Chemical abundances in the dwarf galaxy NGC 4163 based on the nebular and auroral emission lines

TL;DR

This study maps the chemical abundances in dwarf galaxy NGC 4163 using nebular and auroral emission lines, revealing low metallicity, unusual N/O ratios, and insights into its evolutionary history.

Contribution

First direct measurement of chemical abundances in NGC 4163 using auroral lines, highlighting the galaxy's low metallicity and unusual N/O trend.

Findings

Oxygen abundance varies from 7.3 to 7.8 in the galaxy.

N/O ratio is about -1.5, slightly higher than in nearby galaxy HII regions.

Negative correlation between N/O and oxygen abundance detected.

Abstract

We constructed an oxygen abundance map and N/O ratio map of the unusually low excitation dwarf irregular galaxy NGC 4163 based on publicly available spectroscopy obtained by the MaNGA survey. We detected auroral emission line [OII]$\lambda\lambda$7320,7330 which allows us to measure chemical abundance by direct T$_e$ method. We found that the scatter of the oxygen abundance derived by the strong line method is large. The oxygen abundances 12 + log(O/H) derived by strong line method vary from ~7.3 to ~7.8 with a mean value of ~7.55. The oxygen abundances derived in two apertures of 2 arcseconds by the direct T$_e$ method using our measurements of the O$^+$ auroral line is about 7.8 dex. The nitrogen-to-oxygen ratio log(N/O) of about -1.5 is typical value for a low metallicity galaxy, maybe slightly shifted towards higher N/O ratios with respect to the N/O values in the HII regions in nearby galaxies. An unusual negative trend between log(N/O) and oxygen abundance is detected. NGC 4163 is a gas-poor galaxy with a neutral atomic gas mass fraction of around 0.25. The oxygen abundance in the galaxy is only around 0.1 of the oxygen abundance potentially attainable in a galaxy with such a gas mass fraction. The low metallicity coupled with the low gas mass fraction implies that either the metallicity of the interstellar medium of the galaxy was reduced by pristine gas infall in the recent epoch or the evolution of this galaxy was accompanied by strong galactic winds.