Enhanced Nitrogen in Morphologically Disturbed Blue Compact Galaxies at 0.20 < z < 0.35: Probing Galaxy Merging Features

TL;DR

This study reveals that morphologically disturbed blue compact galaxies at z=0.20-0.35 exhibit higher nitrogen-to-oxygen ratios and longer star formation histories, likely due to galaxy merging processes, challenging previous assumptions about nitrogen enrichment sources.

Contribution

It provides new evidence linking galaxy merging features with enhanced nitrogen enrichment and star formation history in blue compact galaxies at intermediate redshifts.

Findings

Disturbed BCGs show higher N/O ratios at given O/H compared to undisturbed ones.

Disturbed BCGs have lower H alpha to UV star formation rate ratios.

Longer star formation timescales are inferred for disturbed BCGs.

Abstract

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.