828 research outputs found
The VLA Low-frequency Sky Survey
The Very Large Array (VLA) Low-frequency Sky Survey (VLSS) has imaged 95% of
the 3*pi sr of sky north of declination = -30 degrees at a frequency of 74 MHz
(4 meter wavelength). The resolution is 80" (FWHM) throughout, and the typical
RMS noise level is ~0.1 Jy/beam. The typical point-source detection limit is
0.7 Jy/beam and so far nearly 70,000 sources have been catalogued. This survey
used the 74 MHz system added to the VLA in 1998. It required new imaging
algorithms to remove the large ionospheric distortions at this very low
frequency throughout the entire ~11.9 degree field of view. This paper
describes the observation and data reduction methods used for the VLSS and
presents the survey images and source catalog. All of the calibrated images and
the source catalog are available online (http://lwa.nrl.navy.mil/VLSS) for use
by the astronomical community.Comment: 53 pages, including 3 tables and 15 figures. Has been accepted for
publication in the Astronomical Journa
Limits on the Peculiar Velocities of Two Distant Clusters Using the Kinematic Sunyaev-Zel'dovich Effect
We report millimeter-wavelength observations of the Sunyaev-Zel'dovich (S-Z)
effect in two distant galaxy clusters. A relativistically correct analysis of
the S-Z data is combined with the results of X-ray observations to determine
the radial peculiar velocities v_r of the clusters. We observed Abell 2163
(z=.201) in three mm-wavelength bands centered at 2.1, 1.4, and 1.1 mm. We
report a significant detection of the thermal component of the S-Z effect seen
as both a decrement in the brightness of the CMB at 2.1 mm, and as an increment
at 1.1 mm. Including uncertainties due to the calibration of the instrument,
distribution and temperature of the IC gas, and astrophysical confusion, a
simultaneous fit to the data in all three bands gives v_r=+490 +1370/-880 km/s
at 68% confidence. We observed Abell 1689 (z=.181) in the 2.1 and 1.4 mm bands.
Including the same detailed accounting of uncertainty, a simultaneous fit to
the data in both bands gives v_r=+170 +815/-630 km/s. The limits on the
peculiar velocities of A2163 and A1689 correspond to deviations from the
uniform Hubble flow of <= 2-3%.Comment: 21 pages, 13 postscript figures, LaTeX(aaspptwo.sty), ApJ(in press
Poststarburst Models of LINERs
Since the discovery of low-ionization nuclear emission-line regions in many
galaxies (LINERs), it has been recognized that they constitute a class of
active galactic nuclei (AGNs) which are thought to be powered by gas accretion
onto a central, supermassive black hole. LINERs are observed in approximately
one third of galaxies in the local universe and it has been often thought that
they harbor an AGN-like central engine with moderate activity. However, some
LINERs show no direct evidence for AGNs such as broad emission lines, radio
jets, hard X-ray emission, spectral energy distributions which are inconsistent
with starlight, and so on. For such LINERs (a subset of type 2 LINERs), we
present new poststarburst models which explain some of their most important
optical narrow emission-line ratios. In these models, the ionization sources
are planetary nebula nuclei (PNNs) with temperature of ~ 10^5 K which appear in
the late-phase evolution of intermediate-mass stars with mass between ~ 3
M_solar and ~ 6 M_solar. Such PNNs left in a typical starburst nucleus can
produce an Halpha luminosity of L(Halpha) ~ 10^38 ergs s^-1 for typical
poststarburst LINERs and ~ 10^39 ergs s^-1 only in exceptionally bright cases.
The PNN phase lasts until the death of the lowest-mass stars formed in the
starburst, which is ~ 5 x 10^8 yr for an assumed lower limit of the initial
mass function of 3 M_solar. This long duration appears consistent with the
observed higher frequency of occurrence of LINERs if every galaxy could
experience the starburst activity several times in its life. We therefore
propose that some LINERs which show no direct evidence for AGNs may be
poststarburst nuclei powered by a cluster of PNNs.Comment: 8 pages, 6 figures, emulateapj.sty; To appear in the September 2000
issue of the Astronomical Journa
Radio morphology and spectral analysis of cD galaxies in rich and poor galaxy clusters
We present a radio morphological study and spectral analysis for a sample of
13 cD galaxies in rich and poor clusters of galaxies.} Our study is based on
new high sensitivity Giant Metrewave Radio Telescope (GMRT) observations at
1.28 GHz, 610 MHz and 235 MHz, and on archival data. From a statistical sample
of cluster cD galaxies we selected those sources with little information
available in the literature and promising for the detection of aged radio
emission. Beyond the high sensitivity images for all 13 radio galaxies, we
present also a detailed spectral analysis for 7 of them. We found a variety of
morphologies and linear sizes, as typical for radio galaxies in the radio power
range sampled here (low to intermediate power radio galaxies). The spectral
analysis shows that 10/13 radio galaxies have steep radio spectrum, with
spectral index . In general, the radiative ages and growth
velocities are consistent with previous findings that the evolution of radio
galaxies at the cluster centres is affected by the dense external medium (i.e.
low growth velocities and old ages. We suggest that the dominant galaxies in A
2622 and MKW 03s are dying radio sources, which at present are not fed by
nuclear activity. On the other hand, the spectacular source at the centre of A
2372 might be a very interesting example of restarted radio galaxy. For this
source we estimated a life cycle of the order of 10 yr.Comment: Accepted by A&A, 25 pages, 28 figures, 6 tables and appendix Full
version including high quality images available at
http://www.ira.inaf.it/~tventuri/pap/Venturi.pd
The prevalence of sarcopenia in fallers and those at risk of falls in a secondary care falls unit as measured by bioimpedance analysis
Objectives: Sarcopenia is characterised by loss of skeletal muscle mass and strength with adverse outcomes: physical disability, poor quality of life and death. Low muscle mass and strength are risk factors for falls, although there are few data available on the prevalence of sarcopenia in fallers. This study aimed to determine prevalence of sarcopenia in older people referred to a falls clinic. /
Methods: Consecutive patients referred to a secondary care falls unit were recruited. Sarcopenia was diagnosed using the European Working Group on Sarcopenia definition (low muscle mass and function) and cut-off points. Bio-impedance measured appendicular skeletal muscle mass. Gait speed and grip strength were functional measures. /
Results: Fifty-eight patients were recruited. Mean (SD) grip strength for women and men respectively were 17.9 (4.9) and 29.9(8.7) kg, mean (SD) gait speeds were 0.61(0.18) and 0.72 (0.4) m/s, mean (SD) appendicular skeletal muscle index in women and men were 6.98(1.0) and 7.85 (1.0) kg/m2 (p=0.018). Prevalence of sarcopenia was 9.8% (95% CI=1.6%-18%). /
Conclusions: Sarcopenia, as measured by bio-impedance is not uncommon in older people accessing a secondary care falls clinic. Bio-impedance was simple to perform, although further validation against gold standard methods is needed. As nutritional and exercise interventions for sarcopenia are available, simple methods for diagnosing sarcopenia in fallers should be considered
What Powers the Compact Radio Emission in Nearby Elliptical and S0 Galaxies?
Many nearby early-type (elliptical and S0) galaxies contain weak
(milli-Jansky level) nuclear radio sources on scales a few hundred parsecs or
less. The origin of the radio emission, however, has remained unclear,
especially in volume-limited samples that select intrinsically less luminous
galaxies. Both active galactic nuclei and nuclear star formation have been
suggested as possible mechanisms for producing the radio emission. This paper
utilizes optical spectroscopic information to address this issue. A substantial
fraction of the early-type galaxies surveyed with the Very Large Array by
Wrobel & Heeschen (1991) exhibits detectable optical emission lines in their
nuclei down to very sensitive limits. Comparison of the observed radio
continuum power with that expected from the thermal gas traced by the optical
emission lines implies that the bulk of the radio emission is nonthermal. Both
the incidence and the strength of optical line emission correlate with the
radio power. At a fixed line luminosity, ellipticals have stronger radio cores
than S0s. The relation between radio power and line emission observed in this
sample is consistent with the low-luminosity extension of similar relations
seen in classical radio galaxies and luminous Seyfert nuclei. A plausible
interpretation of this result is that the weak nuclear sources in nearby
early-type galaxies are the low-luminosity counterparts of more powerful AGNs.
The spectroscopic evidence supports this picture. Most of the emission-line
objects are optically classified as Seyfert nuclei or low-ionization nuclear
emission-line regions (LINERs), the majority of which are likely to be
accretion-powered sources.Comment: LaTex, 16 pages including embedded figures. Accepted for publication
in the Astrophysical Journa
The discovery of diffuse steep spectrum sources in Abell 2256
Context: Hierarchical galaxy formation models indicate that during their
lifetime galaxy clusters undergo several mergers. Here we report on the
discovery of three diffuse radio sources in the periphery of Abell 2256, using
the GMRT.
Aims: The aim of the observations was to search for diffuse ultra-steep
spectrum radio sources within the galaxy cluster Abell 2256.
Methods: We have carried out GMRT 325 MHz radio continuum observations of
Abell 2256. V, R and I band images of the cluster were taken with the 4.2m WHT.
Results: We have discovered three diffuse elongated radio sources located
about 1 Mpc from the cluster center. Two are located to the west of the cluster
center, and one to the southeast. The sources have a measured physical extent
of 170, 140 and 240 kpc, respectively. The two western sources are also visible
in deep low-resolution 115-165 MHz WSRT images, although they are blended into
a single source. For the combined emission of the blended source we find an
extreme spectral index of -2.05\pm 0.14 between 140 and 351 MHz. The extremely
steep spectral index suggests these two sources are most likely the result of
adiabatic compression of fossil radio plasma due to merger shocks.
Conclusions: The discovery of the steep spectrum sources implies the
existence of a population of faint diffuse radio sources in (merging) clusters
with such steep spectra that they have gone unnoticed in higher frequency
(\gtrsim 1 GHz) observations. An exciting possibility therefore is that such
sources will determine the general appearance of clusters in low-frequency high
resolution radio maps as will be produced by for example LOFAR or LWA.Comment: 5 pages, 2 figures, accepted for publication in A&A on October 16,
200
The 74MHz System on the Very Large Array
The Naval Research Laboratory and the National Radio Astronomy Observatory
completed implementation of a low frequency capability on the VLA at 73.8 MHz
in 1998. This frequency band offers unprecedented sensitivity (~25 mJy/beam)
and resolution (~25 arcsec) for low-frequency observations. We review the
hardware, the calibration and imaging strategies, comparing them to those at
higher frequencies, including aspects of interference excision and wide-field
imaging. Ionospheric phase fluctuations pose the major difficulty in
calibrating the array. Over restricted fields of view or at times of extremely
quiescent ionospheric ``weather'', an angle-invariant calibration strategy can
be used. In this approach a single phase correction is devised for each
antenna, typically via self-calibration. Over larger fields of view or at times
of more normal ionospheric ``weather'' when the ionospheric isoplanatic patch
size is smaller than the field of view, we adopt a field-based strategy in
which the phase correction depends upon location within the field of view. This
second calibration strategy was implemented by modeling the ionosphere above
the array using Zernike polynomials. Images of 3C sources of moderate strength
are provided as examples of routine, angle-invariant calibration and imaging.
Flux density measurements indicate that the 74 MHz flux scale at the VLA is
stable to a few percent, and tied to the Baars et al. value of Cygnus A at the
5 percent level. We also present an example of a wide-field image, devoid of
bright objects and containing hundreds of weaker sources, constructed from the
field-based calibration. We close with a summary of lessons the 74 MHz system
offers as a model for new and developing low-frequency telescopes. (Abridged)Comment: 73 pages, 46 jpeg figures, to appear in ApJ
Radio Sources in Galaxy Clusters at 28.5 GHz
We present serendipitous detections of radio sources at 28.5 GHz (1 cm),
which resulted from our program to image thermal Sunyaev-Zeldovich (SZ) effect
in 56 galaxy clusters. We find 64 radio sources with fluxes down to 0.4 mJy,
and within 250 arcseconds from the pointing centers. The spectral indices (S ~
\nu^-\alpha) of 54 sources with published low frequency flux densities range
from -0.6 to 2 with a mean of 0.77, and a median of 0.84. Extending low
frequency surveys of radio sources towards galaxy clusters CL 0016+16, Abell
665, and Abell 2218 to 28.5 GHz, and selecting sources with 1.4 GHz flux
density greater than 7 mJy to form an unbiased sample, we find a mean spectral
index of 0.71 and a median of 0.71. We find 4 to 7 times more sources predicted
from a low frequency survey in areas without galaxy clusters. This excess
cannot be accounted for by gravitational lensing of a background radio
population by cluster potentials, indicating most of the detected sources are
associated with galaxy clusters. For the cluster Abell 2218, the presence of
unsubtracted radio sources with 28.5 GHz flux densities less than 0.5 mJy, can
only contribute to temperature fluctuations at a level of 10 to 25 \muK. The
corresponding error due to radio point source contamination in the Hubble
constant derived through a combined analysis of 28.5 GHz SZ images and X-ray
emission observations ranges from 1% to 6%.Comment: 18 pages, 8 figures, to appear in April 1998 issue of A
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