Consistent Gas-Phase Temperatures and Metallicities from UV and Optical Nebular Emission: A Reliable Foundation from z=0 to Cosmic Dawn

TL;DR

This study introduces a new method using HeII emission lines to accurately compare UV and optical nebular properties in galaxies, establishing a reliable foundation for high-redshift galaxy studies.

Contribution

A novel approach leveraging HeII lines for precise aperture and reddening corrections, enabling consistent UV and optical nebular temperature and metallicity measurements.

Findings

UV and optical electron temperatures agree within 0.1 dex for nearby galaxies.

Two galaxies show lower UV-based electron temperatures, challenging existing models.

The method improves the reliability of nebular property comparisons across wavelengths.

Abstract

The rest-frame UV spectra of star-forming galaxies are increasingly important as they become one of the primary windows to probe the physical properties of cosmic dawn (z>8) galaxies with the James Webb Space Telescope. However, the systematic discrepancies between UV and optical gas-phase metallicity measurements remain poorly understood in the local universe, partly due to challenges in achieving precise comparisons between UV and optical spectra for the same objects. In this work, we introduce a novel method that leverages the HeII 1640 and HeII 4686 nebular emission lines to achieve accurate aperture and reddening corrections between UV and optical spectra. Here we apply this method to three nearby Blue Compact Dwarf (BCD) galaxies. Our results demonstrate that this approach enables precise measurements, with electron temperatures ($T_e$) derived from UV and optical spectra exhibiting closer agreement compared to previous studies, and O/H abundance agreeing within 0.1 dex. However, two BCDs appear to have lower UV-based electron temperatures $T_{e~1666} < T_{e~4363}$, in contrast to expectations from the temperature fluctuation model. We consider a variety of possible explanations for these unphysical temperatures - differential dust attenuation, aperture differences, and spatial extent of emission lines - but no suitable cause is identified. These findings suggest a complex gaseous environment associated with star formation, and underscore the need for additional observations to further investigate the nature of HeII nebular emission and address the systematic issues between UV and optical nebular properties. Nonetheless, the close empirical agreement of these results indicates that UV- and optical-based nebular temperature and abundance measurements can be reliably compared within 0.1 dex, providing a solid foundation for evolutionary studies from the local Universe to cosmic dawn.