# The Chemical Evolution of Carbon, Nitrogen, and Oxygen in Metal-Poor Dwarf Galaxies

## TL;DR

This study uses UV spectroscopy of low-metallicity dwarf galaxies to analyze the chemical evolution of carbon, nitrogen, and oxygen, revealing constant abundance ratios and the influence of star formation history and feedback on C/O ratios.

## Contribution

It provides new analytic relationships for ionization correction factors and models the chemical evolution, highlighting the sensitivity of C/O ratios to star formation and feedback processes.

## Key findings

- C/O ratio is flat with O/H, with average log(C/O) = -0.71
- C/N ratio remains constant at log(C/N) = 0.75 with scatter
- C/O ratios vary with star formation history and supernova feedback

## Abstract

Ultraviolet nebular emission lines are important for understanding the time evolution and nucleosynthetic origins of their associated elements, but the underlying trends of their relative abundances are unclear. We present UV spectroscopy of 20 nearby low-metallicity, high-ionization dwarf galaxies obtained using the Hubble Space Telescope. Building upon previous studies, we analyze the C/O relationship for a combined sample of 40 galaxies with significant detections of the UV O+2/C+2 collisionally-excited lines and direct-method oxygen abundance measurements. Using new analytic carbon ionization correction factor relationships, we confirm the flat trend in C/O versus O/H observed for local metal-poor galaxies. We find an average log(C/O) = -0.71 with an intrinsic dispersion of {\sigma} = 0.17 dex. The C/N ratio also appears to be constant at log(C/N) = 0.75, plus significant scatter ({\sigma} = 0.20 dex), with the result that carbon and nitrogen show similar evolutionary trends. This large and real scatter in C/O over a large range in O/H implies that measuring the UV C and O emission lines alone does not provide a reliable indicator of the O/H abundance. By modeling the chemical evolution of C, N, and O of individual targets, we find that the C/O ratio is very sensitive to both the detailed star formation history and to supernova feedback. Longer burst durations and lower star formation efficiencies correspond to low C/O ratios, while the escape of oxygen atoms in supernovae winds produces decreased effective oxygen yields and larger C/O ratios. Further, a declining C/O relationship is seen with increasing baryonic mass due to increasing effective oxygen yields.