Young Star Clusters in The Antennae: A Clue to their Nature from Evolutionary Synthesis

We analyse the population of bright star clusters in the interacting galaxy pair NGC 4038/39 detected with HST WFPC1 by Whitmore & Schweizer (1995) using our spectrophotometric evolutionary synthesis models for various initial metallicities. We derive individual ages for all clusters with V-I colour and follow the time evolution of their luminosity function (LF). The age distribution clearly reveals the presence of a number of old Globular Clusters (GCs) from the parent galaxies. Their effective radii do not significantly differ from those of the young star cluster population. We confirm Meurer's (1995) conjecture that the shape of the LF changes in the course of evolution if age spread effects are accounted for. We find that over a Hubble time the observed exponential LF of the young star clusters will evolve into a typical Gaussian GCLF with parameters M$_{V_0} = -7.1$ mag and $\sigma (M_{V_0}) = 1.3$ mag. We discuss the influence of metallicity, the effects of an inhomogeneous internal dust distribution, as well as the possible influence of internal -- through stellar mass loss -- and external dynamical effects on the secular evolution of the LF. Referring YSC luminosities to a uniform age and combining with model M/L, we recover the intrinsic mass distribution of the YSC system. It is Gaussian in shape to good approximation thus representing a quasi-equilibrium distribution that $-$ according to Vesperini's (1997) dynamical modelling for the Milky Way GC system $-$ will not be altered in shape over a Hubble time of dynamical evolution, allthough a substantial number of clusters will be destroyed. Comparing young star cluster populations in an age sequence of interacting/merged and merger remnant galaxies will directly reveal the role of external dynamical effects.Comment: 15 pages Latex, figures embedded in the text Submitted to Astron. & Astrophy

Violent star and star cluster formation in nearby and distant galaxies

I present recent observations and analyses of star cluster formation in a wide variety of environments -- from young star clusters and super star clusters in normal actively star-forming spirals and irregulars to starbursting dwarfs and spiral-spiral mergers. Star cluster formation in interacting galaxies can be restricted to central starburst region, extend over the entire body of the merger, or even all along extended tidal structures. I address methods and results for the determination of star cluster ages, metallicities, masses, and sizes and discuss the nature, possible lifetimes and future signatures of these star cluster populations, as well as the relative importance of field star formation vs. star cluster formation.Comment: will appear in proceedings of "XXVth Moriond Astrophysics Meeting 2005: When UV meets IR: a History of Star Formation", ed. by D. Elbaz & H. Aussel; corrected for my misquotation of the work by Zhang & Fall (1999) on p.

Abundance Ratios in Composite Stellar Populations with special emphasis on Elliptical Galaxies

Our unified chemical and spectrophotometric evolution code allows to simultaneously study the ISM abundances of a series of elements and the spectral properties of the stellar population in our model galaxies. We use stellar evolutionary tracks, yields, spectra, color and absorption index calibrations for 5 different metallicities and account for the increase in initial metallicity of successive generations of stars. For any kind of stellar system, as described by its star formation history and IMF, we thus can directly compare the time evolution of gaseous and stellar abundance ratios. Spiral galaxy models that successfully reproduce spectral properties as well as ISM abundances of nearby templates are combined with a cosmological model and compared to damped Lyman $\alpha$ absorbers. For early type galaxies various formation scenarii -- initial monolithic collapse, spiral-spiral merger, hierarchical formation -- are tested with respect to their predicted spectral energy distributions from UV to NIR and absorption indices and index ratios, as e.g. [MgFe].Comment: 4 pages Latex, no figures, using jd.sty To appear in the proceedings of IAU JD1 `Abundance Ratios of the Oldest Stars', August 1997, Kyoto, Japan, Highlights of Astronom

Chemically Consistent Evolutionary Synthesis Models

Any stellar system with a star formation history (SFH) more extended than a massive star's lifetime will be composite in metallicity. Our method of chemically consistent evolutionary synthesis tries to account for the increasing initial metallicity of successive generations of stars. Using various sets of input physics for a range of metallicities $10^{-4} \leq Z \leq 0.05$ we keep track of the ISM enrichment and follow successive generations of stars using stellar evolutionary tracks, yields, model atmosphere spectra, index calibrations, etc., appropriate for their respective initial metallicities. Since the SFH determines the evolution not only of the metallicity, but, in particular, of abundance ratios of specific elements, stellar evolution and galaxy evolution become intimately coupled. I review the concept of chemically consistent evolution, present results for the photometric, spectral, and chemical evolution of galaxies of various types in the local Universe and at high redshift, and discuss its advantages as well as its current limitations.Comment: 10 pages, using paspconf.sty, Invited Talk, Annapolis Conference ` Spectrophotometric Dating of Stars and Galaxies', April 199

Bright Star Clusters in the Antennae analysed with Evolutionary Synthesis

HST WFPC1 observations revealed 738 bright star clusters spread over the inner 12 kpc of the interacting galaxy pair NGC 4038/39. If the young star clusters (YSCs) formed in the global starburst are (proto-) globular or open clusters has far-reaching consequences for the formation of elliptical galaxies. Effective radii of the YSCs are difficult to measure on WFPC1 images, their present luminosity function (LF) looks exponential. Evolutionary synthesis models for star clusters of various metallicities are used to individually age-date these clusters and describe their future luminosity and color evolution. We identify a small but plausible number of old globular clusters (GCs). Properly accounting for age spread effects significantly changes the shape of the YSC LF from an exponential towards a Gaussian during further evolution. This suggests that -- at least a major part of -- the YSCs may well be GCs, implying that during a spiral-spiral merger+starburst the specific GC frequency can increase significantly. Being preferentially observed around those galactocentric radii where they formed, the YSCs serve as tracers for the dynamical evolution of the global starburst in NGC 4038/39. We tentatively report several radial trends in the YSCs' properties to be verified by WFPC2 reobservations.Comment: 4 pages Latex, 2 POSTSCRIPT figures, using psfig and crckapb.sty To appear in the proceedings of IAU Symposium 186 `Interacting Galaxies at Low and High Redshift', August 18-22, Kyoto, Japa

Tidal Dwarf Galaxies: Their Present State and Future Evolution

Evolutionary synthesis models for Tidal Dwarf Galaxies (TDGs) are presented that allow to have varying proportions of young stars formed in the merger-induced starburst and of stars from the merging spirals' disks. Comparing model grids with observational data (see e.g. P.-A. Duc this conference for a review) we try to identify the present evolutionary state of TDGs. The influence of their specific metallicities as well as of the gaseous emission of actively star forming TDGs on their luminosity and colour evolution are studied.Comment: 5 pages Latex, 4 POSTSCRIPT figures, using psfig To appear in The Magellanic Clouds and other Dwarf Galaxies, eds. J. M. Braun, T. Richtler Proceedings of Workshop of the Graduiertenkolleg Bonn-Bochum, Bad Honnef (Jan. 18-22, 1998

The Virtual Observatory as a Tool to Study Star Cluster Populations in Starburst Galaxies

The cluster luminosity function (CLF) is one of the most important diagnostics in the study of old globular and young compact star cluster populations. We are currently using ASTROVIRTEL to obtain CLFs in several optical and/or near-infrared passbands, and colour distributions. This will provide us with a powerful analytical tool for the determination of the violent star and cluster formation history of galaxies: we will address questions related to the universality of the globular CLF, the time-scale of low-mass, low-luminosity star cluster depletion and its observability, and environmental effects affecting the shape of the CLFs and the efficiency of the depletion process. This has required the development of complex data mining tools, which are currently being incorporated in ASTROVIRTEL's "querator".Comment: 2-page poster paper, to appear in "Toward an International Virtual Observatory", June 2002, Garching bei Muenchen (Germany), eds. Gorski K.M., et al., ESO conference series; full poster available from http://www.ast.cam.ac.uk/~grijs/VOposter.pd

Predicting Spectral Properties of DLA Galaxies

Comparison of our chemically consistent models for spiral galaxies with observed DLA abundances shows that at high redshift DLA galaxies may well be the progenitors of normal spiral disks of all types from Sa through Sd. Towards lower redshifts z </= 1.5 however, early type spirals drop out of DLA samples due to low gas or/and high dust content. We use the spectrophotometric aspects of our unified spectral, chemical, and cosmological evolution models to predict expected luminosities in different bands for DLA galaxies at various redshifts and compare to the few optical identifications available.Comment: 5 pages, using LAMUPHYS LaTeX macro Springer, version 1.0, ESO Workshop "Chemical Evolution from Zero to High Redshift

Chemically Consistent Evolution of Galaxies on Cosmological Timescales and the DLA Galaxy Population

We describe the evolution of galaxies in a chemically consistent way accounting for the increasing initial metallicity of successive generations of stars. The enrichment of various types of model galaxies is compared with Damped Ly$\alpha$ (= DLA) abundances over the redshift range from ${\rm z \sim 0}$ through ${\rm z > 4.4}$. We discuss properties and composition of the galaxy population giving rise to DLA absorption. Beyond evolution of individual galaxies a change is found in the composition of DLA galaxy samples from high to low redshift. Spectrophotometric properties are predicted for optical identifications of DLA galaxies.Comment: 4 pages, using moriond.sty, Proceedings of the XIXth Moriond Astrophysics Meeting `Building Galaxies: from the Primordial Universe to the Present', Les Arcs, March 199

Analysing Multi-Color Observations of Young Star Clusters in Mergers

We present a new evolutionary synthesis (ES) tool specifically developed for the analysis of multi-color data of young star clusters (YSCs) in interacting galaxies. Our ESO/ST-ECF ASTROVIRTEL project provides an unprecedented database of UV--optical--NIR observations from the HST/WFPC2, HST/NICMOS and VLT data archives. Comparison of these data with ES models for clusters of various metallicities which include gaseous emission as well as various degrees of dust extinction, allows one to independently determine metallicities, ages and extinction for individual clusters. These, in turn, are the basis to derive the mass functions of the YSC systems and to predict the future evolution of their luminosity functions and color distributions. Comparing YSC systems of various ages will, we anticipate, allow us to ``see cluster disruption processes at work''.Comment: 2 pages, to appear in Conference Proceedings 'New Horizons in Globular Cluster Astronomy', Padova 2002, at ASP Conf. Ser., by G. Piotto, G. Meylan, G. Djorgovski, M. Riello, requires 'newpasp' styl