Zinchenko I. Chemical composition of active starforming galaxies based on the Sloan Survey data

The dissertation is devoted to the study of the oxygen and nitrogen abundances evolution in galaxies over the last ~ 4 Gyr based on SDSS spectra. One-dimensional N calibration, the relation between the nitrogen abundance and the strong nitrogen line intensities, proposed. The parametric OS calibration for estimation of the nitrogen and oxygen abundances has been proposed. The ON and NS calibrations for the oxygen and nitrogen abundances determination have been revised. It was shown that the revised ON and NS calibrations provide a more accurate determination of oxygen and nitrogen abundances from global spectra as compared with the original ON and NS calibrations. The oxygen and nitrogen abundances were estimated through different calibrations aiming test the validity of those abundances. Our data shows clear evidence for galaxy downsizing, where the sites of metal enrichment and hence of active star formation shift from high-mass galaxies at early cosmic times to lower-mass galaxies at later epochs. A significant time delay between the release of oxygen and that of nitrogen into the interstellar medium can be also seen from our data. With the aim of investigating galaxies with two strong simultaneous starbursts, we have extracted a sample of galaxies with double-peaked emission lines in their global spectra from the SDSS spectral database. Using a standard BPT classification diagram, we have been able to divide the galaxies from our sample into two subsamples: galaxies where both components belong to the photoionised class of objects, and galaxies which show non-thermal ionisation (AGNs).