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

Polar-ring galaxies are multi-spin 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 polar-ring galaxies (PRGs) from the Sloan Digital Sky Survey. Each object was fit with a S\'ersic host galaxy and a S\'ersic ring. Our general results are: (i) The central (host) galaxies of the PRGs are non-dwarf sub-$L^{\ast}$ galaxies with colors 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 $\sim 1^m$ to fainter magnitudes in comparison with E/S0 galaxies. It means that the PRGs hosts are more similar to quenched disks 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$\mu$m luminosity their star formation rate (SFR). Their SFR/M$_*$ ratio is larger than for the early-type galaxy sample of Atlas$^{\rm 3D}$, showing that the star forming disk brings a significant contribution to the new stars. Globally, PRGs appear frequently on the green valley in the mass-color diagram, revealing the symbiotic character between a red-sequence host and a blue cloud ring.