Polar-ring galaxies: the SDSS view on the symbiotic galaxies

Polar-ring galaxies (PRGs) are multispin systems, showing star formation in a blue late-type component, perpendicular to a red early-type one, revealing how galaxy formation can sometimes occur in successive steps. We perform two-dimensional decomposition in the g, r, i bandpasses of 50 PRGs from the Sloan Digital Sky Survey. Each object was fit with a Sérsic host galaxy and a Sérsic ring. Our general results are: (i) the central (host) galaxies of the PRGs are non-dwarf sub-L* galaxies with colours typical for early-type galaxies. (ii) Polar structures in our sample are, on average, fainter and bluer than their host galaxies. (iii) In most galaxies, the stellar mass M_* of the polar component is not negligible in comparison with that of the host. (iv) The distributions of the host galaxies on the size - luminosity and Kormendy diagrams are shifted by ∼1 mag to fainter magnitudes in comparison with E/S0 galaxies. It means that the PRGs hosts are more similar to quenched discs than to ordinary early-type galaxies. (v) All the PRGs in our sample are detected in mid-infrared by WISE, and we derive from the 22 μm luminosity their star formation rate (SFR). Their SFR/M_* ratio is larger than for the early-type galaxy sample of Atlas^3D, showing that the star-forming disc brings a significant contribution to the new stars. Globally, PRGs appear frequently on the green valley in the mass-colour diagram, revealing the symbiotic character between a red-sequence host and a blue cloud ring.