Abstract. We include gaseous continuum and line emission into our  models for the spectral and photometric evolution of Simple Stellar Populations (SSPs) for various metallicities in the range 0.02 ≤ Z/Z ≤ 2.5. This allows to extend them to significantly younger ages than before. They now cover the age range from 4 Myr all through 14 Gyr. We point out the very important contributions of gaseous emission to broad band fluxes and their strong metallicity dependence during very early evolutionary stages of star clusters, galaxies or subgalactic fragments with vigorous ongoing star formation. Emissionline contributions are commonly seen in these actively star-forming regions. Models without gaseous emission cannot explain their observed colors at all, or lead to wrong age estimates. We use up-to-date Lyman continuum (=Lyc) emission rates and decided to use recent empirical determinations of emission line ratios relative to H β for subsolar metallicities. We justify this approach for all situations where no or not enough spectral information is available to determine all the parameters required by photoionization models. The effects of gaseous line and continuum emission on broad band fluxes are shown for different metallicities and as a function of age. In addition to the many filter systems already included in our earlier models, we here also include the HST NICMOS and Advanced Camera for Surveys (=ACS) filter systems.

Abstract.A new set of evolutionary synthesis spectra are presented for Simple Stellar Populations (SSPs) covering ranges in metallicity from 0.02 ≤ Z/Z ≤ 2.5 and ages from 4 × 10 6 yr ≤ t ≤ 16 Gyr. They are based on the most recent isochrones from the Padova group that extend earlier models by the inclusion of the thermal pulsing AGB phase for stars in the mass range 2 M ≤ m ≤ 7 M in accordance with the fuel consumption theorem. We show that with respect to earlier models, inclusion of the TP-AGB phase leads to significant changes in the (V − I) and (V − K) colors of SSPs in the age range from 10 8 to ∼ > 10 9 yr. Using model atmosphere spectra from Lejeune et al. (1997Lejeune et al. ( , 1998, we calculate the spectral evolution of single burst populations of various metallicities covering the wavelength range from 90 Å through 160 µm. Isochrone spectra are convolved with filter response functions to describe the time evolution of luminosities and colors in Johnson, Thuan & Gunn, Koo, HST, Washington and Strömgren filters. The models and their results are not only intended for use in the interpretation of star clusters but also for combination with any kind of dynamical galaxy formation and/or evolution model that contains a star formation criterion. Moreover, the evolution of these single burst single metallicity stellar populations is readily folded with any kind of star formation -and eventually chemical enrichment -history to describe the evolutionary spectral synthesis of composite stellar populations like galaxies of any type with continuous or discontinuous star formation. For these latter purposes we also present the time evolution of ejection rates for gas and metals for two different Initial Mass Functions (IMFs) as well as cosmological and evolutionary corrections for all the filters as a function of redshift for 0 ≤ z ≤ 5 and two different cosmologies. Extensive data files are provided in the electronic version, at CDS, and at our above www-address.

Abstract. We construct evolutionary synthesis models for simple stellar populations using the evolutionary tracks from the Padova group (1993, 1994), theoretical colour calibrations from Lejeune et al. (1997Lejeune et al. ( , 1998 and fit functions for stellar atmospheric indices from Worthey et al. (1994).A Monte-Carlo technique allows us to obtain a smooth time evolution of both broad band colours in UBVRIK and a series of stellar absorption features for Single Burst Stellar Populations (SSPs). We present colours and indices for SSPs with ages from 1 10 9 yrs to 1.

The luminosity functions (LFs) of star cluster (SC) systems (i.e. the number of clusters per luminosity interval) are vital diagnostics to probe the conditions of SC formation. Early studies have revealed a clear dichotomy between old globular clusters and young clusters, with the former characterized by Gaussian-shaped LFs, and the latter following a power law. Recently, this view was challenged by studies of galaxy merger remnants and post-starburst galaxies. In this paper, we re-evaluate the young ( few hundreds of Myrs, with the majority few tens of Myrs) SC system in the ongoing spiral-spiral major merger system NGC 4038/39, the 'Antennae' galaxies. The Antennae galaxies represent a very active and complex star-forming environment, which hampers cluster selection and photometry as well as the determination of observational completeness fractions. A main issue of concern is the large number of bright young stars contained in most earlier studies, which we carefully exclude from our cluster sample by accurately determining the source sizes. The resulting LFs are fitted both with Gaussian and with power-law distributions, taking into account both the observational completeness fractions and the photometric errors, and compared using a likelihood-ratio test. The likelihood-ratio results are rigidly evaluated using Monte Carlo simulations. We perform a number of additional tests, for example, with subsets of the total sample, all confirming our main result: that a Gaussian distribution fits the observed LFs of clusters in this preferentially very young cluster system significantly better than a power-law distribution, at a (statistical) error probability of less than 0.5 per cent.

Abstract. HST has opened the possibility to decompose the surface brightness profiles of galaxies up to significant redshifts and look-back times into r 1/4 −bulge and exponential disk components. This should allow to study the redshift evolution of bulge and disk luminosity contributions and discriminate between the different formation scenarios for these galaxy components currently discussed, i.e. decide if star formation in bulges and disks started at the same time or was delayed in either of the two components. An indispensable prerequisite for the comparison of bulge-to-disk ratios of galaxies at different redshifts is to properly account for cosmological band shift and evolutionary effects. We present evolutionary synthesis models for both components and add their spectra in various proportions to obtain the full range of local galaxies' B-band bulge-to-total light ratios. Bulge star formation is assumed to occur on a short timescale of 10 9 yr, disk star formation proceeds at a constant rate. We study the evolution of the relative light contributions of both components backward in time and, for a given cosmological model, as a function of redshift. This allows us to see how far back into the past the locally well-established correlation between galaxy morphologies and spectral properties can hold. To cope with the present uncertainty about the formation epochs of bulge and disk components we present models for three scenarios: bulges and disks of equal age, old bulges and delayed disk star formation, and old disks with subsequent bulge star formation. We quantitatively show the wavelength dependence of bulge-to-total (=B/T ) light ratios for local galaxies. The different star formation timescales for bulge and disk components lead to B/T ratios that significantly increase from U through I-bands (by factors 4-6 for weak bulge systems ∼Sc) with the rate of increase slightly depending on the relative ages of the two components. The redshift evolution of B/T -ratios in various bands U, B, V, I, H is calculated accounting both for cosmological and evolutionary corrections assuming a standard cosmology (H 0 = 65, Ω 0 = 0.1, Λ 0 = 0). In particular, for the two scenarios with old bulges and old or younger disks, the redshift evolution of B/T -ratios is dramatic in every band and both for galaxies ending up at z ∼ 0 with low and high B-band B/T light ratios. Our results clearly show that it does not make any sense to compare B/T ratios measured in one and the same band for galaxies at different redshifts without fully accounting for evolutionary and cosmological effects. These, unfortunately, significantly depend on the relative ages of the two components and, hence, on the galaxy formation scenario adopted. We also show that simultaneous decomposition of galaxy profiles in several bands can give direct information about these relative ages and constrain formation scenarios for the different galaxy components. Of the wavelength bands we explore (U, B, V, I, H), the I-and H-bands show the smoothest redshift evolut...