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