Stellar population models of Lick indices with variable element abundance ratios

By D. Thomas, C. Maraston, R. B. Mpe, Garching, Usm, Munich. 2002

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We provide the whole set of Lick indices fromCN1 to TiO2 in the wavelength range 4000≲λ≲6500 A of simple stellar population models with, for the first time, variable element abundance ratios, [α/Fe ]=0.0, 0.3, 0.5, [α/Ca ]=−0.1, 0.0, 0.2, 0.5 and [α/N]=−0.5, 0.0. The models cover ages between 1 and 15 Gyr, metallicities between 1/200 and 3.5 solar. The impact from the element abundance changes on the absorption-line indices are taken from Tripicco & Bell, using an extension of the method introduced by Trager et al. Our models are free from the intrinsic α/Fe bias that was imposed by the Milky Way template stars up to now, hence they reflect well-defined α/Fe ratios at all metallicities. The models are calibrated with Milky Way globular clusters for which metallicities and α/Fe ratios are known from independent spectroscopy of individual stars. The metallicities that we derive from the Lick indices Mgb and Fe5270 are in excellent agreement with the metallicity scale by Zinn & West, and we show that the latter provides total metallicity rather than iron abundance.We can reproduce the relatively strong CN-absorption featuresCN1 andCN2 of galactic globular clusters with models in which nitrogen is enhanced by a factor of 3. An enhancement of carbon, instead,would lead to serious inconsistencies with the indices Mg1 and C24668. The calcium sensitive index Ca4227 of globular clusters is well matched by our models with [Ca/Fe] = 0.3, including the metal-rich bulge clusters NGC 6528 and 6553. From our α/Fe-enhanced models we infer that the index [MgFe] defined by Gonzalez is quite independent of α/Fe but still slightly decreases with increasing α/Fe.We find that the index [MgFe] ≡ √ Mgb(0.72 × Fe5270 + 0.28 × Fe5335), instead, is completely independent of α/Fe and serves best as a tracer of total metallicity. Searching for blue indices that give similar information as Mg b and Fe , we find that CN1 and Fe4383 may be best suited to estimating α/Fe ratios of objects at redshifts z ∼ 1.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/844985b05f65da9df2a8bec8826ed621fa632ffa

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Citations in the corpus

Population Synthesis in the Blue. IV. Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations (2007, methodology): “The method, first proposed by Trager et al. (2000) and further developed by Thomas et al. (2003a) and Korn et al. (2005), is based on estimates of how line indices in the spectra of three types of stars with relevant atmospheric parameters (a turnoff, a giant, and a lower main sequence star) change…”
From star clusters to dwarf galaxies: the properties of dynamically hot stellar systems (2008, methodology): “…by Carney (1996) and Evstigneeva et al. (2007) and the assumption that the iron abundance characterises the metallicity of the MCOs, we adopt [α/Fe] = 0.3 for each one of them and use the relation [Z/H] = [Fe/H] + 0.94 [α/Fe] (7) found by Thomas et al. (2003) to calculate [Z/H] from [Fe/H].”
VLT spectroscopy of globular cluster systems - II. Spectroscopic ages, metallicities and [α/Fe] ratios of globular clusters in early-type galaxies (2005, methodology): “Hence, among all Lick Balmer line indices, the least [α/Fe]-sensitive index is Hβ, followed by HγF, HγA, HδA, HδF (see also Thomas et al. 2003, 2004).”
Observational evidence for AGN feedback in early-type galaxies (2007, methodology): “We derive luminosity-weighted SSP ages and metallicities byχ2 fitting the Thomas et al. (2003, 2004) models to the 25 Lick absorption line indices and compute a second probability distribution through the comparison of these quantities with theV -luminosity weighted ages and metallicities of the…”
Higher‐order Balmer line indices in α/Fe‐enhanced stellar population models (2004, background): “Hence, even though relative variations of the elem nt abundances within the enhanced group exist in early-type galaxies (Worthey 1998; Saglia et al. 2002; Sanchez-Blazquez et al. 2003; Thomas et al. 2003b), these do not significantly affect theα/Fe ratio.”
The ages and metallicities of galaxies in the local universe (2005, methodology): “When estimating metallicity, we therefore consider only those spectral features that have been shown by previous studies to depend negligibly on α/Fe abundance ratio (Trager et al. 2000; Thomas et al. 2003b; Tantalo & Chiosi 2004).”
Re-evaluating old stellar populations (2018, methodology): “The NaD index is problematic in that it is also affected by Na absorption in interstellar material along the line of sight to the target (Thomas et al. 2003).”
Stellar population models at high spectral resolution (2011, methodology): “The models by Thomas et al. (2003), that account explicitly for various elemental ratios (see the same Figure), match well the Bulge globular cluster indices for [α/Fe] ∼ 0.3, consistent with the measurements from integrated-light spectroscopy.”
Spatially resolved spectroscopy of Coma cluster early-type galaxies III. The stellar population gradients ? (2003, background): “It is important to take into account that correlated errors actually lead to the opposite trend, namely higher α/Fe ratios at younger ages (Thomas et al. 2003b).”
NOAO Fundamental Plane Survey. II. Age and Metallicity along the Red Sequence from Line-Strength Data (2005, methodology): “However, attempts to fit their spectra with stellar population models suggest that the formation of early-types may be more complicated, and that there may be a spread in their ages (Worthey 1994; Thomas et al. 2003; Trager et al. 2000a; Caldwell et al. 2003).”
STECMAP: STEllar Content from high-resolution galactic spectra via Maximum A Posteriori (2005, neutral): “We assume that metallicity describes the stellar abundances, mainly because our population synthesis model does not allow for abundance variations (Thomas et al. (2003) specifically address this issue).”
Evolutionary stellar population synthesis with MILES – I. The base models and a new line index system (2010, methodology): “Therefore these clusters show CN2 values that are larger than expected according to their total metallicities, as traced by the [MgFe] index (e.g. Thomas et al. 2003).”
Stellar population synthesis at the resolution of 2003 (2003, methodology): “Based on the model with variable element abundance ratios of Thomas et al. (2003), we have identified a few spectral features that depend negligibly on element abundance ratios and are well reproduced by our model: [MgFe]′, [Mg1Fe] and [Mg2Fe] (equations 4 and 5).”