An Empirical Calibration of the Helium Abundance in HII Regions based in Literature and CALIFA Survey data

TL;DR

This paper develops an empirical calibration for helium abundance in HII regions using emission line ratios, enabling large-scale studies of chemical evolution in galaxies and revealing trends consistent with inside-out galaxy formation models.

Contribution

It introduces a new empirical calibration for helium abundance based on emission line ratios, applied to a large galaxy sample from CALIFA, linking helium variation with oxygen abundance.

Findings

Helium abundance correlates with oxygen abundance in HII regions.

The calibration aligns with inside-out galaxy formation models.

Provides a tool for estimating helium abundance at kiloparsec scales.

Abstract

Helium is the second most common chemical species in the Universe. The study of helium abundance has the potential to unravel the chemical evolution of and within galaxies. In this study, we provide an empirical calibration for the singly ionized helium abundance: $12+\log_{10}({\rm He}^+/{\rm H}^+)$, based on the emission line flux ratio He$_{\lambda5876}$/H$\alpha$ from Galactic and extragalactic HII regions compiled from the literature. Based on this calibrator, we explore for the first time the helium abundance in a large sample of HII regions located in galaxies representative of the nearby Universe from the CALIFA survey. Furthermore, this calibrator allows us to explore the variations of the helium abundance with respect to the oxygen abundance. The observed trends are in agreement with a change in the chemical enrichment with mass/oxygen abundance similar to the one observed due to the inside-out model in a MW-galaxy (highlighting the connection between resolved and global trends in galaxies). Our calibrator provides an empirical proxy to estimate the helium abundance at kpc scales as well as to constrain chemical evolutionary models.