HST UV Spectroscopy of the Dwarf Starburst Galaxy Pox 186

TL;DR

This study presents deep UV spectroscopy of the low-mass dwarf galaxy Pox 186, revealing its stellar and ionization properties, and compares these to other local starbursts to understand conditions relevant to reionization-era galaxies.

Contribution

It provides detailed UV spectral analysis of Pox 186, highlighting its neutral gas content, chemical abundances, and ionization features, offering insights into low-mass galaxy conditions during reionization.

Findings

Detection of neutral gas via Lyα and low-ionization absorption.

C/O ratio consistent with low-metallicity dwarf galaxies.

Excellent match between observed UV line ratios and photoionization models.

Abstract

Studying the galaxies responsible for reionization is often conducted through local reionization-era analogs; however, many of these local analogs are too massive to be representative of the low-mass star-forming galaxies that are thought to play a dominant role in reionization. The local, low-mass dwarf starburst galaxy Pox 186 is one such system with physical conditions representative of a reionization-era starburst galaxy. We present deep ultraviolet (UV) spectroscopy of Pox 186 to study its stellar population and ionization conditions and to compare these conditions to other local starburst galaxies. The new Cosmic Origins Spectrograph data are combined with archival observations to cover $\sim$1150-2000 A and allow for an assessment of Pox 186's stellar population, the relative enrichment of C and O, and the escape of ionizing photons. We detect significant Ly$\alpha$ and low-ionization state absorption features, indicative of previously undetected neutral gas in Pox 186. The C/O relative abundance, log(C/O) = -0.62$\pm$0.02, is consistent with other low-metallicity dwarf galaxies and suggests a comparable star formation history in these systems. We compare UV line ratios in Pox 186 to those of dwarf galaxies and photoionization models, and we find excellent agreement for the ratios utilizing the intense C III], O III], and double-peaked C IV lines. However, the UV and optical He II emission is faint and distinguishes Pox 186 from other local starburst dwarf galaxies. We explore mechanisms that could produce faint He II, which have implications for the low-mass reionization-era galaxies which may have similar ionization conditions.