Chemical abundances in the polar disk of NGC4650A: implications for cold
  accretion scenario
  The aim of the present study is to test whether the cold accretion of gas
through a "cosmic filament" Macci\`o et al. 2006 is a possible formation
scenario for the polar disk galaxy NGC 4650A. If polar disks form from cold
accretion of gas, the abundances of the HII regions may be similar to those of
very late-type spiral galaxies, regardless of the presence of a bright central
stellar spheroid, with total luminosity of few 10^9 Lsun. We use deep long slit
spectra obtained with the FORS2 spectrograph at the VLT in the optical and
near-infrared wavelength ranges for the brightest HII regions in the disk polar
disk of NGC 4650A. The strongest emission lines ([OII] Hbeta, [OIII], Halpha)
were used to derived oxygen abundances, metallicities and the global star
formation rates for the disk. The deep spectra available allowed us to measure
the Oxygen abundances (12 + log (O/H)) using the "Empirical method" based on
intensities of the strongest emission lines, and the "Direct method", based on
the determination of the electron temperature from the detection of weak
auroral lines, as the [OIII] at 4363 Angstrom. The Oxygen abundance measured
for the polar disk is then compared with those measured for different galaxy
types of similar total luminosities, and then compared against the predictions
of different polar ring formation scenarios. The average metallicity values for
the polar disk in NGC 4650A is Z=0.2 Zsun, and it is lower that the values
measured for ordinary spirals of similar luminosity. Moreover the gradient of
the metallicity is flat along the polar disk major axis, which implies none or
negligible metal enrichment from the stars in the older central spheroid. The
low metallicity value in the polar disk NGC 4650A and the flat metallicity
gradient are both consistent with a later infall of metal-poor gas, as expected
in the cold accretion processes.
