Source Matching in the SDSS and RASS: Which Galaxies are Really X-ray Sources?

摘要

The current view of galaxy formation holds that all massive galaxies harbor amassive black hole at their center, but that these black holes are not alwaysin an actively accreting phase. X-ray emission is often used to identifyaccreting sources, but for galaxies that are not harboring quasars(low-luminosity active galaxies), the X-ray flux may be weak, or obscured bydust. To aid in the understanding of weakly accreting black holes in the localuniverse, a large sample of galaxies with X-ray detections is needed. Wecross-match the ROSAT All Sky Survey (RASS) with galaxies from the SloanDigital Sky Survey Data Release 4 (SDSS DR4) to create such a sample. Becauseof the high SDSS source density and large RASS positional errors, thecross-matched catalog is highly contaminated by random associations. Weinvestigate the overlap of these surveys and provide a statistical test of thevalidity of RASS-SDSS galaxy cross-matches. SDSS quasars provide a test of ourcross-match validation scheme, as they have a very high fraction of true RASSmatches. We find that the number of true matches between the SDSS main galaxysample and the RASS is highly dependent on the optical spectral classificationof the galaxy; essentially no star-forming galaxies are detected, while morethan 0.6% of narrow-line Seyferts are detected in the RASS. Also, galaxies withambiguous optical classification have a surprisingly high RASS detectionfraction. This allows us to further constrain the SEDs of low-luminosity activegalaxies. Our technique is quite general, and can be applied to anycross-matching between surveys with well-understood positional errors.