# Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST

## TL;DR

This study uses ultraviolet spectra of nearby metal-poor star-forming galaxies to understand stellar populations and nebular emissions, providing insights relevant for interpreting high-redshift galaxy observations with JWST.

## Contribution

It presents new UV spectral data of nearby dwarf galaxies, revealing how decreasing metallicity enhances nebular emission and ionizing photon production beyond standard models.

## Key findings

- Metal-poor stellar populations can produce extreme high-ionization UV lines.
- A transition in UV spectral features occurs below 12+log(O/H) ≈ 8.0.
- Ionizing photon production increases significantly at low metallicity.

## Abstract

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below $Z_\odot/2$. Such tests are particularly important for interpreting the surprising high-ionization UV line emission detected at $z>6$ in recent years. We present HST/COS ultraviolet spectra of ten nearby metal-poor star-forming galaxies selected to show He II emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance ($7.8<12+\log\mathrm{O/H}<8.5$) and present uniformly large specific star formation rates (sSFR $\sim 10^2$ $\mathrm{Gyr}^{-1}$). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C III] equivalent widths comparable to those seen in systems at $z\sim 6-7$. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below $12+\log\mathrm{O/H}\lesssim 8.0$ ($Z/Z_\odot \lesssim 1/5$) presenting minimal stellar wind features and prominent nebular emission in He II and C IV. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the $\mathrm{He^+}$-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C IV and He II powered by these stars may provide useful metallicity constraints in the reionization era.