Fossil groups in the Sloan Digital Sky Survey

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' in all cases). A search for neighbors of the selected elliptical galaxies within a radius of 0: 5 h(70)(-1) Mpc was conducted, taking into account the r- band magnitudes and spectroscopic or photometric redshifts of all objects within this area, leading to a sample of 34 candidate fossil groups. Considering this sample, the estimated space density of fossil systems is n = (1.0 +/- 0.6) x 10(-6) h(50)(3) Mpc(-3).