EARLY-TYPE GALAXY ARCHEOLOGY: AGES, ABUNDANCE RATIOS, AND EFFECTIVE TEMPERATURES FROM FULL-SPECTRUM FITTING

Abstract

The stellar populations of galaxies hold vital clues to their formation\nhistories. In this paper we present results based on modeling stacked spectra\nof early-type galaxies drawn from the Sloan Digital Sky Survey (SDSS) as a\nfunction of velocity dispersion, sigma, from 90 km/s to 300 km/s. The spectra\nare of extremely high quality, with typical S/N of 1000/A, and a wavelength\ncoverage of 4000A-8800A. Our population synthesis model includes variation in\n16 elements from C to Ba, the shift in effective temperature, Delta(Teff), of\nthe stars with respect to a solar metallicity isochrone, amongst other\nparameters. In our approach we fit the full optical spectra rather than a\nselect number of spectral indices and are able to, for the first time, measure\nthe abundances of the elements V, Cr, Mn, Co, and Ni from the integrated light\nof distant galaxies. Our main results are as follows: 1) light-weighted stellar\nages range from 6-12 Gyr from low to high sigma; 2) [Fe/H] varies by less than\n0.1 dex across the entire sample; 3) Mg closely tracks O, and both increase\nfrom ~0.0 at low sigma to ~0.25 at high sigma; Si and Ti show a shallower rise\nwith sigma, and Ca tracks Fe rather than O; 4) the iron peak elements V, Cr,\nMn, and Ni track Fe, while Co tracks O, suggesting that Co forms primarily in\nmassive stars; 5) C and N track O over the full sample and [C/Fe] and [N/Fe]\nexceed 0.2 at high sigma; and 6) the variation in Delta(Teff) with total\nmetallicity follows theoretical predictions based on stellar evolution theory.\nOur derived [Mg/Fe] and [O/Fe] abundance ratios are 0.05-0.1 dex lower than\nmost previous determinations. Under the conventional interpretation that the\nvariation in these ratios is due to star formation timescale variations, our\nresults suggest longer star formation timescales for massive early-type\ngalaxies than previous studies. (ABRIDGED)\n