Chemical abundances of the PRGs UGC7576 and UGC9796. I. Testing the
  formation scenario
  The study of the chemical abundances of HII regions in polar ring galaxies
and their implications for the evolutionary scenario of these systems has been
a step forward both in tracing the formation history of the galaxy and giving
hints on the mechanisms at work during the building of disk by cold accretion
process. It's now important to establish whether such results are typical for
the class of polar disk galaxies as whole. The present work aims at checking
the cold accretion of gas through a "cosmic filament" as a possible scenario
for the formation of the polar structures in UGC7576 and UGC9796. If these form
by cold accretion, we expect the HII regions abundances and metallicities to be
lower than those of same-luminosity spiral disks, with values of the order of Z
~ 1/10 Zsun, as predicted by cosmological simulations. We have used deep
long-slit spectra, obtained with DOLORES@TNG in the optical wavelengths, of the
brightest HII regions associated with the polar structures to derive their
chemical abundances and star formation rate. We used the "Empirical methods",
based on the intensities of easily observable lines, to derive the oxygen
abundance 12+log(O/H) of both galaxies. Such values are compared with those
typical for different morphological galaxy types of comparable luminosity. The
average metallicity values for UGC7576 and UGC9796 are Z = 0.4 Zsun and Z = 0.1
Zsun respectively. Both values are lower than those measured for ordinary
spirals of similar luminosity and UGC7576 presents no metallicity gradient
along the polar structure. These data, toghether with other observed features,
available for the two PRGs in previous works, are compared with the predictions
of simulations of tidal accretion, cold accretion and merging, to disentangle
between these scenarios.
