Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Northern Local Star-forming Dwarf Galaxies. Analogues of the First Galaxies and Probes of the Cosmic Metallicity Scale

TL;DR

The paper argues that deep spectroscopy of local SFDGs is essential to anchor the metallicity scale across cosmic time by measuring faint RLs and CELs to map $T_{ m e}$ and $n_{ m e}$ structure and to quantify the abundance discrepancy factor. It advocates a tenfold gain in photon-collecting power with a $30$-m telescope on La Palma, equipped with AO-fed high-throughput spectrographs to detect RLs (e.g., O II multiplet 1 near 4650 Å, C II 4267) and faint CELs (e.g., [N II] 5755, [Cl III] 5518, 5538, [Fe III] 4702) in EMPGs and H II regions. It further states that such data would yield empirical RL-based calibrations for strong-line indicators across wide ionization ranges, enabling robust recalibration of metallicity scales and improved interpretation of JWST/ELT surveys and the mass–metallicity relation. It would also illuminate the origin of high-ionization lines (He II, [Ne V], C IV) and the possible presence of Pop III-like ionizing sources in metal-poor galaxies, thereby informing reionization-era models.

Abstract

Star-forming dwarf galaxies in the local Universe, especially extremely metal-poor ones, can be considered analogous to early galaxies of the Epoch of Reionization (z >= 6). Currently available telescopes cannot adequately detect and measure heavy element recombination lines and certain faint collisionally excited lines, which are essential for exploring the effects and biases that potential inhomogeneities in electron temperature and density of the ionized gas may have on determining the chemical composition of these galaxies. On the other hand, the origin of very high-ionization lines (e.g. He II, [Ne V], C IV]) measured in the spectra of an important fraction of these objects remains unknown and a challenge to current stellar models, suggesting the presence of Population III-like stars and/or the existence of non-conventional ionizing sources. Obtaining very deep spectra for a selected sample of local star-forming dwarf galaxies would provide unprecedented constraints on their nature, ionization and true chemical abundances, and could change the metallicity scale we assume to understand the chemical evolution of galaxies over cosmic time.