850 research outputs found
MERLIN/VLA imaging of the gravitational lens system B0218+357
Gravitational lenses offer the possibility of accurately determining the
Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the
most promising systems for an application of this technique. In particular this
system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999)
and preliminary mass modelling has given a value for H_0 of 69 +13/-19
km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the
mass modelling. As this system contains an Einstein ring it should be possible
to constrain the model better by imaging the ring at high resolution. To
achieve this we have combined data from MERLIN and the VLA at a frequency of 5
GHz. In particular MERLIN has been used in multi-frequency mode in order to
improve substantially the aperture coverage of the combined data set. The
resulting map is the best that has been made of the ring and contains many new
and interesting features. Efforts are currently underway to exploit the new
data for lensing constraints using the LensClean algorithm (Kochanek & Narayan
1992).Comment: Accepted for publication in MNRAS. 6 pages, 4 included PostScript
figure
A deep, high resolution survey of the low frequency radio sky
We report on the first wide-field, very long baseline interferometry (VLBI)
survey at 90 cm. The survey area consists of two overlapping 28 deg^2 fields
centred on the quasar J0226+3421 and the gravitational lens B0218+357. A total
of 618 sources were targeted in these fields, based on identifications from
Westerbork Northern Sky Survey (WENSS) data. Of these sources, 272 had flux
densities that, if unresolved, would fall above the sensitivity limit of the
VLBI observations. A total of 27 sources were detected as far as 2 arcdegrees
from the phase centre. The results of the survey suggest that at least 10% of
moderately faint (S~100 mJy) sources found at 90 cm contain compact components
smaller than ~0.1 to 0.3 arcsec and stronger than 10% of their total flux
densities. A ~90 mJy source was detected in the VLBI data that was not seen in
the WENSS and NRAO VLA Sky Survey (NVSS) data and may be a transient or highly
variable source that has been serendipitously detected. This survey is the
first systematic (and non-biased), deep, high-resolution survey of the
low-frequency radio sky. It is also the widest field of view VLBI survey with a
single pointing to date, exceeding the total survey area of previous higher
frequency surveys by two orders of magnitude. These initial results suggest
that new low frequency telescopes, such as LOFAR, should detect many compact
radio sources and that plans to extend these arrays to baselines of several
thousand kilometres are warranted.Comment: Accepted by The Astrophysical Journal. 39 pages, 4 figure
A Spectroscopic Study of the Environments of Gravitational Lens Galaxies
(Abridged) We present the first results from our spectroscopic survey of the
environments of strong gravitational lenses. The lens galaxy belongs to a poor
group of galaxies in six of the eight systems in our sample. We discover three
new groups associated with the lens galaxies of BRI 0952-0115 (five members),
MG 1654+1346 (seven members), and B2114+022 (five members). We more than double
the number of members for another three previously known groups around the
lenses MG 0751+2716 (13 total members), PG 1115+080 (13 total members), and
B1422+231 (16 total members). We determine the kinematics of the six groups,
including their mean velocities, velocity dispersions, and projected spatial
centroids. The velocity dispersions of the groups range from 110 +170, -80 to
470 +100, -90 km/s. In at least three of the lenses -- MG0751, PG1115, and
B1422 -- the group environment significantly affects the lens potential. These
lenses happen to be the quadruply-imaged ones in our sample, which suggests a
connection between image configuration and environment. The lens galaxy is the
brightest member in fewer than half of the groups. Our survey also allows us to
assess for the first time whether mass structures along the line of sight are
important for lensing. We first show that, in principle, the lens potential may
be affected by line-of-sight structures over a wide range of spatial and
redshift offsets from the lens. We then quantify real line-of-sight effects
using our survey and find that at least four of the eight lens fields have
substantial interloping structures close in projection to the lens, and at
least one of those structures (in the field of MG0751) significantly affects
the lens potential.Comment: Accepted for publication in the Astrophysical Journal. Figure 6
posted as a JPEG image. Requires emulateapj.st
Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548
The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor
the radio continuum counterpart to the optical broad line region (BLR) in the
Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained
at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited
to observations spanning 10-60 days and 3-4 yr, and acquired at matched
resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA
observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is
photometrically variable at 8.4 GHz by % and % between VLA
observations separated by 41 days and 4.1 yr, respectively. The 41-day changes
are milder (%) at 4.9 GHz and exhibit an inverted spectrum (, ) from 4.9 to 8.4 GHz. The nucleus
is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between
VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc
between VLBA observations separated by 3.1 yr. Such photometric variability and
astrometric stability is consistent with a black hole being the ultimate energy
source for the BLR, but is problematic for star cluster models that treat the
BLR as a compact supernova remnant and, for NGC 5548, require a new supernova
event every 1.7 yr within an effective radius 42 mas = 22 pc. A deep
image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data
from quiescent VLA observations. This image shows faint bipolar lobes
straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These
synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from
the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March
10, volume 53
Redshifts of CLASS Radio Sources
Spectroscopic observations of a sample of 42 flat-spectrum radio sources from
the Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of with an RMS spread of 0.95, at a completeness level of 64%. The sample
consists of sources with a 5-GHz flux density of 25-50 mJy, making it the
faintest flat-spectrum radio sample for which the redshift distribution has
been studied. The spectra, obtained with the Willam Herschel Telescope (WHT),
consist mainly of broad-line quasars at and narrow-line galaxies at
. Though the mean redshift of flat-spectrum radio sources exhibits
little variation over more than two orders of magnitude in radio flux density,
there is evidence for a decreasing fraction of quasars at weaker flux levels.
In this paper we present the results of our spectroscopic observations, and
discuss the implications for constraining cosmological parameters with
statistical analyses of the CLASS survey.Comment: 10 pages, AJ accepte
Effects of Dust on Gravitational Lensing by Spiral Galaxies
Gravitational lensing of an optical QSO by a spiral galaxy is often
counteracted by dust obscuration, since the line-of-sight to the QSO passes
close to the center of the galactic disk. The dust in the lens is likely to be
correlated with neutral hydrogen, which in turn should leave a Lyman-alpha
absorption signature on the QSO spectrum. We use the estimated dust-to-gas
ratio of the Milky-Way galaxy as a mean and allow a spread in its values to
calculate the effects of dust on lensing by low redshift spiral galaxies. Using
a no-evolution model for spirals at z<1 we find (in Lambda=0 cosmologies) that
the magnification bias due to lensing is stronger than dust obscuration for QSO
samples with a magnitude limit B<16. The density parameter of neutral hydrogen,
Omega_HI, is overestimated in such samples and is underestimated for fainter
QSOs.Comment: 18 pages, 4 figures, ApJ, in pres
Laser induced breakdown of the magnetic field reversal symmetry in the propagation of unpolarized light
We show how a medium, under the influece of a coherent control field which is
resonant or close to resonance to an appropriate atomic transition, can lead to
very strong asymmetries in the propagation of unpolarized light when the
direction of the magnetic field is reversed. We show how EIT can be used to
mimic effects occuring in natural systems and that EIT can produce very large
asymmetries as we use electric dipole allowed transitions. Using density matrix
calculations we present results for the breakdown of the magnetic field
reversal symmetry for two different atomic configurations.Comment: RevTex, 6 pages, 10 figures, Two Column format, submitted to Phys.
Rev.
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