2,459 research outputs found
Supernovae and Cosmology with Future European Facilities
Prospects for future supernova surveys are discussed, focusing on the ESA
Euclid mission and the European Extremely Large Telescope(E-ELT), both expected
to be in operation around the turn of the decade. Euclid is a 1.2m space survey
telescope that will operate at visible and near-infrared wavelengths, and has
the potential to find and obtain multi-band lightcurves for thousands of
distant supernovae. The E-ELT is a planned general-purpose ground-based
40m-class optical-IR telescope with adaptive optics built in, which will be
capable of obtaining spectra of Type Ia supernovae to redshifts of at least
four. The contribution to supernova cosmology with these facilities will be
discussed in the context of other future supernova programs such as those
proposed for DES, JWST, LSST and WFIRST.Comment: To appear in proceedings of the Royal Society scientific discussion
meeting "New windows on transients across the Universe", 23-24 April 201
Automated optical identification of a large complete northern hemisphere sample of flat spectrum radio sources with S_6cm > 200 mJy
This paper describes the automated optical APM identification of radio
sources from the Jodrell Bank - VLA Astrometric Survey (JVAS), as used for the
search for distant radio-loud quasars. The sample has been used to investigate
possible relations between optical and radio properties of flat spectrum radio
sources. From the 915 sources in the sample, 756 have an optical APM
identification at a red (e) and/or blue (o) plate,resulting in an
identification fraction of 83% with a completeness and reliability of 98% and
99% respectively. About 20% are optically identified with extended APM objects
on the red plates, e.g. galaxies. However the distinction between galaxies and
quasars can not be done properly near the magnitude limit of the POSS-I plates.
The identification fraction appears to decrease from >90% for sources with a 5
GHz flux density of >1 Jy, to <80% for sources at 0.2 Jy. The identification
fraction, in particular that for unresolved quasars, is found to be lower for
sources with steeper radio spectra. In agreement with previous studies, we find
that the quasars at low radio flux density levels also tend to have fainter
optical magnitudes, although there is a large spread. In addition, objects with
a steep radio-to-optical spectral index are found to be mainly highly polarised
quasars, supporting the idea that in these objects the polarised synchrotron
component is more prominent. It is shown that the large spread in
radio-to-optical spectral index is possibly caused by source to source
variations in the Doppler boosting of the synchrotron component [Abridged].Comment: LaTex, 17 pages, 5 gif figures, 4 tables. Accepted for publication in
MNRAS. High resolution figures can be found at http://www.roe.ac.uk/~ignas
The Parkes quarter-Jansky flat-spectrum sample 3. Space density and evolution of QSOs
We analyze the Parkes quarter-Jansky flat-spectrum sample of QSOs in terms of
space density, including the redshift distribution, the radio luminosity
function, and the evidence for a redshift cutoff. With regard to the luminosity
function, we note the strong evolution in space density from the present day to
epochs corresponding to redshifts ~ 1. We draw attention to a selection effect
due to spread in spectral shape that may have misled other investigators to
consider the apparent similarities in shape of luminosity functions in
different redshift shells as evidence for luminosity evolution. To examine the
evolution at redshifts beyond 3, we develop a model-independent method based on
the V_max test using each object to predict expectation densities beyond z=3.
With this we show that a diminution in space density at z > 3 is present at a
significance level >4 sigma. We identify a severe bias in such determinations
from using flux-density measurements at epochs significantly later than that of
the finding survey. The form of the diminution is estimated, and is shown to be
very similar to that found for QSOs selected in X-ray and optical wavebands.
The diminution is also compared with the current estimates of star-formation
evolution, with less conclusive results. In summary we suggest that the
reionization epoch is little influenced by powerful flat-spectrum QSOs, and
that dust obscuration does not play a major role in our view of the QSO
population selected at radio, optical or X-ray wavelengths.Comment: 18 pages, 11 figures, accepted 18 Dec 2004, Astron. & Astrophys. The
accepted version is expanded to include an analysis of the form of the
decline in radio-QSO space density at high redshifts. This is compared with
the forms of epoch dependence derived for optically-selected QSOs, for
X-ray-selected QSOs, and for star formation rat
VLBI imaging of extremely high redshift quasars at 5 GHz
We present very long baseline interferometry (VLBI) images of ten very high
redshift (z>3) quasars at 5 GHz. The sources 0004+139, 0830+101, 0906+041,
0938+119 and 1500+045 were observed in September 1992 using a global VLBI
array, while 0046+063, 0243+181, 1338+381, 1428+423 and 1557+032 were observed
in October 1996 with the European VLBI Network and Hartebeesthoek, South
Africa. Most of the sources are resolved and show asymmetric structure. The
sample includes 1428+423, the most distant radio loud quasar known to date
(z=4.72). It is barely resolved with an angular resolution of about 2.0*1.4
mas.Comment: Astronomy and Astrophysics, in press, Latex2e, 10 pages, 3 figures
(and lots of sub-figures
A Direct Upper Limit on the Density of Cosmological Dust from the Absence of an X-ray Scattering Halo around the z=4.3 QSO 1508+5714
We report on the results of a search for an intergalactic X-ray dust
scattering halo in a deep observation of the bright, high-redshift quasar QSO
1508+5714 with the Chandra X-ray Observatory. We do not detect such a halo. Our
result implies an upper limit on the density of diffuse, large-grained
intergalactic dust of Omega_ dust < 2 x 10^-6, assuming a characteristic grain
size of 1micron. The result demonstrates the sensitivity of this technique for
detecting very small amounts of intergalactic dust which are very hard to
detect otherwise. This will allow us to put important constraints on systematic
effects induced by extinction on the interpretation of the SN Ia Hubble
Diagram, as well as on the amount and properties of cosmological dust being
expelled into the intergalactic medium at early z~2 times.Comment: 14 pages, 2 figures. to appear in ApJ, vol. 651, Nov. 200
- …