Evolutionary population synthesis: models, analysis of the ingredients and application to high-z galaxies

Evolutionary population synthesis models for a wide range of metallicities, ages, star formation histories, and Horizontal Branch morphologies, including blue morphologies at high metallicity, are computed. The energetics of the post Main Sequence evolutionary phases are evaluated with the fuel consumption theorem. The impact on the models of the stellar evolutionary tracks is assessed. We find modest differences in synthetic broad-band colours as induced by the use of different tracks in our code (e.g. d(V-K) ~ 0.08 mag; d(B-V) ~ 0.03 mag). These differences are smaller than the scatter among other models in the literature, even when the latter adopt the same evolutionary tracks. The models are calibrated with globular cluster data from the Milky Way for old ages, and the Magellanic Clouds plus the merger remnant galaxy NGC 7252, for ages of ~ 0.1 - 2 Gyr, in a large wavelength range from U to K. Particular emphasis is put on the contribution from the Thermally-Pulsing Asymptotic Giant Branch phase. We show that this phase is crucial for the modelling of young stellar populations by the comparison with observed spectral energy distributions of Magellanic Clouds clusters, which are characterised by high fluxes both blueward and redward the V-band. We find that the combination of the near-IR spectral indices C2 and H2O can be used to determine the metallicity of ~ 1 Gyr stellar populations. We re-analyze the spectral energy distributions of some of the high-z galaxies (2.4 < z < 2.9) observed with the Spitzer Space Telescope by Yan et al. (2004). Their high rest-frame near-IR fluxes are reproduced very well with the models including Thermally-Pulsing Asymptotic Giant Branch stars for ages in the range 0.6-1.5 Gyr, suggesting formation redshifts around z ~ 3-6.Comment: 29 pages, 30 figures, MNRAS, in press. Very few minor revisions with respect to the submitted version. Improved Figures 2, 18, 19, 23 and 27. More models available at http://www-astro.physics.ox.ac.uk/~maraston

Evolutionary synthesis of stellar populations: a modular tool

A new tool for the Evolutionary Synthesis of Stellar Populations (EPS) is presented, which is based on three independent matrices, giving respectively: 1) the fuel consumption during each evolutionary phase as a function of stellar mass; 2) the typical temperatures and gravities during such phases; 3) colours and bolometric corrections as functions of gravity and temperature. The modular structure of the code allows to easily assess the impact on the synthetic spectral energy distribution of the various assumptions and model ingredients, such as, for example, uncertainties in stellar evolutionary models, mixing length, the temperature distribution of horizontal branch (HB) stars, AGB mass loss, and colour-temperature transformations. The so-called ``AGB-Phase Transition'' in Magellanic Cloud clusters is used to calibrate the contribution of the Thermally Pulsing Asymptotic Giant Branch phase (TP-AGB) to the synthetic integrated luminosity. As an illustrative example, solar metallicity ($Y=0.27,Z=0.02$) models, with ages ranging between 30 {\rm Myr} and 15 {\rm Gyr} and various choices for the slope of the Initial Mass Function (IMF), are presented. Synthetic broad band colours and the luminosity contributions of the various evolutionary stages are compared with LMC and Galactic globular cluster data. In all these cases, a good agreement is found. Finally, we show the evolution of stellar mass-to-light ratios in the bolometric and U,B,V,R, and K passbands, in which the contribution of stellar remnants is accounted for.Comment: 21 pages, 26 figures, accepted for publication in MNRA

The impact of alpha/Fe enhanced stellar evolutionary tracks on the ages of elliptical galaxies

We complement our study of alpha/Fe enhanced stellar population models of Lick absorption indices (Thomas et al. 2003) by comparing two sets of alpha/Fe enhanced models. In both models the impact on Lick indices due to alpha/Fe enhancement is accounted for through a modification of the stellar absorption line-strengths using the response functions of Tripicco & Bell (1995). One set of models, however, uses solar-scaled, the other alpha/Fe enhanced stellar evolutionary tracks. Since the alpha/Fe enhanced tracks are hotter than the solar-scaled ones (Salasnich et al. 2000), the correspondent stellar population models have slightly weaker metallic indices (i.e. Mgb, etc.) and stronger Balmer line indices (Hbeta) (Maraston et al 2003). Here we explore quantitatively the impact of this effect on the alpha/Fe ratios, metallicities and ages that are derived for elliptical galaxies. We find that the modest decrease of the metallic indices Mgb and balance each other, such that fully consistent alpha/Fe ratios are derived for stellar systems using alpha/Fe enhanced models with either solar-scaled or alpha/Fe enhanced stellar tracks. The decrease of the metallic indices and the increase of Hbeta conspire in a way that also consistent metallicities are obtained. The derived ages, instead, are significantly different. The inclusion of alpha/Fe enhanced stellar tracks leads to the derivation of ages as high as 30 Gyr for elliptical galaxies. For the same objects, ages not older than 15 Gyr are obtained, if alpha/Fe enhanced models using solar-scaled tracks are adopted. This may indicate that current stellar evolutionary models overestimate the bluing of stellar evolutionary tracks due to alpha/Fe enhanced chemical mixtures at super-solar metallicities.Comment: 5 pages, 2 figures. Accepted by A&

Modelling stellar populations at high redshift

Stellar populations carry information about the formation of galaxies and their evolution up to the present epoch. A wealth of observational data are available nowadays, which are analysed with stellar population models in order to obtain key properties such as ages, star formation histories, stellar masses. Differences in the models and/or in the assumptions regarding the star formation history affect the derived properties as much as differences in the data. I shall review the interpretation of high-redshift galaxy data from a model perspective. While data quality dominates galaxy analysis at the highest possible redshifts (z>5), population modelling effects play the major part at lower redshifts. In particular, I discuss the cases of both star-forming galaxies at the peak of the cosmic star formation history as well as passive galaxies at redshift below 1 that are often used as cosmological probes. Remarks on the bridge between low and high-z massive galaxies conclude the contribution.Comment: 7 pages, 7 figures, invited review at the IAU Symposium 277 "Tracing the Ancestry of Galaxies (on the land of our ancestors)", Ouagadougou (Burkina Faso), December 2010, Editors: Claude Carignan, Francoise Combes, Ken Freema

Search Instructions for Globular Clusters in Formation at High Redshifts

The formation of globular clusters (GC), with their multiple stellar generations, is still an unsolved puzzle. Thus, interest is rising on the possibility to detect their precursors at high redshift, hence directly witnessing their formation. A simple set of assumptions are empirically justified and then used to predict how many such precursors formed between redshift 3 and 10 could actually be detected by the NIRCam instrument on board of JWST. It is shown that the near power-law shape of the rest-frame UV continuum of young globular cluster precursors (GCP) implies that both colours and luminosities in NIRCam long-wavelength passbands depend remarkably weakly on formation redshift. Thus, the predicted number counts depend only little on the actual formation redshifts in the mentioned range, with the exception of the bluest passbands for which counts can be strongly suppressed by intergalactic absorption along the line of sight. Instead, counts depend strongly on the actual mass of GCPs, in such a way that one NIRCam pointing should detect of the order of 10 GCPs to mag$\sim 30$ if their mass distribution was the same of today GCs, or over 1,000 if their mass was 10 times higher. Therefore, GCP number counts will set fairly tight constraints on the initial mass of GCs. An encouraging agreement with the number density of candidate GCPs at $z=6-8$, revealed by the Hubble Frontier Fields (HFF) program, suggests that their initial mass could be at least 4 times higher than that of their local descendants if all were to end up as GCs.Comment: MNRAS accepte

New Clues on the Calcium Underabundance in Early-Type Galaxies

We use our new stellar population models, which include effects from variable element abundance ratios, to model the Ca4227 absorption line indices of early-type galaxies (Trager et al.), and to derive calcium element abundances. We find that calcium, although being an alpha-element, is depressed with respect to the other alpha-elements by up to a factor 2. This confirms quantitatively earlier speculations that early-type galaxies are calcium underabundant. We find a clear correlation between alpha/Ca ratio and central velocity dispersion, which implies that more massive galaxies are more calcium underabundant. Interestingly this correlation extends down to the dwarf spheroidal galaxies of the Local Group for which alpha/Ca ratios have been measured from high-resolution spectroscopy of individual stars (Shetrone et al.). The increase of the calcium underabundance with galaxy mass balances the higher total metallicities of more massive galaxies, so that calcium abundance in early-type galaxies is fairly constant and in particular does not increase with increasing galaxy mass. This result may be the key to understand why the CaII triplet absorption of early-type galaxies at 8600 A is constant to within 5 per cent over a large range of velocity dispersions (Saglia et al.; Cenarro et al.). The origin of the calcium underabundance in early-type galaxies remains yet to be understood. We argue that formation timescales are disfavoured to produce calcium underabundance, and that the option of metallicity dependent supernova yields may be the most promising track to follow.Comment: 5 pages, 2 figures. Accepted by MNRA

Stellar population models of Lick indices with variable element abundance ratios

We provide the whole set of Lick indices from CN1 to TiO2 of Simple Stellar Population models with, for the first time, variable element abundance ratios, [alpha/Fe]=0.0, 0.3, 0.5, [alpha/Ca]=-0.1, 0.0, 0.2, 0.5, and [alpha/N]=-0.5, 0.0. The models cover ages between 1 and 15 Gyr, metallicities between 1/200 and 3.5 solar. Our models are free from the intrinsic alpha/Fe bias that was imposed by the Milky Way template stars up to now, hence they reflect well-defined alpha/Fe ratios at all metallicities. The models are calibrated with Milky Way globular clusters for which metallicities and alpha/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 (1984), and we show that the latter provides total metallicity rather than iron abundance. We can reproduce the relatively strong CN-absorption features CN1 and CN2 of galactic globular clusters with models in which nitrogen is enhanced by a factor three. 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 NGC 6553. From our alpha/Fe enhanced models we infer that the index [MgFe] defined by Gonzalez (1993) is quite independent of alpha/Fe, but still slightly decreases with increasing alpha/Fe. We define a slight modification of this index that is completely independent of alpha/Fe and serves best as a tracer of total metallicity. Searching for blue indices that give similar information as Mgb and Fe, we find that CN1 and Fe4383 may be best suited to estimate alpha/Fe ratios of objects at redshifts z~1. (Abridged)Comment: 17 pages, 7 figures, plus 8 pages model tables. Accepted by MNRAS. Models are also available in at ftp://ftp.mpe.mpg.de/people/dthomas/SSP

Empirical calibrations of optical absorption line indices based on the stellar library MILES

Stellar population models of absorption line indices are an important tool for the analysis of stellar population spectra. They are most accurately modelled through empirical calibrations of absorption line indices with the stellar parameters effective temperature, metallicity, and surface gravity, the so-called fitting functions. Here we present new empirical fitting functions for the 25 optical Lick absorption line indices based on the new stellar library MILES. The major improvements with respect to the Lick/IDS library are the better sampling of stellar parameter space, a generally higher signal- to-noise, and a careful flux calibration. In fact we find that errors on individual index measurements in MILES are considerably smaller than in Lick/IDS. Instead we find the rms of the residuals between the final fitting functions and the data to be dominated by errors in the stellar parameters. We provide fitting functions for both Lick/IDS and MILES spectral resolutions, and compare our results with other fitting functions in the literature. A Fortran 90 code is available online in order to simplify the implementation in stellar population models. We further calculate the offsets in index measurements between the Lick/IDS system to a flux calibrated system. For this purpose we use the three libraries MILES, ELODIE, and STELIB. We find that offsets are negligible in some cases, most notably for the widely used indices Hbeta, Mgb, Fe5270, and Fe5335. In a number of cases, however, the difference between flux calibrated library and Lick/IDS is significant with the offsets depending on index strengths. Interestingly, there is no general agreement between the three libraries for a large number of indices, which hampers the derivation of a universal offset between the Lick/IDS and flux calibrated systems.Comment: 19 pages, MNRAS in press, online material available at http://www.icg.port.ac.uk/~johanss