Population Synthesis in the Blue Iv . Accurate Model Predictions for Lick Indices and Ubv Colors in Single Stellar Populations

Content Provider	Semantic Scholar
Author	Schiavon, Ricardo P.
Copyright Year	2006
Abstract	We present a new set of model predictions for 16 Lick absorption line indices from Hδ through Fe5335, and UBV colors for single stellar populations with ages ranging between 1 and 15 Gyr, and [Fe/H] ranging from –1.3 to +0.3, and variable abundance ratios. The models are based on accurate stellar parameters for the Jones library stars and a new set of fitting functions describing the behavior of line indices as a function of effective temperature, surface gravity, and iron abundance. The abundances of several key elements in the library stars have been obtained from the literature in order to characterize the abundance pattern of the stellar library, thus allowing us to produce model predictions for any set of abundance ratios desired. We develop a method to estimate mean ages and abundances of iron, carbon, nitrogen, magnesium and calcium that explores the sensitivity of the various indices modeled to those parameters. The models are compared to high S/N data for Galactic clusters spanning the range of ages, metallicities and abundance pattern of interest. Essentially all line indices are matched when the known cluster parameters are adopted as input. Cluster spectroscopic ages determined from different Balmer line indices are consistent to within ∼ 1 Gyr. The models can predict confidently the above elemental abundances to within ± 0.1 dex and ages to within ± 1 (0.5) Gyr for old (intermediate-age) stellar populations. Comparing the models to high-quality data for galaxies in the nearby universe, we reproduce previous results regarding the enhancement of light elements and the spread in the mean luminosity-weighted ages of early-type galaxies. When the results from the analysis of blue and red indices are contrasted, we find good consistency in the [Fe/H] that is inferred from different Fe indices. Applying our method to stacked SDSS spectra of early-type galaxies brighter than L, we find mean luminosity-weighted ages of the order of ∼ 8 Gyr and iron abundances slightly below solar. Abundance ratios, [X/Fe], tend to be higher than solar, and are positively correlated with galaxy luminosity. Of all elements, nitrogen is the more strongly correlated with galaxy luminosity, which seems to indicate secondary nitrogen enrichment. If that interpretation is correct, this result may impose a lower limit of 50-200 Myr to the timescale of star formation in early-type galaxies. Unlike clusters, galaxies show a systematic effect whereby higherorder, bluer, Balmer lines yield younger ages than Hβ. This age discrepancy is stronger for lower luminosity galaxies. We examine four possible scenarios to explain this trend. Contamination of the bluer indices by a metal-poor stellar population with a blue horizontal branch cannot account for the data. Blue stragglers and abundance-ratio effects cannot be ruled out, as they can potentially satisfy the data, even though this can only be achieved by resorting to extreme conditions, such as extremely high [O/Fe] or specific blue-straggler frequencies. The most likely explanation is the presence of small amounts of a young/intermediate-age stellar population component. We simulate this effect by producing two-component models and show that they provide a reasonably good match to the data when the mass fraction of the young component is typically a few %. If confirmed, this result implies star formation has been extended in early-type galaxies, and more so in less massive galaxies, which seems to lend support to the “downsizing” scenario. Moreover, it implies that stellar population synthesis models are capable of constraining not only the mean ages of stellar populations in galaxies, but also their age spread. Subject headings: galaxies: abundances – galaxies: evolution – galaxies: elliptical and lenticular, cD – galaxies: stellar content – Galaxy: globular clusters – stars: fundamental parameters
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Alternate Webpage(s)	http://cds.cern.ch/record/1003798/files/0611464.pdf
Alternate Webpage(s)	http://arxiv.org/pdf/astro-ph/0611464v1.pdf
Language	English
Access Restriction	Open
Subject Keyword	Calcium Color Computer cluster Dex Dietary Iron Discrepancy function Elemental Elements Entity Name Part Qualifier - adopted File spanning GUCY2C protein, human Galaxy Gene Ontology Term Enrichment Inference Jones calculus Lenticular printing Magnesium Mass fraction Medical Subject Headings Notepad2 Population Simulation Spatial decision support system Star Wars Galaxies Stars, Celestial Stellar (payment network)
Content Type	Text
Resource Type	Article