The Soft X-Ray Properties of Nearby Low-Luminosity Active Galactic Nuclei and their Contribution to the Cosmic X-Ray Background

We have examined ROSAT soft X-ray observations of a complete,distance-limited sample of Seyfert and LINER galaxies. X-ray data are availablefor 46 out of 60 such objects which lie within a hemisphere of radius 18 Mpc.We have constructed radial profiles of the nuclear sources in order tocharacterize their spatial extent and, in some cases, to help constrain theamount of flux associated with a nuclear point source. PSPC data from ROSAThave been used to explore the spectral characteristics of the objects withsufficient numbers of detected counts. Based on the typical spectral parametersof these sources, we have estimated the luminosities of the weaker sources inthe sample. We then explore the relationship between the soft X-ray and H alphaluminosities of the observed objects; these quantities are correlated forhigher-luminosity AGNs. We find a weak correlation at low luminosities as well,and have used this relationship to predict L_X for the 14 objects in our samplethat lack X-ray data. Using the results of the spatial and spectral analyses,we have compared the X-ray properties of Seyferts and LINERs, finding nostriking differences between the two classes of objects. However, both types ofobjects often exhibit significant amounts of extended emission, which couldminimize the appearance of differences in their nuclear properties. The softX-ray characteristics of the type 1 and type 2 active galaxies in the sampleare also discussed. We then compute the local X-ray volume emissivity oflow-luminosity Seyferts and LINERs and investigate their contribution to thecosmic X-ray background. The 0.5-2.0 keV volume emissivity of 2.2e38ergs/s/Mpc^3 we obtain for our sample suggests that low-luminosity AGNs produceat least 9% of the soft X-ray background.Comment: 39 pages (AASTeX), including 8 figures. Accepted for publication in the Astronomical Journal, August 200