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Exploring the Star Formation History of Elliptical Galaxies Rogers, Ben; Ferreras, Ignacio; Peletier, Reynier F.; Silk, Joseph

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Exploring the Star Formation History of Elliptical Galaxies Rogers, Ben; Ferreras, Ignacio; Peletier, Reynier F.; Silk, Joseph University of Groningen Exploring the Star Formation History of Elliptical Galaxies Rogers, Ben; Ferreras, Ignacio; Peletier, Reynier F.; Silk, Joseph Published in: Monthly Notices of the Royal Astronomical Society DOI: 10.1111/j.1365-2966.2009.15892.x IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2010 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Rogers, B., Ferreras, I., Peletier, R. F., & Silk, J. (2010). Exploring the Star Formation History of Elliptical Galaxies: Beyond Simple Stellar Populations with a New Estimator of Line Strengths. Monthly Notices of the Royal Astronomical Society, 402(1), 447-460. https://doi.org/10.1111/j.1365-2966.2009.15892.x Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license. More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne- amendment. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 04-10-2021 Mon. Not. R. Astron. Soc. 402, 447–460 (2010) doi:10.1111/j.1365-2966.2009.15892.x Exploring the star formation history of elliptical galaxies: beyond simple stellar populations with a new line strength estimator Ben Rogers,1 Ignacio Ferreras,1,2 Reynier Peletier3 and Joseph Silk4 Downloaded from https://academic.oup.com/mnras/article-abstract/402/1/447/1033534 by University of Groningen user on 22 January 2020 1Department of Physics, King’s College London, Strand, London WC2R 6LS 2Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT 3Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, the Netherlands 4Physics Department, The Denys Wilkinson Building, Keble Road, Oxford OX1 3RH Accepted 2009 October 16. Received 2009 October 13; in original form 2008 December 10 ABSTRACT We study the stellar populations of a sample of 14 elliptical galaxies in the Virgo cluster. Using spectra with high signal-to-noise ratio (S/N 100 Å−1) we propose an alternative approach to the standard side-band method to measure equivalent widths (EWs). Our boosted median continuum is shown to map the EWs more robustly than the side-band method, minimizing the effect from neighbouring absorption lines and reducing the uncertainty at a given S/N. Our newly defined line strengths are more successful at disentangling the age–metallicity degeneracy. We concentrate on Balmer lines (Hβ,Hγ ,Hδ), the G band and the combination [MgFe] as the main age and metallicity indicators. We go beyond the standard comparison of the observations with simple stellar populations (SSPs) and consider four different models to describe the star formation histories, either with a continuous star formation rate or with a mixture of two different SSPs. These models improve the estimates of the more physically meaningful mass-weighted ages. Composite models are found to give more consistent fits among individual line strengths and agree with an independent estimate using the spectral energy distribution. A combination of age- and metallicity-sensitive spectral features allows us to constrain the average age and metallicity. For a Virgo sample of elliptical galaxies our age and metallicity estimates correlate well with stellar mass or velocity dispersion, with a significant threshold around 5 × 1010 M above which galaxies are uniformly old and metal rich. This threshold is reminiscent of the one found by Kauffmann et al. in the general population of Sloan Digital Sky Survey galaxies at a stellar mass 3 × 1010 M.Inamore speculative way, our models suggest that it is formation epoch and not formation time-scale what drives the mass–age relationship of elliptical galaxies. Key words: galaxies: elliptical and lenticular, cD – galaxies: evolution – galaxies: formation – galaxies: stellar content. et al. 2005; Kaviraj et al. 2007) reveals a complex history of star 1 INTRODUCTION formation that requires proper estimates of mass-weighted ages, in Unveiling the star formation histories (SFHs) of elliptical galaxies is contrast with the luminosity-weighted ages that simple stellar pop- key to our understanding of galaxy formation. Being able to resolve ulations (SSPs) can only achieve. The majority of papers dealing their seemingly homogenous distribution is hampered by the fact with age estimates of the stellar populations of elliptical galaxies that their light is dominated by old, i.e. low-mass stars, which do rely on such SSPs (see e.g. Kuntschner & Davies 1998; Trager et al. not evolve significantly even over cosmological times. Furthermore, 2000; Thomas et al. 2005), and it is only recently that special em- the presence of small amounts of young stars as recently discov- phasis has been made on the need to go beyond simple populations ered in near-ultraviolet (NUV) studies (Ferreras & Silk 2000; Yi (Ferreras & Yi 2004; Idiart, Silk & de Freitas Pacheco 2007; Serra & Trager 2007). Dating the (old) stellar populations of elliptical galaxies has been fraught with difficulties, the most prominent being the age– E-mail: [email protected] metallicity degeneracy, whereby the photospectroscopic properties C 2009 The Authors. Journal compilation C 2009 RAS 448 B. Rogers et al. of a galaxy of a given age and metallicity can be replicated by 2THESAMPLE a younger or older galaxy at a suitably higher or lower metallic- ity, respectively. To some extent, this problem has been overcome We use a sample of 14 elliptical galaxies in the Virgo cluster, for by the measurement of pairs of absorption line indices (Worthey which moderate-resolution spectroscopy is available at high signal- 1994), one whose change in equivalent width (EW) is dominated to-noise ratio (S/N 100 Å−1; Yamada et al. 2006, 2008). Eight by the average metallicity of the population and the other dominated galaxies were observed with Faint Object Camera and Spectro- by the average age of the population. Typical metal-sensitive line graph (FOCAS) at the 8-m Subaru Telescope; the other six were strengths are the Mg feature at 5170 Å, the iron lines around 5300 Å observed with Intermediate dispersion Spectrograph and Imaging or a combination such as [MgFe] (Gonzalez´ 1993). Balmer lines System (ISIS) at the 4.2-m William Herschel Telescope (WHT). Downloaded from https://academic.oup.com/mnras/article-abstract/402/1/447/1033534 by University of Groningen user on 22 January 2020 are more age sensitive and are often combined with metal-sensitive Observations from Subaru span the spectral range λ 3800– lines to break the degeneracy. However, measurements of EWs of 5800 Å, whereas the spectra taken at the WHT span a narrower Balmer lines can be affected by the age–metallicity degeneracy be- window, namely λ 4000–5500 Å. The resolution [full width at cause of the presence of nearby absorption lines. Such is the case half-maximum (FWHM)] of both data sets is similar: 2 Å (Subaru) of Hγ , with the prominent G band at 4300 Å or the CN bands in and 2.4 Å (WHT). We refer the interested reader to Yamada et al. the vicinity of the Hδ line. This paper is partly motivated by the (2006) for details about the data reduction process. We compare need to define a method to estimate EWs that minimize the sen- those spectra with composites of the R ∼ 2000 synthetic models sitivity of metallicity on Balmer lines by a proper estimate of the of Bruzual & Charlot (2003), updated to the 2007 version (Bruzual continuum. 2007). We resampled the observed spectra from the original 0.3 Even at relatively younger ages (a few Gyr) where such uncer- to 1 Å pixel−1, performing an average of the spectra over a 1.5 Å tainties are reduced, considerable degeneracies still remain. The window, in order to have a sampling more consistent with the actual confirmation of recent star formation (RSF) occurring in early-type resolution. galaxies (Yi et al. 2005; Kaviraj et al. 2007) has raised the problem In this paper we use two alternative sets of information, either the that the existence of a young population can considerably affect the full spectral energy distribution (SED) or targeted absorption lines. parameters derived through SSP analysis. Trager et al. (2000) and For the former, we consider a spectral window around the 4000 Å later Serra & Trager (2007) showed that even relatively small mass break, which is a strong age indicator (albeit with a significant de- fractions (∼1 per cent) of young stars can distort age and metallicity generacy in metallicity, especially for evolved stellar populations). estimates and in moderate cases (∼10 per cent) completely over- In order to minimize the effect of an error in the flux calibration, we shadow the older population. In addition, the age and mass fraction do not choose the full spectral range of the spectra, restricting the of any younger subpopulation will also be degenerate, with larger analysis to 3800–4500 Å for the Subaru spectra, and 4000–4500 Å mass fraction of relatively older subpopulations having the same for the WHT spectra.
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