Chemical abundances in the polar disk of NGC4650A: Implications for cold accretion scenario

Abstract

The aim of the present study is to test whether the cold accretion of gas through a "cosmic filament" is a possible formation scenario for the polar disk galaxy NGC4650A. If polar disks form from cold accretion of gas, the abundances of the H II 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 109 L·. We use deep long-slit spectra obtained with the FORS2 spectrograph at the Very Large Telescope in the optical and near-infrared wavelength ranges for the brightest H II regions in the polar disk of NGC4650A. The strongest emission lines ([O II] Hβ, [O III], Hα) were used to derive oxygen abundances, metallicities, and the global star formation rates for the disk. The available deep spectra 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 electron temperature from the detection of weak auroral lines, as the [O III] at 4363 . 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 NGC4650A is Z = 0.2 Z ·, and it is lower than 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 NGC4650A and the flat metallicity gradient are both consistent with a latter infall of metal-poor gas, as expected in the cold accretion processes. © 2010. The American Astronomical Society. All rights reserved.