1,813 research outputs found
Parsec-scale HI absorption structure in a low-redshift galaxy seen against a Compact Symmetric Object
We present global VLBI observations of the 21-cm transition of atomic
hydrogen seen in absorption against the radio source J0855+5751. The foreground
absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at = 0.026. As the
background source is heavily resolved by VLBI, the data allow us to map the
properties of the foreground HI gas with a spatial resolution of 2pc. The
absorbing gas corresponds to a single coherent structure with an extent
35pc, but we also detect significant and coherent variations, including a
change in the HI optical depth by a factor of five across a distance of
6pc. The large size of the structure provides support for the Heiles &
Troland model of the ISM, as well as its applicability to external galaxies.
The large variations in HI optical depth also suggest that caution should be
applied when interpreting measurements from radio-detected DLAs. In
addition, the distorted appearance of the background radio source is indicative
of a strong jet-cloud interaction in its host galaxy. We have measured its
redshift ( = 0.54186) using optical spectroscopy on the William Herschel
Telescope and this confirms that J0855+5751 is a FRII radio source with a
physical extent of 1kpc and supports the previous identification of this
source as a Compact Symmetric Object. These sources often show absorption
associated with the host galaxy and we suggest that both HI and OH should be
searched for in J0855+5751.Comment: 14 pages and 10 figures. Accepted for publication in MNRA
An HI survey of the Centaurus and Sculptor Groups - Constraints on the space density of low mass galaxies
We present results of two 21-cm HI surveys performed with the Australia
Telescope Compact Array in the nearby Centaurus A and Sculptor galaxy groups.
These surveys are sensitive to compact HI clouds and galaxies with HI masses as
low as 3E+06 Msun, and are therefore among the most sensitive extragalactic HI
surveys to date. The surveys consist of sparsely spaced pointings that sample
approximately 2% of the groups' area on the sky. We detected previously known
group members, but we found no new HI clouds or galaxies down to the
sensitivity limit of the surveys. If the HI mass function had a faint end slope
of alpha = 1.5 below M_{HI} = 10^{7.5} Msun in these groups, we would have
expected ~3 new objects. Cold dark matter theories of galaxy formation predict
the existence of a large number low mass DM sub-halos that might appear as tiny
satellites in galaxy groups. Our results support and extend similar conclusions
derived from previous HI surveys that a HI rich population of these satellites
does not exist.Comment: Accepted for publication in A&
Exploring the causes of adverse events in hospitals and potential prevention strategies
Objectives
To examine the causes of adverse events
(AEs) and potential prevention strategies to minimise the
occurrence of AEs in hospitalised patients.
Methods
For the 744 AEs identified in the patient record
review study in 21 Dutch hospitals, trained reviewers
were asked to select all causal factors that contributed
to the AE. The results were analysed together with data
on preventability and consequences of AEs. In addition,
the reviewers selected one or more prevention strategies
for each preventable AE. The recommended prevention
strategies were analysed together with four general
causal categories: technical, human, organisational and
patient-related factors.
Results
Human causes were predominantly involved in
the causation of AEs (in 61% of the AEs), 61% of those
being preventable and 13% leading to permanent
disability. In 39% of the AEs, patient-related factors were
involved, in 14% organisational factors and in 4%
technical factors. Organisational causes contributed
relatively often to preventable AEs (93%) and AEs
resulting in permanent disability (20%). Recommended
strategies to prevent AEs were quality assurance/peer
review, evaluation of safety behaviour, training and
procedures. For the AEs with human and patient-related
causes, reviewers predominantly recommended quality
assurance/peer review. AEs caused by organisational
factors were considered preventable by improving
procedures.
Discussion
Healthcare interventions directed at human
causes are recommended because these play a large
role in AE causation. In addition, it seems worthwhile to
direct interventions on organisational causes because the
AEs they cause are nearly always believed to be
preventable. Organisational factors are thus relatively
easy to tackle. Future research designs should allow
researchers to interview healthcare providers that were
involved in the event, as an additional source of
information on contributing factors.
Comparing Galaxies and Lyman Alpha Absorbers at Low Redshift
A scenario is explored in which Lyman alpha absorbers at low redshift arise
from lines of sight through extended galaxy disks, including those of dwarf and
low surface brightness galaxies. A population of galaxies is simulated based
upon observed distributions of galaxy properties, and the gas disks are modeled
using pressure and gravity confinement. Some parameter values are ruled out by
comparing simulation results with the observed galaxy luminosity function, and
constraints may be made on the absorbing cross sections of galaxies. Simulation
results indicate that it is difficult to match absorbers with particular
galaxies observationally since absorption typically occurs at high impact
parameters (>200 kpc) from luminous galaxies. Low impact parameter absorption
is dominated by low luminosity dwarfs. A large fraction of absorption lines is
found to originate from low surface brightness galaxies, so that the absorbing
galaxy is likely to be misidentified. Low redshift Lyman alpha absorber counts
can easily be explained by moderately extended galaxy disks when low surface
brightness galaxies are included, and it is easily possible to find a scenario
which is consistent with observed the galaxy luminosity function, with low
redshift Lyman limit absorber counts, and with standard nucleosynthesis
predictions of the baryon density, Omega_Baryon.Comment: 17 pages, 8 figures, accepted to the Astrophysical Journa
Detection of chloronium and measurement of the 35Cl/37Cl isotopic ratio at z=0.89 toward PKS1830-211
We report the first extragalactic detection of chloronium (H2Cl+), in the
z=0.89 absorber in front of the lensed blazar PKS1830-211. The ion is detected
through its 1_11-0_00 line along two independent lines of sight toward the
North-East and South-West images of the blazar. The relative abundance of H2Cl+
is significantly higher (by a factor ~7) in the NE line of sight, which has a
lower H2/H fraction, indicating that H2Cl+ preferably traces the diffuse gas
component. From the ratio of the H2^35Cl+ and H2^37Cl+ absorptions toward the
SW image, we measure a 35Cl/37Cl isotopic ratio of 3.1 (-0.2; +0.3) at z=0.89,
similar to that observed in the Galaxy and the solar system.Comment: Accepted for publication in A&A Lette
Probing the jet base of the blazar PKS1830-211 from the chromatic variability of its lensed images. Serendipitous ALMA observations of a strong gamma-ray flare
The launching mechanism of the jets of active galactic nuclei is
observationally poorly constrained, due to the large distances to these objects
and the very small scales (sub-parsec) involved. In order to better constrain
theoretical models, it is especially important to get information from the
region close to the physical base of the jet, where the plasma acceleration
takes place. In this paper, we report multi-epoch and multi-frequency continuum
observations of the z=2.5 blazar PKS1830-211 with ALMA, serendipitously
coincident with a strong -ray flare reported by Fermi-LAT. The blazar
is lensed by a foreground z=0.89 galaxy, with two bright images of the compact
core separated by 1". Our ALMA observations individually resolve these two
images (although not any of their substructures), and we study the change of
their relative flux ratio with time (four epochs spread over nearly three times
the time delay between the two lensed images) and frequency (between 350 and
1050 GHz, rest-frame of the blazar), during the -ray flare. In
particular, we detect a remarkable frequency-dependent behaviour of the flux
ratio, which implies the presence of a chromatic structure in the blazar (i.e.,
a core-shift effect). We rule out the possiblity of micro- and milli-lensing
effects and propose instead a simple model of plasmon ejection in the blazar's
jet to explain the time and frequency variability of the flux ratio. We suggest
that PKS1830-211 is likely one of the best sources to probe the activity at the
base of a blazar's jet at submillimeter wavelengths, thanks to the peculiar
geometry of the system. The implications of the core-shift in absorption
studies of the foreground z=0.89 galaxy (e.g., constraints on the cosmological
variations of fundamental constants) are discussed.Comment: Accepted for publication in A&
A search for 21 cm HI absorption in AT20G compact radio galaxies
We present results from a search for 21 cm associated HI absorption in a
sample of 29 radio sources selected from the Australia Telescope 20 GHz survey.
Observations were conducted using the Australia Telescope Compact Array
Broadband Backend, with which we can simultaneously look for 21 cm absorption
in a redshift range of 0.04 < z < 0.08, with a velocity resolution of 7 km/s .
In preparation for future large-scale H I absorption surveys we test a
spectral-line finding method based on Bayesian inference. We use this to assign
significance to our detections and to determine the best-fitting number of
spectral-line components. We find that the automated spectral-line search is
limited by residuals in the continuum, both from the band-pass calibration and
spectral-ripple subtraction, at spectral-line widths of \Deltav_FWHM > 103 km/s
. Using this technique we detect two new absorbers and a third, previously
known, yielding a 10 per cent detection rate. Of the detections, the
spectral-line profiles are consistent with the theory that we are seeing
different orientations of the absorbing gas, in both the host galaxy and
circumnuclear disc, with respect to our line-of-sight to the source. In order
to spatially resolve the spectral-line components in the two new detections,
and so verify this conclusion, we require further high-resolution 21 cm
observations (~0.01 arcsec) using very long baseline interferometry.Comment: 16 pages, 8 figures and 5 tables; accepted for publication in MNRAS
(version 2 based on proof corrections
Discovery of a Nearby Low-Surface-Brightness Spiral Galaxy
During the course of a search for compact, isolated gas clouds moving with
anomalous velocities in or near our own Galaxy (Braun and Burton 1998 A&A, in
press), we have discovered, in the data of the Leiden/Dwingeloo survey
(Hartmann and Burton 1997, Atlas of Galactic Neutral Hydrogen, CUP) of Galactic
hydrogen, the HI signature of a large galaxy, moving at a recession velocity of
282 km/s, with respect to our Galaxy. Deep multicolor and spectroscopic optical
observations show the presence of star formation in scattered HII regions;
radio HI synthesis interferometry confirms that the galaxy is rich in HI and
has the rotation signature of a spiral galaxy; a submillimeter observation
failed to detect the CO molecule. The radio and optical evidence combined
suggest its classification as a low-surface-brightness spiral galaxy. It is
located in close spatial and kinematic proximity to the galaxy NGC 6946. The
newly-discovered galaxy, which we call Cepheus 1, is at a distance of about 6
Mpc. It is probably to be numbered amongst the nearest few LSB spirals.Comment: 13 page LaTeX, requires aastex, 4 GIF figures. Accepted for
publication in the AJ, January 199
A Structural and Dynamical Study of Late-Type, Edge-On Galaxies: I. Sample Selection and Imaging Data
We present optical (B & R) and infrared (K_s) images and photometry for a
sample of 49 extremely late-type, edge-on disk galaxies selected from the Flat
Galaxy Catalog of Karenchentsev et al. (1993). Our sample was selected to
include galaxies with particularly large axial ratios, increading the
likelihood that the galaxies in the sample are truly edge-on. We have also
concentrated the sample on galaxies with low apparent surface brightness, in
order to increase the representation of intrinisically low surface brightness
galaxies. Finally, the sample was chosen to have no apprarent bulges or optical
warps so that the galaxies represent undisturbed, ``pure disk'' systems. The
resulting sample forms the basis for a much larger spectroscopic study designed
to place constraints on the physical quantities and processes which shape disk
galaxies. The imaging data presented in this paper has been painstakingly
reduced and calibrated to allow accurate surface photometry of features as
faint as 30 mag/sqr-arcsec in B and 29 mag/sqr-arcsec in R on scales larger
than 10 arcsec. Due to limitations in sky subtraction and flat fielding, the
infrared data can reach only to 22.5 mag/sqr-arcsec in K_s on comparable
scales. As part of this work, we have developed a new method for quantifying
the reliability of surface photometry, which provides useful diagnostics for
the presence of scattered light, optical emission from infrared cirrus, and
other sources of non-uniform sky backgrounds.Comment: scheduled to appear in the Astronomical Journal, LaTeX, 36 pages
including 7 pages of figures (fig 1-2,4). A low resolution version of Figure
3 is included in JPEG format; contours are seriously degraded. A full
resolution Postscript version of Figure 3 (10.6Mb,gzipped) is available
through anonymous ftp at
ftp://ftp.astro.washington.edu/pub/users/jd/FGC/dalcanton.f3.ps.g
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