KISS Active Galactic Nuclei in the Soft X-Ray Band: Correlation with the [ITAL]ROSAT[/ITAL] All-Sky Survey

Abstract

We present a study of the X-ray properties of a volume-limited sample of optically selected emission-line galaxies. The sample is derived from a correlation between the KPNO International Spectroscopic Survey (KISS), an Hα-selected objective-prism survey of active galactic nuclei (AGNs) and starbursting galaxies, and the ROSAT All-Sky Survey (RASS). After elimination of all spurious matches, we identify 18 ROSAT-detected X-ray sources within the KISS sample in the 0.1–2.4 keV band. Because of soft X-ray selection effects, the majority of the ROSAT sources are Seyfert 1 galaxies. The majority (54%) of the ROSAT-KISS Seyfert galaxies are classified as narrow-line Seyfert 1 objects, a relatively high percentage compared with previous objective-prism–selected Seyfert galaxy samples. We estimate the X-ray luminosities of the ROSAT-detected KISS objects and derive volume emissivities of 6.63 × 1038 and 1.45 × 1038 ergs s-1 Mpc-3 for the 30° and 43° survey strips, respectively. For those KISS AGNs not detected by the RASS, we use the median LX/LHα ratio derived from a previous study to estimate LX. The total 0.5–2 keV volume emissivity we predict for the overall KISS AGN sample is sufficient to account for 22.1% ± 8.9% of the soft X-ray background, averaged over both survey strips. The KISS AGN sample is made up predominantly of intermediate-luminosity Seyfert 2's and LINERs, which tend to be weak sources of soft X-rays. They may, however, represent a much more significant contribution to the hard X-ray background.