The RASS-6dFGS catalogue: a sample of X-ray selected AGN from the 6dF Galaxy Survey

Abstract

We present a catalogue of 3405 X-ray sources from the ROSAT All Sky Survey (RASS) Bright Source Catalogue which fall within the area covered by the 6dF Galaxy Survey (6dFGS). The catalogue is count-rate limited at 0.05 cts s−1 in the X-ray and covers the area of sky with δ < 0° and |b| > 10°. The RASS–6dFGS sample was one of the additional target catalogues of the 6dFGS and as a result we obtained optical spectra for 2224 (65 per cent) RASS sources. Of these, 1715 (77 per cent) have reliable redshifts with a median redshift of z= 0.16 (excluding the Galactic sources). For the optically bright sources (bJ≤ 17.5) in the observed sample, over 90 per cent have reliable redshifts. The catalogue mainly comprises quasi-stellar objects (QSOs) and active galaxies but also includes 238 Galactic sources. Of the sources with reliable redshifts the majority are type 1 active galactic nuclei (AGN, 69 per cent), while 12 per cent are type 2 AGN, 6 per cent absorption-line galaxies and 13 per cent are stars. We also identify a small number of optically faint, very low redshift, compact objects which fall outside the general trend in the bJ−z plane. The RASS–6dFGS catalogue complements a number of Northern hemisphere samples, particularly the ROSAT Bright Source Catalogue–NRAO VLA Sky Survey (RBSC–NVSS) sample (Bauer et al. 2000), and furthermore, in the same region of sky (−40° < δ < 0°) reveals an additional 561 sources that were not identified as part of that sample. We detect 918 sources (27 per cent) of the RASS–6dFGS sample in the radio using either the 1.4 GHz NVSS or the 843 MHz Sydney University Molonglo Sky Survey (SUMSS) catalogues and find that the detection rate changes with redshift. At redshifts larger than 1 virtually all of these sources have radio counterparts and with a median flux density of 1.15 Jy, they are much stronger than the median flux density of 28.6 mJy for the full sample. We attribute this to the fact that the X-ray flux of these objects is being boosted by a jet component, possibly Doppler boosted, that is only present in radio-loud AGN. The RASS–6dFGS sample provides a large set of homogeneous optical spectra ideal for future studies of X-ray emitting AGN.