On the Nature of Fossil Galaxy Groups: Are they really fossils ?

Understand

We use SDSS-DR4 photometric and spectroscopic data out to redshift z~0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of twenty-five fossil groups (FGs), defined as bound systems dominated by a single, luminous elliptical galaxy with extended X-ray emission.

• We examine possible biases introduced by varying the parameters used to define the sample and the main pitfalls are discussed.
• The spatial density of FGs, estimated via the V/V_ MAX} test, is 2.83 x 10^{-6} h_{75}^3 Mpc^{-3} for L_x > 0.89 x 10^42 h_{75}^-2 erg/s consistent with Vikhlinin et al.
• (1999), who examined an X-ray overluminous elliptical galaxy sample (OLEG).