Enhanced Nitrogen in Morphologically Disturbed Blue Compact Galaxies at
  0.20 < z < 0.35: Probing Galaxy Merging Features
  We present a study of correlations between the elemental abundances and
galaxy morphologies of 91 blue compact galaxies (BCGs) at z=0.20-0.35 with
Sloan Digital Sky Survey (SDSS) DR7 data. We classify the morphologies of the
galaxies as either 'disturbed' or 'undisturbed', by visual inspection of the
SDSS images, and using the Gini coefficient and M_20. We derive oxygen and
nitrogen abundances using the T_e method. We find that a substantial fraction
of BCGs with disturbed morphologies, indicative of merger remnants, show
relatively high N/O and low O/H abundance ratios. The majority of the disturbed
BCGs exhibit higher N/O values at a given O/H value compared to the
morphologically undisturbed galaxies, implying more efficient nitrogen
enrichment in disturbed BCGs. We detect Wolf-Rayet (WR) features in only a
handful of the disturbed BCGs, which appears to contradict the idea that WR
stars are responsible for high nitrogen abundance. Combining these results with
Galaxy Evolution Explorer (GALEX) GR6 ultraviolet (UV) data, we find that the
majority of the disturbed BCGs show systematically lower values of the H alpha
to near-UV star formation rate ratio. The equivalent width of the H beta
emission line is also systematically lower in the disturbed BCGs. Based on
these results, we infer that disturbed BCGs have undergone star formation over
relatively longer time scales, resulting in a more continuous enrichment of
nitrogen. We suggest that this correlation between morphology and chemical
abundances in BCGs is due to a difference in their recent star formation
histories.
