H-delta in the Integrated Light of Galaxies: What Are We Actually Measuring?

Details H-delta in the Integrated Light of Galaxies: What Are We Actually Measuring? H-delta in the Integrated Light of Galaxies: What Are We Actually Measuring?

2007-02-21

arxiv.org/abs/astro-ph/0702589v1

Astron.J.134:321-333,2007

Astronomy and Astrophysics

Astrophysics

8 figures, 2 tables

Scientific paper

10.1086/516754

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

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