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 histories. In this paper we present results based on modeling stacked spectra of early-type galaxies drawn from the Sloan Digital Sky Survey (SDSS) as a function of velocity dispersion, σ, from 90 km s −1 to 300 km s −1 . The spectra are of extremely high quality, with typical signal-to-noise ratio, S/N, of 1000 Å −1 , and a wavelength coverage of 4000Å−8800Å. Our population synthesis model includes variation in 16 elements from C to Ba, a two-component star formation history, the shift in effective temperature, ∆ T eff , of the stars with respect to a solar metallicity isochrone, and the stellar initial mass function (IMF), amongst other parameters. In our approach we fit the full optical spectra rather than a select number of spectral indices and are able to, for the first time, measure the abundances of the elements V, Cr, Mn, Co, and Ni from the integrated light of distant galaxies. Our main results are as follows: 1) lightweighted stellar ages range from 6 − 12 Gyr from low to high σ; 2) [Fe/H] varies by less than 0.1 dex across the entire sample; 3) Mg closely tracks O, and both increase from ≈ 0.0 at low σ to ∼ 0.25 at high σ; Si and Ti show a shallower rise with σ, and Ca tracks Fe rather than O; 4) the iron peak elements V, Cr, Mn, and Ni track Fe, while Co tracks O, suggesting that Co forms primarily in massive stars; 5) C and N track O over the full sample and [C/Fe] and [N/Fe] exceed 0.2 at high σ ; and 6) the variation in ∆ T eff with total metallicity closely follows theoretical predictions based on stellar evolution theory. This last result is significant because it implies that we are robustly solving not only for the detailed abundance patterns but also the detailed temperature distributions (i.e., isochrones) of the stars in these galaxies. A variety of tests reveal that the systematic uncertainties in our measurements are probably 0.05 dex or less. Our derived [Mg/Fe] and [O/Fe] abundance ratios are 0.05 − 0.1 dex lower than most previous determinations. Under the conventional interpretation that the variation in these ratios is due to star formation timescale variations, our results suggest longer star formation timescales for massive early-type galaxies than previous studies. Detailed chemical evolution models are necessary in order to translate the abundance ratio distributions of these galaxies into constraints on their formation histories. Alternatively, these data may provide useful constraints on the nucleosynthetic pathways for elements whose production is not well understood.

Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models

“…Although other α-elements are expected to follow magnesium there are exceptions to the rule such as calcium, which nearly tracks Fe (e.g., Vazdekis et al 1997;Worthey 1998;Saglia et al 2002;Cenarro et al 2003;Falcón-Barroso et al 2003a, Thomas, Maraston & Bender 2003bYamada et al 2006;Worthey, Ingermann & Serven 2011;Conroy & van Dokkum 2012b;Johansson, Thomas & Maraston 2012. These abundances and the abundance ratios of other elements can therefore be used to investigate the starformation timescales of ETGs (e.g., Conroy, Graves & van Dokkum 2014).…”

Element abundance ratios in stellar population modelling

“…A key result seen in all these studies consistently is that the light-element to Fe-peak abundance ratios [C/Fe] (Smith et al 2009;Johansson et al 2012;Conroy et al 2014), leading to significantly super-solar ratios in massive galaxies. The heavier elements Ca and Ti deviate from this trend.…”

“…More recently, a number of other element abundance sensitive models have been produced increasing the number of chemical elements considered, implementing other stellar libraries, and extending the approach to full spectral response (Annibali et al 2007;Schiavon 2007;Coelho et al 2007;Lee et al 2009;Worthey et al 2011;Conroy et al 2014).…”

“…Conroy et al (2014) follow the method of Thomas et al (2003a) in that they use response functions from model atmosphere calculations to assess the relative effect of element abundance variations, while the basic stellar population model is footed on empirical stellar libraries. Rather than using response functions for key absorption features, though, they calculate wavelength-dependent spectral response functions and apply those to the full model spectrum.…”

“…The abundance of oxygen cannot be derived directly from absorption features, but can be estimated through its role as dominating element among the light α-elements (Johansson et al 2012). Conroy et al (2014) recently extended this procedure to full spectral fits adding the element Si.…”