Consistent gas-phase C/O abundances from UV and optical emission lines: a robust scale for chemical evolution across cosmic time

TL;DR

This paper demonstrates that UV and optical emission line methods for measuring C/O ratios in H II regions are consistent within uncertainties, providing a reliable scale for studying chemical evolution across cosmic time.

Contribution

It establishes that C/O abundance ratios derived from UV collisionally excited lines and optical recombination lines are consistent, confirming the robustness of these methods for galaxy evolution studies.

Findings

C/O ratios from UV and optical methods agree within 0.1 dex

A consistent abundance discrepancy factor (ADF) is found for C$^{2+}$ and O$^{2+}$

Results support using UV-based measurements for high-redshift galaxy evolution studies

Abstract

The carbon to oxygen (C/O) abundance ratio is a valuable tracer of star formation history, as C and O enrichment occurs on different timescales. However, measurements based on ultraviolet (UV) collisionally excited lines and those based on optical recombination lines may be subject to biases from the abundance discrepancy factor (ADF), which is well established for oxygen but uncertain for carbon. We present precise UV-based measurements of gas-phase C$^{2+}$/O$^{2+}$ ionic abundance in four H II regions which have prior optical-based measurements, combined with archival UV data for two additional H II regions, in order to establish a reliable abundance scale and to investigate biases between the two methods. We find a clear ADF for the C$^{2+}$ ion which is consistent with that of O$^{2+}$, assuming a similar temperature structure in the zones of the nebula which these ions occupy. The C/O abundance derived from UV collisional lines and optical recombination lines is therefore also consistent to within $<0.1$ dex, with an offset of $0.05\pm0.03$ dex in C$^{2+}$/O$^{2+}$ for the standard T$_e$ method. While the absolute C/H and O/H abundances are subject to large uncertainty from the ADF, our results establish that C/O abundances measured from these different methods can be reliably compared. Thus we confirm the robustness of gas-phase C/O measurements for studying galaxy evolution and star formation timescales, including from rest-UV observations of high redshift galaxies with JWST.