expression of PGC-1α has been shown to increase levels of utrophin mRNA and improve the histology of mdx muscles. Other reports have shown that PGC-1α signaling can lead to increased oxidative capacity and a fast to slow fiber type shift. Given that it has been shown that slow fibers produce and maintain more utrophin than fast skeletal muscle fibers, we hypothesized that over-expression of PGC-1α in post-natal mdx mice would increase utrophin levels via a fiber type shift, resulting in more slow, oxidative fibers that are also more resistant to contraction-induced damage. To test this hypothesis, neonatal mdx mice were injected with recombinant adeno-associated virus (AAV) driving expression of PGC-1α. PGC-1α over-expression resulted in increased utrophin and type I myosin heavy chain expression as well as elevated mitochondrial protein expression. Muscles were shown to be more resistant to contraction-induced damage and more fatigue resistant. Sirt-1 was increased while p38 activation and NRF-1 were reduced in PGC-1α over-expressing muscle when compared to control. We also evaluated if the use a pharmacological PGC-1α pathway activator, resveratrol, could drive the same physiological changes. Resveratrol administration (100 mg/kg/day) resulted in improved fatigue resistance, but did not achieve significant increases in utrophin expression. These data suggest that the PGC-1α pathway is a potential target for therapeutic intervention in dystrophic skeletal muscle
100 mum and 160 mum emission as resolved star-formation rate estimators in M33 (HERM33ES)
Over the past few years several studies have provided estimates of the SFR
(star-formation rate) or the total infrared luminosity from just one infrared
band. However these relations are generally derived for entire galaxies, which
are known to contain a large scale diffuse emission that is not necessarily
related to the latest star-formation episode. We provide new relations to
estimate the SFR from resolved star-forming regions at 100 mum and 160 mum. We
select individual star-forming regions in the nearby (840 kpc) galaxy M33. We
estimate the SFR combining the emission in Halpha and at 24 mum to calibrate
the emission at 100 mum and 160 mum as SFR estimators, as mapped with
PACS/Herschel. The data are obtained in the framework of the HERM33ES open time
key project. There is less emission in the HII regions at 160 mum than at 100
mum. Over a dynamic range of almost 2 dex in Sigma(SFR) we find that the 100
mum emission is a nearly linear estimator of the SFR, whereas that at 160 mum
is slightly superlinear. The behaviour of individual star-forming regions is
surprisingly similar to that of entire galaxies. At high Sigma(SFR), star
formation drives the dust temperature, whereas uncertainties and variations in
radiation-transfer and dust-heated processes dominate at low Sigma(SFR).
Detailed modelling of both galaxies and individual star forming regions will be
needed to interpret similarities and differences between the two and assess the
fraction of diffuse emission in galaxies.Comment: 5 pages, 3 figures, accepted for publication in the A&A Herschel
special issu
Pseudomonas syringae pv. actinidiae (PSA) Isolates from Recent Bacterial Canker of Kiwifruit Outbreaks Belong to the Same Genetic Lineage
Intercontinental spread of emerging plant diseases is one of the most serious threats to world agriculture. One emerging disease is bacterial canker of kiwi fruit (Actinidia deliciosa and A. chinensis) caused by Pseudomonas syringae pv. actinidiae (PSA). The disease first occurred in China and Japan in the 1980s and in Korea and Italy in the 1990s. A more severe form of the disease broke out in Italy in 2008 and in additional countries in 2010 and 2011 threatening the viability of the global kiwi fruit industry. To start investigating the source and routes of international transmission of PSA, genomes of strains from China (the country of origin of the genus Actinidia), Japan, Korea, Italy and Portugal have been sequenced. Strains from China, Italy, and Portugal have been found to belong to the same clonal lineage with only 6 single nucleotide polymorphisms (SNPs) in 3,453,192 bp and one genomic island distinguishing the Chinese strains from the European strains. Not more than two SNPs distinguish each of the Italian and Portuguese strains from each other. The Japanese and Korean strains belong to a separate genetic lineage as previously reported. Analysis of additional European isolates and of New Zealand isolates exploiting genome-derived markers showed that these strains belong to the same lineage as the Italian and Chinese strains. Interestingly, the analyzed New Zealand strains are identical to European strains at the tested SNP loci but test positive for the genomic island present in the sequenced Chinese strains and negative for the genomic island present in the European strains. Results are interpreted in regard to the possible direction of movement of the pathogen between countries and suggest a possible Chinese origin of the European and New Zealand outbreaks
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