I. Snellen — Page 2

arXiv1999

The CERES Astronomical Database

The CERES Astronomical Archive was created initially in order to make the large amount of data that were collected from the two surveys, the Jodrell-VLA Astrometric Survey (JVAS) and the Cosmic Lens All-Sky Survey (CLASS), easily accessible to the partner members of the Consortium for European Research on Extragalactic Surveys (CERES) through the web. Here we describe the web based archive of the 15,000 flat spectrum radio sources. There is a wealth of information that one can access including downloading the raw and processed data for each of the sources. We also describe the search engines that were developed so that a) one can search through the archive for an object or a sample of objects by setting specific criteria and b) select new samples. Since new data are continually gathered on the same sources from follow up programs, an automatic update engine was created so that the new information and data can be added easily in the archive.

astro-ph↗

arXiv1998

Optical and near-infrared imaging of faint Gigahertz Peaked Spectrum sources

A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected from the Westerbork Northern Sky Survey (WENSS, Rengelink et al. 1997), has been imaged in the optical and near infrared, resulting in an identification fraction of 87%. The R-I and R-K colours of the faint optical counterparts are as expected for passively evolving elliptical galaxies, assuming that they follow the R band Hubble diagram as determined for radio-bright GPS galaxies. We have found evidence that the radio spectral properties of the GPS quasars are different from those of GPS galaxies: The observed distribution of radio spectral peak frequencies for GPS sources optically identified with bright stellar objects (presumably quasars) is shifted compared with GPS sources identified with faint or extended optical objects (presumably galaxies), in the sense that a GPS quasar is likely to have a higher peak frequency than a GPS galaxy. This means that the true peak frequency distribution is different for the GPS galaxies and quasars, because the sample selection effects are independent of optical identification. The correlation between peak frequency and redshift as has been suggested for bright sources has not been found in this sample; no correlation exists between R magnitude (and therefore redshift) and peak frequency for the GPS galaxies. We therefore believe that the claimed correlation is actually caused by the dependence of the peak frequency on optical host, because the GPS galaxies are in general at lower redshifts than the quasars. The difference in the peak frequency distributions of the GPS galaxies and quasars is further evidence against the hypothesis that they form a single class of object.

astro-ph↗