A UNIONS view of the brightest central galaxies of candidate fossil groups

The formation process of fossil groups (FGs) is still under debate, and large samples of such objects are still missing. The aim of this paper is to increase the sample of known FGs, and to analyse the properties of their brightest group galaxies (BGG) and compare them with a control sample of non-FG BGGs. Based on the Tinker spectroscopic catalogue of haloes and galaxies, we extract 87 FG and 100 non-FG candidates. For all the objects with data available in UNIONS in the u and r bands, and/or in an extra r-band processed to preserve all low surface brightness features (rLSB), we made a 2D photometric fit of the BGG with GALFIT with one or two Sersic components and analysed how the subtraction of intracluster light contribution modifies the BGG properties. From the SDSS spectra available for the BGGs of 65 FGs and 82 non-FGs, we extracted the properties of their stellar populations with Firefly. We also investigated the origin of the emission lines in a nearby FG, NGC 4104, that has an AGN. A single Sersic profile can fit most objects in the u band, while two Sersics are needed in the r and rLSB bands, both for FGs and non-FGs. Non-FG BGGs cover a larger range of Sersic index. FG BGGs follow the Kormendy relation derived for almost one thousand brightest cluster galaxies (BCGs) by Chu et al. (2022) while non-FGs BGGs are mostly located below this relation, suggesting that FG BGGs have evolved similarly to BCGs, while non-FG BGGs have evolved differently. The above properties can be strongly modified by the subtraction of intracluster light contribution. The stellar populations of FG and non-FG BGGs do not differ significantly. Our results suggest FG and non-FG BGGs have had different formation histories, but it is not possible to trace differences in their stellar populations or large scale distributions.