Fossil Groups in the Sloan Digital Sky Survey

Abstract

A search for fossil groups in the Sloan Digital Sky Survey was performed using virtual observatory tools. A cross-match of the positions of all SDSS luminous red galaxies (with r < 19 and measured spectroscopic redshifts) with sources in the ROSAT All-Sky Survey catalog resulted in a list of elliptical galaxies with extended X-ray emission (with a galaxy/ROSAT-source distance of less than 0.5 0 in all cases). A search for neighbors of the selected elliptical galaxies within a radius of 0:5 h À1 70 Mpc was conducted, taking into account the r-band magnitudes and spectroscopic or photo-metric redshifts of all objects within this area, leading to a sample of 34 candidate fossil groups. Considering this sam-ple, the estimated space density of fossil systems is n ¼ (1:0 AE 0:6) ; 10 À6 h 3 50 Mpc À3 .