Metallicity and ionization state of the gas in polar-ring galaxies
  The ionization state and oxygen abundance distribution in a sample of
polar-ring galaxies (PRGs) were studied from the long-slit spectroscopic
observations carried out with the SCORPIO-2 focal reducer at the Russian 6-m
telescope. The sample consists of 15 PRGs classified as `the best candidates'
in the SDSS-based Polar Ring Catalogue. The distributions of line-of-sight
velocities of stellar and gaseous components have given kinematic confirmation
of polar structures in 13 galaxies in the sample. We show that ionization by
young stars dominates in the external parts of polar discs, while shocks have a
significant contribution to gas excitation in the inner parts of polar
structures. This picture was predicted earlier in a toy model implying the
collision between gaseous clouds on polar orbits with the stellar disc
gravitational potential well. The exception is a moderately inclined ring to
the host galaxy NGC 5014: the accreted gas in the centre has already settled on
the main plane and ionized by young stars, while the gas in the internal part
of the ring is excited by shocks. The present study three times increases the
number of polar structures with an available oxygen abundance estimation. The
measured values of the gas metallicity almost do not depend on the galaxy
luminosity. The radial [O/H] gradient in the considered polar rings is shallow
or absent. No metal-poor gas was detected. We ruled out the scenario of the
formation of polar rings due to cold accretion from cosmic filaments for the
considered sample of PRGs.
