Large Binocular Telescope observations of new six compact star-forming galaxies with [NeV] 3426A emission

TL;DR

This study reports the discovery of [NeV]3426 emission in six compact star-forming galaxies, revealing high ionization states and suggesting the presence of shocks or active galactic nuclei, expanding understanding of ionization mechanisms in such galaxies.

Contribution

It presents new observations of six galaxies with [NeV] emission, highlighting the role of shocks and AGN activity in ionizing dense star-forming regions, which was not previously documented.

Findings

All galaxies show high ionization ratios and dense HII regions.

Most galaxies exhibit Wolf-Rayet features indicating young starbursts.

[NeV] emission suggests contributions from shocks or AGN activity.

Abstract

We report the discovery of [NeV]3426 emission, in addition to HeII4686 emission, in six compact star-forming galaxies. These observations considerably increase the sample of eight such galaxies discovered earlier by our group. For four of the new galaxies, the optical observations are supplemented by near-infrared spectra. All galaxies, but one, have HII regions that are dense, with electron number densities of ~300-700 cm-3. They are all characterised by high Hbeta equivalent widths EW(Hbeta)~190-520A and high O32=[OIII]5007/[OII]3727 ratios of 10-30, indicating young starburst ages and the presence of high ionization radiation. All are low-metallicity objects with 12+logO/H=7.46-7.88. The spectra of all galaxies show a low-intensity broad component of the Halpha line and five out of six objects show Wolf-Rayet features. Comparison with photoionization models shows that pure stellar ionization radiation from massive stars is not hard enough to produce such strong [NeV] and HeII emission in our galaxies. The [NeV]3426/HeII4686 flux ratio of ~1.2 in J1222+3602 is consistent with some contribution of active galactic nucleus ionizing radiation. However, in the remaining five galaxies, this ratio is considerably lower, <0.4. The most plausible models are likely to be non-uniform in density, where HeII and [NeV] lines are emitted in low-density channels made by outflows and illuminated by harder ionizing radiation from radiative shocks propagating through these channels, whereas [OIII] emission originates in denser regions exposed to softer stellar ionizing sources.