We use SDSS-DR4 photometric and spectroscopic data out to redshift z \sim 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 \times 10 ^ { -6 } h _ { 75 } ^ { 3 } Mpc ^ { -3 } for L _ { X } > 0.89 \times 10 ^ { 42 } h _ { 75 } ^ { -2 } erg s ^ { -1 } consistent with Vikhlinin et al .
( 1999 ) , who examined an X-ray overluminous elliptical galaxy sample ( OLEG ) .
We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same dataset .
These two samples show no differences in the distribution of neighboring faint galaxy density excess , distance from the red sequence in the color-magnitude diagram , and structural parameters such as a _ { 4 } and internal color gradients .
Furthermore , examination of stellar populations shows that our twenty-five FGs have similar ages , metallicities , and \alpha -enhancement as the bright field ellipticals , undermining the idea that these systems represent fossils of a physical mechanism that occurred at high redshift .
Our study reveals no difference between FGs and field ellipticals , suggesting that FGs might not be a distinct family of true fossils , but rather the final stage of mass assembly in the Universe .