An X-ray and optical investigation of the environments around nearby radiogalaxies

Investigations of the cluster environment of radio sources have not shown a correlation between radio power and degree of clustering. However, it has been demonstrated that extended X-ray luminosity and galaxy clustering do e xhibit a positive correlation. This study investigates a complete sample of 25 nearby (z less than or equal to 0.06) radio galaxies that are not catal oged members of Abell clusters. The environment of these radio galaxies is studied in both the X-ray and the optical by means of the ROSAT All-Sky Sur vey (RASS), ROSAT pointed observations, and the Palomar optical Digitized S ky Survey (DSS). X-ray luminosities and extents are determined from the RAS S, and the DSS is used to quantify the degree of clustering via the spatial two-point correlation coefficient, B-gg. Of the 25 sources, 20 are greater than or equal to 3 sigma detections in the X-ray and 11 possessed B-gg's s ignificantly in excess of that expected for an isolated galaxy. Adding the criterion that the X-ray emission be resolved, 10 of the radio galaxies do appear to reside in poor clusters with extended X-ray emission suggestive o f the presence of an intracluster medium. Eight of these galaxies also poss ess high spatial correlation coefficients. Taken together, these data sugge st that the radio galaxies reside in a low-richness extension of the Abell clusters. The unresolved X-ray emission from the other galaxies is most lik ely associated with active galactic nucleus phenomena. Furthermore, althoug h the sample size is small, it appears that the environments of FR I and FR II sources differ. FR I's tend to be more frequently associated with exten ded X-ray emission (10 of 18), whereas FR II's are typically point sources or nondetections in the X-ray (none of the seven sources exhibit extended X -ray emission).