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

Policy Advice Derived from Simulation Models

When advising policy we face the fundamental problem that economic processes are uncertain. Consequently, policy can err. In this paper we show how the use of simulation models can reduce policy errors by inferring empirically reliable and meaningful statements about economic processes. We suggest that policy is best based on so-called abductive simulation models, which help to better understand how policy measures can influence economic processes. We show that abductive simulation models use a combination of theoretical and empirical analysis based on different data sets. By way of example we show what policy can learn with the help of abductive simulation models, namely how policy measures can influence the emergence of a regional cluster.Policy Advice, Simulation Models, Uncertainty, Methodology

Policy Advice Derived From Simulation Models

When advising policy we face the fundamental problem that economic processes are connected with uncertainty and thus policy can err. In this paper we show how the use of simulation models can reduce policy errors. We suggest that policy is best based on so-called abductive simulation models, which help to better understand how policy measures can influence economic processes. We show that abductive simulation models use a combination of theoretical and empirical analysis based on different data sets. This helps inferring empirically reliable and meaningful statements about how policy measures influence economic processes. By way of example we show how research subsidies by the government influence the likelihood that a regional cluster emerges.Policy Advice, Simulation Models, Uncertainty, Methodology

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

Flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios

We present new stellar population models of Lick absorption-line indices with variable element abundance ratios. The models are based on our new calibrations of absorption-line indices with stellar parameters derived from the MILES stellar library. The key novelty compared to our previous models is that they are now available at the higher spectral resolution of MILES (~2.7A FWHM) and flux-calibrated, hence not tied anymore to the Lick/IDS system. This is essential for the interpretation of galaxy spectra where calibration stars are not available, such as large galaxy redshift surveys or other high-redshift observations. We note that the MILES resolution appears to be comparable to SDSS resolution, so that our models can be applied to SDSS data without any corrections for instrumental spectral resolution. For the first time we provide random errors for the model predictions based on the uncertainties in the calibration functions and the underlying stellar parameter estimates. We show that random errors are small except at the edges of the parameter space (high/low metallicities and young ages <1 Gyr) where the stellar library is under-sampled. We calibrate the base model for the parameters age, metallicity and alpha/Fe ratio with galactic globular cluster and galaxy gradient data. We discuss two model flavours with different input stellar evolutionary tracks from the Frascati and Padova groups. The new model release now includes abundance variations of the elements C, N, Mg, Na, Si, Ca, Ti, Cr, and Fe. The individual elements that are best accessible with these models and the standard set of Lick absorption features are C, N, Mg, Ca, Ti, and Fe. The model data is available at www.icg.port.ac.uk/~thomasd.Comment: MNRAS, re-submitted including referee's comments (moderate revision

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

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