Evolutionary stellar population synthesis with MILES - I. The base models and a new line index system

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Vazdekis, A.
Sánchez Blázquez, P.
Falcón Barroso, J.
Cenarro, A. J.
Beasley, M. A.
Peletier, R. F.
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We present synthetic spectral energy distributions (SEDs) for single-age, single-metallicity stellar populations (SSPs) covering the full optical spectral range at moderately high resolution [full width at half-maximum (FWHM) = 2.3 A]. These SEDs constitute our base models, as they combine scaled-solar isochrones with an empirical stellar spectral library [Medium resolution INT Library of Empirical Spectra (MILES)], which follows the chemical evolution pattern of the solar neighbourhood. The models rely as much as possible on empirical ingredients, not just on the stellar spectra, but also on extensive photometric libraries, which are used to determine the transformations from the theoretical parameters of the isochrones to observational quantities. The unprecedented stellar parameter coverage of the MILES stellar library allowed us to safely extend our optical SSP SED predictions from intermediate- to very-old-age regimes and the metallicity coverage of the SSPs from super-solar to [M/H] = -2.3. SSPs with such low metallicities are particularly useful for globular cluster studies. We have computed SSP SEDs for a suite of initial mass function shapes and slopes. We provide a quantitative analysis of the dependence of the synthesized SSP SEDs on the (in)complete coverage of the stellar parameter space in the input library that not only shows that our models are of higher quality than those of other works, but also in which range of SSP parameters our models are reliable. The SSP SEDs are a useful tool to perform the analysis of stellar populations in a very flexible manner. Observed spectra can be studied by means of full spectrum fitting or by using line indices. For the latter, we propose a new line index system to avoid the intrinsic uncertainties associated with the popular Lick/IDS system and provide more appropriate, uniform, spectral resolution. Apart from constant resolution as a function of wavelength, the system is also based on flux-calibrated spectra. Data can be analysed at three different resolutions: 5, 8.4 and 14 A (FWHM), which are appropriate for studying globular cluster, low- and intermediate-mass galaxies, and massive galaxies, respectively. Furthermore, we provide polynomials to transform current Lick/IDS line index measurements to the new system. We provide line index tables in the new system for various popular samples of Galactic globular clusters and galaxies. We apply the models to various stellar clusters and galaxies with high-quality spectra, for which independent studies are available, obtaining excellent results. Finally, we designed a web page from which not only these models and stellar libraries can be downloaded but which also provides a suite of on-line tools to facilitate the handling and transformation of the spectra.
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society © 2010 The Authors © 2010 RAS. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. We are indebted to the Padova group for making available their isochrone calculations. We are grateful to P. Coelho, J. L. Cervantes, I. G. de la Rosa, M. Koleva, E. Ricciardelli and J. González Hernández for very useful discussions. We would also like to thank J. A. Pérez Prieto for helping us in the construction of the web page for the models. We thank the referee for pointing out relevant aspects that were useful for improving the original manuscript. The MILES library was observed at the INT on the island of La Palma, operated by the Isaac Newton Group at the Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This research has made an extensive use of the SIMBAD data base ( operated at CDS, Strasbourg, France), the NASA's Astrophysics Data System Article Service and the Hipparcos Input Catalogue. PSB acknowledges the support of a Marie Curie Intra-European Fellowship within the 6th European Community Framework Programme. JFB acknowledges support from the Ramón y Cajal Program financed by the Spanish Ministry of Science and Innovation. AJC is a Ramón y Cajal Fellow of the Spanish Ministry of Science and Innovation. This work has been supported by the Programa Nacional de Astronomía y Astrofísica of the Spanish Ministry of Science and Innovation under grants AYA2007-67752-C03-01, AYA2007-67752-C03-02 and AYA2006-15698-C02-02.
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