73 research outputs found
Quasars from the 7C Survey - I:sample selection and radio maps
We describe the selection of candidate radio-loud quasars obtained by
cross-matching radio source positions from the low-frequency (151 MHz) 7C
survey with optical positions from five pairs of EO POSS-I plates scanned with
the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied
is centred at RA 10 h 28 m, Dec +41 and covers 0.057 sr. We present VLA
observations of the quasar candidates, and tabulate various properties derived
from the radio maps. We discuss the selection criteria of the resulting `7CQ'
sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of
the 36 unconfirmed candidates, constitute a filtered sample with the following
selection criteria: 151-MHz flux density S151 > 100 mJy; POSS-I E-plate
magnitude E approx R < 20; and POSS-I colour (O - E) < 1.8; the effective area
of the survey drops significantly below S151 approx 200 mJy. We argue that the
colour criterion excludes few if any quasars, but note, on the basis of recent
work by Willott et al. (1998b), that the E magnitude limit probably excludes
more than 50 per cent of the radio-loud quasars.Comment: Latex, 30 pages. Accepted by MNRA
Further Sunyaev-Zel'dovich observations of two Planck ERCSC clusters with the Arcminute Microkelvin Imager
We present follow-up observations of two galaxy clusters detected blindly via
the Sunyaev-Zel'dovich (SZ) effect and released in the Planck Early Release
Compact Source Catalogue. We use the Arcminute Microkelvin Imager, a dual-array
14-18 GHz radio interferometer. After radio source subtraction, we find a SZ
decrement of integrated flux density -1.08+/-0.10 mJy toward PLCKESZ
G121.11+57.01, and improve the position measurement of the cluster, finding the
centre to be RA 12 59 36.4, Dec +60 04 46.8, to an accuracy of 20 arcseconds.
The region of PLCKESZ G115.71+17.52 contains strong extended emission, so we
are unable to confirm the presence of this cluster via the SZ effect.Comment: 4 tables, 3 figures, revised after referee's comments and resubmitted
to MNRA
High resolution AMI Large Array imaging of spinning dust sources: spatially correlated 8 micron emission and evidence of a stellar wind in L675
We present 25 arcsecond resolution radio images of five Lynds Dark Nebulae
(L675, L944, L1103, L1111 & L1246) at 16 GHz made with the Arcminute
Microkelvin Imager (AMI) Large Array. These objects were previously observed
with the AMI Small Array to have an excess of emission at microwave frequencies
relative to lower frequency radio data. In L675 we find a flat spectrum compact
radio counterpart to the 850 micron emission seen with SCUBA and suggest that
it is cm-wave emission from a previously unknown deeply embedded young
protostar. In the case of L1246 the cm-wave emission is spatially correlated
with 8 micron emission seen with Spitzer. Since the MIR emission is present
only in Spitzer band 4 we suggest that it arises from a population of PAH
molecules, which also give rise to the cm-wave emission through spinning dust
emission.Comment: accepted MNRA
Microwave observations of spinning dust emission in NGC6946
We report new cm-wave measurements at five frequencies between 15 and 18GHz
of the continuum emission from the reportedly anomalous "region 4" of the
nearby galaxy NGC6946. We find that the emission in this frequency range is
significantly in excess of that measured at 8.5GHz, but has a spectrum from
15-18GHz consistent with optically thin free-free emission from a compact HII
region. In combination with previously published data we fit four emission
models containing different continuum components using the Bayesian spectrum
analysis package radiospec. These fits show that, in combination with data at
other frequencies, a model with a spinning dust component is slightly preferred
to those that possess better-established emission mechanisms.Comment: submitted MNRA
First results from the Very Small Array -- I. Observational methods
The Very Small Array (VSA) is a synthesis telescope designed to image faint
structures in the cosmic microwave background on degree and sub-degree angular
scales. The VSA has key differences from other CMB interferometers with the
result that different systematic errors are expected. We have tested the
operation of the VSA with a variety of blank-field and calibrator observations
and cross-checked its calibration scale against independent measurements. We
find that systematic effects can be suppressed below the thermal noise level in
long observations; the overall calibration accuracy of the flux density scale
is 3.5 percent and is limited by the external absolute calibration scale.Comment: 9 pages, 10 figures, MNRAS in press (Minor revisions
Searching for non-Gaussianity in the VSA data
We have tested Very Small Array (VSA) observations of three regions of sky
for the presence of non-Gaussianity, using high-order cumulants, Minkowski
functionals, a wavelet-based test and a Bayesian joint power
spectrum/non-Gaussianity analysis. We find the data from two regions to be
consistent with Gaussianity. In the third region, we obtain a 96.7% detection
of non-Gaussianity using the wavelet test. We perform simulations to
characterise the tests, and conclude that this is consistent with expected
residual point source contamination. There is therefore no evidence that this
detection is of cosmological origin. Our simulations show that the tests would
be sensitive to any residual point sources above the data's source subtraction
level of 20 mJy. The tests are also sensitive to cosmic string networks at an
rms fluctuation level of (i.e. equivalent to the best-fit observed
value). They are not sensitive to string-induced fluctuations if an equal rms
of Gaussian CDM fluctuations is added, thereby reducing the fluctuations due to
the strings network to rms . We especially highlight the usefulness
of non-Gaussianity testing in eliminating systematic effects from our data.Comment: Minor corrections; accepted for publication to MNRA
Sunyaev-Zel'dovich observations of galaxy clusters out to the virial radius with the Arcminute Microkelvin Imager
We present observations using the Small Array of the Arcminute Microkelvin
Imager (AMI; 14-18 GHz) of four Abell and three MACS clusters spanning
0.171-0.686 in redshift. We detect Sunyaev-Zel'dovich (SZ) signals in five of
these without any attempt at source subtraction, although strong source
contamination is present. With radio-source measurements from high-resolution
observations, and under the assumptions of spherical -model,
isothermality and hydrostatic equilibrium, a Bayesian analysis of the data in
the visibility plane detects extended SZ decrements in all seven clusters over
and above receiver noise, radio sources and primary CMB imprints. Bayesian
evidence ratios range from 10^{11}:1 to 10^{43}:1 for six of the clusters and
3000:1 for one with substantially less data than the others. We present
posterior probability distributions for, e.g., total mass and gas fraction
averaged over radii internal to which the mean overdensity is 1000, 500 and
200, r_200 being the virial radius. Reaching r_200 involves some extrapolation
for the nearer clusters but not for the more-distant ones. We find that our
estimates of gas fraction are low (compared with most in the literature) and
decrease with increasing radius. These results appear to be consistent with the
notion that gas temperature in fact falls with distance (away from near the
cluster centre) out to the virial radius.Comment: 18 pages, 10 figures, submitted to MNRAS (updated authors and fixed
Figure 1
Very Small Array observations of the Sunyaev-Zel'dovich effect in nearby galaxy clusters
We present VSA observations (~34GHz) on scales ~20 arcmin towards a complete,
X-ray-flux-limited sample of seven clusters at redshift z<0.1. Four have
significant SZ detections in the presence of CMB primordial anisotropy. We use
a bayesian MCMC method for inference from the VSA data, with X-ray priors on
cluster positions and temperatures, and radio priors on sources. We make
assumptions of beta-model gas distributions and of hydrostatic equilibrium, to
evaluate probability densities for the gas mass and total mass out to r_200.
Our combined estimate of the gas fraction is 0.08^{+0.06}_{-0.04}h^{-1} The
random errors are poor (note that the errors are higher than would have been
obtained with the usual chi-squared method) but the control of bias is good. We
have described the MCMC analysis method specifically in terms of SZ but hope
the description will be of more general use. We find that the effects of
primordial CMB contamination tend to be similar in the estimates of both the
gas mass and total mass over our narrow range of angular scales, so that there
is little effect of primordials on the gas fraction determination. Using our
total mass estimates we find a normalisation of the mass-temperature relation
based on the profiles from the VSA cluster pressure maps that is in good
agreement with recent M-T determinations from X-ray cluster measurements.Comment: Replaces earlier version. 16 pages, 5 figures, LaTeX. Minor revisions
to content, accepted by MNRAS for publicatio
Estimating the bispectrum of the Very Small Array data
We estimate the bispectrum of the Very Small Array data from the compact and
extended configuration observations released in December 2002, and compare our
results to those obtained from Gaussian simulations. There is a slight excess
of large bispectrum values for two individual fields, but this does not appear
when the fields are combined. Given our expected level of residual point
sources, we do not expect these to be the source of the discrepancy. Using the
compact configuration data, we put an upper limit of 5400 on the value of f_NL,
the non-linear coupling parameter, at 95 per cent confidence. We test our
bispectrum estimator using non-Gaussian simulations with a known bispectrum,
and recover the input values.Comment: 17 pages, 16 figures, replaced with version accepted by MNRAS.
Primordial bispectrum recalculated and figure 11 change
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