First ranked galaxies of non-elliptical morphology

In the local Universe, the major morphology for first ranked galaxies (FRGs) is elliptical. However, FRGs might not be limited to elliptical or central dominant galaxies. We select galaxies from galaxy group/cluster catalogues based on the Sloan Digital Sky Survey Data Release 10, and we find that more than 35 per cent of FRG morphologies are classified as unclear or even spiral. Spiral FRGs (spFRGs) have several distinctive features. The stellar mass of spFRGs is as massive as the majority of elliptical galaxies and their luminosity is between the elliptical FRGs (elFRGs) and their elliptical member galaxies. There are also non-negligible differences in star formation rate and colour between (both field and member) spirals and spFRGs, and spFRGs show a relatively young formation age compared to elFRGs. In comparison to elFRG hosts, spFRG hosts are poorer and less massive, and spFRGs have large offsets from the luminosity-weighted group/cluster centre. spFRG also has large seperaration between spFRGs and the closest members with r-band absolute magnitude is ≤−19 (mag + 5 log₁₀h). From the analysis, we propose a possible formation scenario for spFRGs. The galaxy number density around spFRGs and the mass ratio between spFRGs and their closest members rule out the possibility of a formation scenario from the galaxy-galaxy minor interaction. This implies that a galaxy has the potential to become the brightest group galaxy or a low-mass brightest cluster galaxy without galaxy-galaxy interactions but instead growing mass by gentle processes. We suggest that the halo cold gas accretion mechanism is plausible for spFRGs to assemble their mass.