Candidate fossil groups in the CFHTLS: a probabilistic approach

Abstract

Context. Fossil groups (FGs) have been discovered 25 years ago, and are now defined as galaxy groups with an X-ray luminosity higher than $ 10^{42}\,h_{50}^{-2} $ erg s−1 and a brightest group galaxy brighter than the other group members by at least two magnitudes. However, the scenario of their formation remains controversial. Aims. We propose here a probabilistic analysis of FGs, extracted from the large catalog of candidate groups and clusters previously detected in the CFHTLS survey based on photometric redshifts to investigate their position in the cosmic web and probe their environment. Methods. Based on spectroscopic and photometric redshifts, we estimated the probability of galaxies to belong to a galaxy structure, and by imposing the condition that the brightest group galaxy is at least brighter than the others by two magnitudes, we computed the probability for a given galaxy structure to be a FG. We analyzed the mass distribution of these candidate FGs, and estimated their distance to the filaments and nodes of the cosmic web in which they are embedded. Results. We find that structures with masses lower than 2.4 × 1014 M⊙ have the highest probabilities of being fossil groups (PFG). Overall, structures with PFG ≥ 50% are located close to the cosmic web filaments (87% are located closer than 1 Mpc to their nearest filament). They are preferentially four times more distant from their nearest node than from their nearest filament. Conclusions. We confirm that FGs have low masses and are rare. They seem to reside closely to cosmic filaments and do not survive in nodes. Being in a poor environment might therefore be the driver of FG formation because the number of nearby galaxies is not sufficient to compensate for the cannibalism of the central group galaxy.