ROSAT public PSPC observations in the Las Campanas Redshift Survey

Abstract

The Las Campanas Redshift Survey, an optically selected survey which contains 26 418 galaxy redshifts, has been correlated with "The First ROSAT Source Catalogue of Pointed Observations with the PSPC," which contains 50408 sources from 2876 ROSAT pointed observations. Ten matches were found. The optical spectra of most of the ten matches show weak narrow emission lines. Due to their high x-ray luminosities, their high x-ray-to-optical flux ratios, and the evidence of rapid x-ray variability in the two brightest matches, we interpret the majority of these objects to be narrow-line Seyfert galaxies or "hidden" active galactic nuclei. Of the ten matches, only one galaxy shows the characteristics of a bona fide starburst.Key words: catalogs -surveys -galaxies: active -galaxies: starburst -x-rays: galaxies A A A subject classification: 157; 158 1. IntroductionGreat strides have been made in recent years in resolving the soft (0.5 -2 keV) extragalactic x-ray background (XRB) into discrete sources (for a review, see Hasinger 1996). These sources include stars, galaxies, quasars and other active galactic nuclei (AGNs), and groups and clusters of galaxies. Most recent surveys of the soft XRB have focused on finding the optical counterparts of x-ray sources from flux-limited x-ray catalogues. In this paper, however, we reverse the process: we take an optically selected galaxy redshift survey -the Las Campanas Redshift Survey (LCRS; Shectinan et al. 1996) -and search for x-ray sources within it, taking for our x-ray sample "The First ROSAT Source Catalogue of Point,ed Observat,ions with t.he PSPC" (ROSATSRC; Zimmermann 1994). That our approach is complement,ary t,o the standard strategy is evident,, since it is based upon a magnitude-limited optical galaxy survey which therefore contains much fainter x-ray sources (down to -3 x erg s-l cm-2) than most presenbday x-ray surveys. With such an approach, we can learn about the x-ray properties of a typical, optically selected catalogue of galaxies. 2. The LCRS and the R.OSATSRCThe LCRS is an optically selected galaxy redshift survey which extends to a redshift of 0.2 and which is composed of a total of 6 alternating 105 x 80' slices in t,lie North and Sout,li Galactic Caps. Accurate R-band photometry and sky positions for program objects have been extracted from CCD drift scans obtained on the Las Campanas Swope l-m telescope; spectroscopy has been performed at the Las Campanas Du Pont 2.5-m telescope, originally via a 50-fiber Multi-Object, Spectrograph (MOS), and later via a 112-fiber MOS. For observing efficiency, all the fibers are used, but each MOS field is observed only once. Hence, the LCRS is a collection of 50-fiber fields (with nominal apparent magnitude limits of 16 < m R < 17.3) and 112-fiber fields (with nominal apparent magnitude limits of 15 < mR < 17.7). Recently completed, the LCRS contains 26418 galaxy redshifts; in this paper, we consider only those 25327 LCRS galaxy redshifts which lie in the 6 published slices (Shectman et al. 1996).Our x-ray sample, ROSAT...