3,409 research outputs found
Low and High Surface Brightness Galaxies at Void Walls
We study the relative fraction of low and high surface brightness galaxies
(LSBGs and HSBGs) at void walls in the SDSS DR7. We focus on galaxies in equal
local density environments. We assume that the host dark-matter halo mass (for
which we use SDSS group masses) is a good indicator of local density. This
analysis allows to examine the behavior of the abundance of LSBG and HSBG
galaxies at a fixed local density and distinguish the large-scale environment
defined by the void geometry. We compare galaxies in the field, and in the void
walls; the latter are defined as the volume of void shells of radius equal to
that of the void. We find a significant decrement, a factor , of the
relative fraction of blue, active star-forming LSBGs in equal mass groups at
the void walls and the field. This decrement is consistent with an increase of
the fraction of blue, active star-forming HSBGs. By contrast, red LSBGs and
HSBGs show negligible changes. We argue that these results are consistent with
a scenario where LSBGs with blue colors and strong star formation activity at
the void walls are fueled by gas from the expanding void regions. This process
could lead to LSBG to HSBG transformations.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
Emergence of methicillin resistance predates the clinical use of antibiotics
The discovery of antibiotics more than 80 years ago has led to considerable improvements in human and animal health. Although antibiotic resistance in environmental bacteria is ancient, resistance in human pathogens is thought to be a modern phenomenon that is driven by the clinical use of antibiotics1. Here we show that particular lineages of methicillin-resistant Staphylococcus aureusâa notorious human pathogenâappeared in European hedgehogs in the pre-antibiotic era. Subsequently, these lineages spread within the local hedgehog populations and between hedgehogs and secondary hosts, including livestock and humans. We also demonstrate that the hedgehog dermatophyte Trichophyton erinacei produces two ÎČ-lactam antibiotics that provide a natural selective environment in which methicillin-resistant S. aureus isolates have an advantage over susceptible isolates. Together, these results suggest that methicillin resistance emerged in the pre-antibiotic era as a co-evolutionary adaptation of S. aureus to the colonization of dermatophyte-infected hedgehogs. The evolution of clinically relevant antibiotic-resistance genes in wild animals and the connectivity of natural, agricultural and human ecosystems demonstrate that the use of a One Health approach is critical for our understanding and management of antibiotic resistance, which is one of the biggest threats to global health, food security and development
On the accuracy of the high redshift cluster luminosity function
We study the reliability of the statistical background subtraction method for
computing the Ks-band luminosity function of cluster galaxies at z~1 using mock
Red-sequence Cluster Survey cluster catalogues constructed from GALFORM
semi-analytic galaxies. The underlying cluster luminosity function in the mocks
are compatible with recent estimates at z~1 by several authors. We simulate
different samples where the number of clusters with Ks-band photometry goes
from 5 to a maximum of 50, in order to find the most suitable observational
sample to carry out this study; the current observational status in the nIR
wavelength range has been reached using 5 real clusters at z~1. We compute the
composite luminosity function for several samples of galaxy clusters with
masses 1.5x10^14 M_sun assuming a flux limited, complete sample of galaxies
down to Ks=21.0 magnitudes. We find that the Schechter fit parameters Ks* and
alpha for a sample of galaxies with no redshift information are rather poorly
constrained if both parameters are allowed to vary freely; if alpha is fixed at
a fiducial value, then Ks* shows significantly improved stochastic
uncertainties but can be influenced by systematic deviations. We find a
significantly improved accuracy in the luminosity function parameters when
adding photometric redshift information for bright cluster galaxies. The
dwarf-to-giant ratios inferred from the luminosity functions of red-sequence
galaxies in the mock catalogue agree very well with the underlying values.
Finally, we find that in order to use estimates of Ks* to study the formation
redshift of cluster galaxies at z=1, the sample would need to contain 520 z~1
clusters, for an accuracy of 2 Gyr at the 68 per cent confidence level.Comment: 12 pages, 8 figures. Accepted for publication in MNRA
Milagro Constraints on Very High Energy Emission from Short Duration Gamma-Ray Bursts
Recent rapid localizations of short, hard gamma-ray bursts (GRBs) by the
Swift and HETE satellites have led to the observation of the first afterglows
and the measurement of the first redshifts from this type of burst. Detection
of >100 GeV counterparts would place powerful constraints on GRB mechanisms.
Seventeen short duration (< 5 s) GRBs detected by satellites occurred within
the field of view of the Milagro gamma-ray observatory between 2000 January and
2006 December. We have searched the Milagro data for >100 GeV counterparts to
these GRBs and find no significant emission correlated with these bursts. Due
to the absorption of high-energy gamma rays by the extragalactic background
light (EBL), detections are only expected for redshifts less than ~0.5. While
most long duration GRBs occur at redshifts higher than 0.5, the opposite is
thought to be true of short GRBs. Lack of a detected VHE signal thus allows
setting meaningful fluence limits. One GRB in the sample (050509b) has a likely
association with a galaxy at a redshift of 0.225, while another (051103) has
been tentatively linked to the nearby galaxy M81. Fluence limits are corrected
for EBL absorption, either using the known measured redshift, or computing the
corresponding absorption for a redshift of 0.1 and 0.5, as well as for the case
of z=0.Comment: Accepted for publication in the Astrophysical Journa
Invasive Pulmonary Adenocarcinoma with Lepidic Growth Pattern in a Pregnant Patient
Among the differential diagnoses that should be considered in acute respiratory failure (ARF) are infectious processes, autoimmune diseases, interstitial pulmonary fibrosis, and pulmonary neoplasia. Timely diagnosis of lung neoplasia is complicated in the early stages. An opportune diagnosis, as well as the specific treatment, decrease mortality. ARF occurs 1 in 500 pregnancies and is most common during the postpartum period. Among the specific etiologies that cause ARF during pregnancy that must be considered are: (1) preeclampsia; (2) embolism of amniotic fluid; (3) peripartum cardiomyopathy; and (4) trophoblastic embolism. The case of a 36-year-old patient with a 33-week pregnancy and ARF is presented. The patient presented dyspnea while exerting moderate effort that progressed to orthopnea and type 1 respiratory insufficiency. Imaging studies showed bilateral alveolar infiltrates and predominantly right areas of consolidation. Blood cultures, a galactomannan assay and IgG antibodies against mycoplasma pneumoniae, were reported as negative. Autoimmune etiology was ruled out through an immunoassay. A percutaneous pulmonary biopsy was performed and an invasive pulmonary adenocarcinoma with lepidic growth pattern (i.e. lepidic pulmonary adenocarcinoma, LPA) result was reported. This etiology is rare and very difficult to recognize in acute respiratory failure cases. After infectious, autoimmune and interstitial lung fibrosis have been excluded the clinician must suspect of lung cancer in a patient with acute respiratory failure and chest imaging compatible with the presence of ground-glass nodular opacities, a solitary nodule or mass with bronchogram, and lung consolidation. In the presence of acute respiratory failure, the suspicion of pulmonary neoplasia in an adult of reproductive age must be timely. Failure to recognize this etiology can lead to fatal results
Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000
In this paper we present the results of simultaneous observations of the TeV
blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the
Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope
system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively.
The source was monitored from February 2nd to February 16th and from May 3rd to
May 8th, 2000. We discuss in detail the temporal and spectral properties of the
source. Remarkably, the TeV observations of February 7th/8th showed
statistically significant evidence for substantial TeV flux variability on 30
min time scale. We show the results of modeling the data with a time dependent
homogeneous Synchrotron Self-Compton (SSC) model. The X-ray and TeV gamma-ray
emission strengths and energy spectra together with the rapid flux variability
strongly suggest that the emission volume is approaching the observer with a
Doppler factor of 50 or higher. The different flux variability time scales
observed at X-rays and TeV Gamma-rays indicate that a more detailed analysis
will require inhomogeneous models with several emission zones.Comment: Accepted for Publication in ApJ, 21 Pages, 5 Figure
Correlated intense X-ray and TeV activity of Mrk~501 in 1998 June
We present exactly simultaneous X-ray and TeV monitoring with {\it RXTE} and
HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial
period of very low flux at both wavelengths, the source underwent a remarkable
flare in the TeV and X-ray energy bands, lasting for about six days and with a
larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV
flare, rapid TeV flux variability on sub-hour timescales is found. Large
spectral variations are observed at X-rays, with the 3--20 keV photon index of
a pure power law continuum flattening from to on a
timescale of 2--3 days. This implies that during the maximum of the TeV
activity, the synchrotron peak shifted to energies keV, a behavior
similar to that observed during the longer-lasting, more intense flare in 1997
April. The TeV spectrum during the flare is described by a power law with
photon index and an exponential cutoff at 4 TeV; an
indication for spectral softening during the flare decay is observed in the TeV
hardness ratios. Our results generally support a scenario where the TeV photons
are emitted via inverse Compton scattering of ambient seed photons by the same
electron population responsible for the synchrotron X-rays. The simultaneous
spectral energy distributions (SEDs) can be fit with a one-zone
synchrotron-self Compton model assuming a substantial increase of the magnetic
field and the electron energy by a factor of 3 and 10, respectively.Comment: Accepted for publication in ApJ, Part
The Energy Spectrum of TeV Gamma-Rays from the Crab Nebula as measured by the HEGRA system of imaging air Cherenkov telescopes
The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray
Astronomy) stereoscopic system of imaging air Cherenkov telescopes (IACTs) for
a total of about 200 hrs during two observational campaigns: from September
1997 to March 1998 and from August 1998 to April 1999. The recent detailed
studies of system performance give an energy threshold and an energy resolution
for gamma-rays of 500 GeV and ~ 18%, respectively. The Crab energy spectrum was
measured with the HEGRA IACT system in a very broad energy range up to 20 TeV,
using observations at zenith angles up to 65 degrees. The Crab data can be
fitted in the energy range from 1 to 20 TeV by a simple power-law, which yields
dJg/dE = (2.79+/-0.02 +/- 0.5) 10^{-7} E^{-2.59 +/- 0.03 +/- 0.05}, ph m^{-2}
s^{-1} TeV^{-1} The Crab Nebula energy spectrum, as measured with the HEGRA
IACT system, agrees within 15% in the absolute scale and within 0.1 units in
the power law index with the latest measurements by the Whipple, CANGAROO and
CAT groups, consistent within the statistical and systematic errors quoted by
the experiments. The pure power-law spectrum of TeV gamma-rays from the Crab
Nebula constrains the physics parameters of the nebula environment as well as
the models of photon emission.Comment: to appear in ApJ, 29 pages, 6 figure
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