Chemical composition and kinematics of ionised gas in low-mass star-forming galaxies with extremely high [OIII]/[OII] ratios

TL;DR

This study analyzes the chemical composition and gas kinematics in low-mass, high [OIII]/[OII] ratio star-forming galaxies, revealing insights into their ionized gas dynamics and potential for Lyman continuum leakage.

Contribution

First detailed spectroscopic analysis of low-mass galaxies with extremely high emission-line ratios, linking gas kinematics to stellar mass and ionization conditions.

Findings

Higher velocity dispersions in Hbeta with increasing stellar mass.

HeII 4686 and HeI 10830 lines broadened by stellar processes.

Outer HII regions show narrower emission lines.

Abstract

We present Very Large Telescope/Xshooter spectrophotometric observations of eleven low-redshift (z<0.085) compact star-forming galaxies (`high O32 sample'). These galaxies are characterised by extremely high emission-line ratios [OIII]$\lambda$5007/[OII]3727, ranging from 11 to 42. Galaxies with such high ratios are thought to be promising candidates for leaking large amounts of Lyman continuum radiation. They are characterized by low oxygen abundances 12+log(O/H)\,=7.5-8.0 and low stellar masses M*~10^6-10^8 Msun. Strong emission lines of various ions in all spectra are used to derive helium and oxygen abundances, and N/O, Ne/O, S/O, Cl/O, Ar/O and Fe/O abundance ratios. We also derived macroscopic velocity dispersions sigma(lambda) from various emission lines of different ions. We find that sigma(4861) of the Hbeta emission line is increased with increasing stellar mass and decreasing O32 ratio. On the other hand, sigma(lambda)/sigma(4861) ratios for various lines are close to 1. Exceptions are sigma(lambda)/sigma(4861) of two lines, HeII 4686 and HeI 10830, which are considerably higher than unity and of four lines, [OII] 3726,3729, [SII] 6717,6731, with sigma(lambda)/sigma(4861) lower than unity. The two former lines are likely produced in the inner parts of HII regions and are broadened by dynamical processes generated by massive stars, and by radiative scattering in the case of the HeI 10830 emission line. Emission in the four latter lines is produced mainly in the outer and likely more quiet parts of HII regions.