Deep extragalactic VLBI-optical survey (DEVOS)^*

II. Efficient VLBI detection of SDSS quasars

¹ FÖMI Satellite Geodetic Observatory, PO Box 585, 1592 Budapest, Hungary e-mail: frey@sgo.fomi.hu
² MTA Research Group for Physical Geodesy and Geodynamics, PO Box 91, 1521 Budapest, Hungary
³ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁴ Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Netherlands Foundation for Research in Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁷ University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK

Received: 20 September 2007
Accepted: 2 November 2007

Abstract

Context.The Deep Extragalactic VLBI-Optical Survey (DEVOS) aims at constructing a large sample of compact radio sources up to two orders of magnitude fainter than those studied in other Very Long Baseline Interferometry (VLBI) surveys. Optical identification of the objects is ensured by selecting them from the Sloan Digital Sky Survey (SDSS) list.

Aims.While continuing to build up the DEVOS data base, we investigated how the VLBI detection rate could be enhanced by refining the initial selection criteria introduced in the first paper of this series.

Methods.We observed 26 sources in two adjacent, slightly overlapping -radius fields with the European VLBI Network (EVN) at 5 GHz frequency on 2 March 2007. The phase-reference calibrator quasars were J1616+3621 and J1623+3909. The objects selected were unresolved both in the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey catalogue and the SDSS Data Release 4.

Results.We present images of milli-arcsecond (mas) scale radio structures and accurate coordinates of 24 extragalactic sources. Most of them have never been imaged with VLBI. Twenty-two compact radio sources (85% of our initial sample) are considered as VLBI detections of the corresponding optical quasars in SDSS. We found an efficient way to identify quasars as potential VLBI targets with mas-scale compact radio stucture at >1 mJy level, based only on the FIRST and SDSS catalogue data by applying simple selection criteria.