Site Map
Contacts
Follow us on Facebook Follow us on Twitter YouTube channel
Centro de Astrofísica da Universidade do Porto

Flat-spectrum radio sources with kpc-scale structure

P. Augusto, P. N. Wilkinson, I. W. A. Browne

Abstract
We have carried out the first systematic study of flat-spectrum radio sources which have significant structure on angular scales 90-300 milliarcsec (mas), corresponding to linear scales of ~ 0.5-1.5 kpc at a redshift of 0.5. The principal aim of the study was to search for multiple gravitational imaging of compact radio components with image separations smaller than that of the smallest galaxy lens system known (335 mas) and corresponding to masses appropriate to compact/dwarf galaxies and to the bulges of spiral galaxies. A secondary aim was the morphological classification of a sample of flat-spectrum sources with kpc-scale structure. We particularly wanted to find out the frequency of occurrence of compact symmetric objects (CSOs: linear size <1 kpc) and medium symmetric objects (MSOs: linear size <15 kpc) which are believed to be the precursors of the large classical double radio sources. The parent sample consisted of 1665 flat-spectrum sources selected from two VLA surveys made with ~200 mas resolution (the Jodrell Bank-VLA Astrometric Survey - JVAS - and the Cosmic Lens All-Sky Survey - CLASS), from which we used visibility data to select sources (55 in total) with significant structure in the size range 90-300 mas. Sources with multiple compact components having a flux density ratio <8:1 could be high-magnification lens systems, CSO/MSOs or objects with strong jets. A step-by-step observational filtering process at successively higher angular resolution was employed to classify the 55-source subsample. Initial MERLIN observations at 5 GHz (50 mas resolution) enabled us to classify ~75 per cent of the sources. The remaining sources were observed with the VLBA at 5 GHz (2 mas resolution) and in a few cases with MERLIN at 22 GHz (10 mas resolution). The resulting maps show that the majority of flat-spectrum sources with kpc-scale structure are asymmetric core-jets. The remaining sources include 23 new CSO/MSOs, a much smaller fraction of the parent sample (1.4 per cent) than is found in VLBI surveys of flat-spectrum sources (5-10 per cent). About half of the new CSO/MSOs constitute a hitherto unknown population: bright core CSO/MSOs. No definite high-magnification gravitational lenses were found. The implication is that the optical depth to lensing with image separations in the range 90-300 mas is several times less than on arcsec scales.

Monthly Notices of the Royal Astronomical Society
Volume 299, Page 1159
October 1998

>> ADS

Institute of Astrophysics and Space Sciences

Institute of Astrophysics and Space Sciences (IA) is a new but long anticipated research infrastructure with a national dimension. It embodies a bold but feasible vision for the development of Astronomy, Astrophysics and Space Sciences in Portugal, taking full advantage and fully realizing the potential created by the national membership of the European Space Agency (ESA) and the European Southern Observatory (ESO). IA resulted from the merging the two most prominent research units in the field in Portugal: the Centre for Astrophysics of the University of Porto (CAUP) and the Center for Astronomy and Astrophysics of the University of Lisbon (CAAUL). It currently hosts more than two-thirds of all active researchers working in Space Sciences in Portugal, and is responsible for an even greater fraction of the national productivity in international ISI journals in the area of Space Sciences. This is the scientific area with the highest relative impact factor (1.65 times above the international average) and the field with the highest average number of citations per article for Portugal.

Proceed on CAUP's website|Go to IA website