Hδ in the Integrated Light of Galaxies: What Are We Actually Measuring?

Abstract

We present a cautionary study exploring the reliability of the Hδ line in the integrated spectra of galaxies for determining galaxy ages. Our database consists of the observed integrated spectra of ~120 early-type galaxies, 7 metal-rich globular clusters in M31 and the Galactic globular cluster 47 Tuc, and the open cluster M67. We have measured Hδ using index definitions designed to assess contamination from the CN molecule in and around Hδ by choosing combinations of bandpasses that both avoid and include a region of CN molecular lines redward of Hδ. We find systematic differences in the ages derived from Hδ measurements among the various definitions when extracting ages from Hδ in old stellar populations with enhanced CN bands due to nonsolar abundance ratios. We propose that neighboring CN lines have a strong effect on pseudocontinuum and central bandpass levels. For stellar populations that have nonsolar abundance ratios in C and/or N, population synthesis models that do not account for abundance ratio variations cannot accurately reproduce the CN 4216 A band, which leads to a corresponding inaccuracy in reproducing the various Hδ indices. Hence, caution must be used when extracting galaxy ages from the Hδ line in old stellar populations with significant nonsolar abundance ratios.