The stellar population histories of local early-type galaxies. I. Population parameters

This paper commences a series of investigations into the stellar population s of local elliptical galaxies as determined from their integrated spectra. The goal of the series is to determine the star formation and chemical evo lution histories of present-day elliptical galaxies. The primary galaxy sam ple analyzed is that of Gonzalez, which consists of 39 elliptical galaxies drawn primarily from the local field and nearby groups, plus the bulge of M essier 31. Single-burst stellar population (SSP)-equivalent ages, metallici ties, and abundance ratios are derived from H beta, Mg b, and [Fe] line str engths using an extension of the Worthey models that incorporates nonsolar line-strength "response functions" by Tripicco & Bell. These functions acco unt for changes in the Lick/IDS indices caused by nonsolar abundance ratios , allowing us to correct the Worthey models for the enhancements of Mg and other alpha-like elements relative to the Fe-peak elements. SSP-equivalent ages of the Gonzalez elliptical galaxies are found to vary w idely, 1.5 Gyr less than or similar to t less than or similar to 18 Gyr, wh ile metallicities [Z/H] and enhancement ratios [E/Fe] are strongly peaked a round [[Z/H]] = +0.26 and [[E/Fe]] = +0.20 (in an aperture of radius r(e)/8 ). The enhancement ratios [E/Fe] are milder than previous estimates because of the application of nonsolar abundance corrections to both Mg b and [Fe] for the first time. While [E/Fe] is usually greater than zero, it is not t he "E" elements that are actually enhanced but rather the Fe-peak elements that are depressed; this serves not only to weaken [Fe] but also to strengt hen Mg b, accounting for the overall generally mild enhancements. Based on index strengths from the Lick/IDS galaxy library (Trager et al.), C is not depressed with Fe but rather seems to be on a par with other elements such as Mg in the E group. Gradients in stellar populations within galaxies are found to be mild, with SSP-equivalent age increasing by 25%, metallicity de creasing by [[Z/H]] = 0.20 dex, and [E/Fe] remaining nearly constant out to an aperture of radius r(e)/2 for nearly all systems. Our ages have an overall zero-point uncertainty of at least similar to 25% because of uncertainties in the stellar evolution prescription, the oxygen abundance, the effect of [E/Fe] not equal 0 on the isochrones, and other un knowns. However, the relative age rankings of stellar populations should be largely unaffected by these errors. In particular, the large spread in age s appears to be real and cannot be explained by contamination of H beta by blue stragglers or hot horizontal-branch stars, or by fill-in of H beta by emission. Correlations between these derived SSP-equivalent parameters and other galaxy observables will be discussed in future papers.