Keck HIRES Spectroscopy of Extragalactic HII Regions: C and O Abundances from Recombination Lines

Abstract

We present very deep spectrophotometry of 14 bright extragalactic HII regions belonging to spiral, irregular, and blue compact galaxies. The data for 13 objects were taken with the HIRES echelle spectrograph on the Keck I telescope. We have measured CII recombination lines in 10 of the objects and OII recombination lines in 8 of them. We have determined electron temperatures from line ratios of several ions, specially of low ionization potential ones. We have found a rather tight linear empirical relation between Te([NII]) and Te([OIII]). We have found that OII lines give always larger abundances than [OIII] lines. Moreover, the difference of both O++ abundance determinations --the so-called abundance discrepancy factor-- is very similar in all the objects, with a mean value of 0.26+/-0.09 dex, independently of the properties of the HII region and of the parent galaxy. Using the observed recombination lines, we have determined the O, C, and C/O radial abundance gradients for 3 spiral galaxies: M33, M101, and NGC2403, finding that C abundance gradients are always steeper than those of O, producing negative C/O gradients accross the galactic disks. This result is similar to that found in the Milky Way and has important implications for chemical evolution models and the nucleosynthesis of C.

Accepted for publication in ApJ, 54 pages, 4 figures

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