Fields of current Research Interests

Evolution of Galaxies of Various Hubble Types

with special attention to the metallicity of their stellar populations.
  • Development, extension and up-dating of the Goettingen Evolutionary Synthesis Code for the unified spectral, chemical and cosmological evolution of galaxies.
  • Comparison of models with observations of galaxies - nearby and at high redshifts.
  • Spectrophotometric properties, metallicity of the stellar population, chemical evolution of ISM element abundances.

Chemically Consistent Evolutionary Synthesis

  • Refinement of the Goettingen chemical and spectrophotometric evolutionary synthesis code to account for the increasing initial metallicity of successive generations of stars.
  • Importance of stellar metallicity distributions and subsolar metallicities in galaxies of various types in the local Universe and, in particular, at high redshifts.
  • Enlargement of the spectral range from X through FIR, consistent inclusion of gas and dust (with R. Kotulla), both in absorption and emission.
  • Interpretation of redshift surveys and galaxy counts over a large wavelength range (with R. Kotulla, in collaboration with C. Leitherer, STScI).

Interacting Galaxies

  • Analyses of starbursts and post-starbursts in interacting galaxies, mergers, and merger remnants. Burst parameters: strength, duration, spatial extent, star formation efficiencies - dependence on galaxy properties, encounter parameters, environment.
  • Analysis of interesting individual objects observed over a wide wavelength range from the ground and with HST, as e.g. NGC 7252, NGC 4038/39, ... (collaboration with F. Schweizer, Pasadena, B. Whitmore, STScI).
  • Age sequence of mergers/starburst.
  • Starbursts and mergers at intermediate and high redshifts: Impact on redshift surveys and deep galaxy counts, relation to IR-bright galaxies and EROs

Formation and Evolution of Star Clusters in Starbursts and Galaxy Mergers

  • ESO - ASTROVIRTEL project: Globular Cluster Luminosity Functions: Evolution and Environmental Dependence (PI R. de Grijs, Sheffield, UK, CoIs G. Gilmore, Cambridge, UFvA):
    • Evolutionary synthesis models for star clusters of various metallicities:
    • luminosity & color evolution, absorption features,
    • spectral evolution from earliest stages - including gaseous emission - all over a Hubble time.
  • Theoretical calibrations of broad band colors (Johnson - Cousins - HST), intermediate band colors (Washington - Stroemgren - HST), and absorption features in terms of metallicity and as a function of age (with P. Anders, T. Lilly in collaboration with F. Schweizer, B. Whitmore & C. Leitherer, STScI, D. Geisler, Univ. de Concepcion, Chile).
  • analysis of multi-band photometry of star cluster systems in terms of individual cluster metallicities, ages, extinction values and masses; age and metallicity distributions of star cluster systems, mass functions of star cluster systems and their evolution (with P. Anders, S. Larsen, H. Lamers, Utrecht).

Star and star cluster formation under "violent" vs "normal" conditions

  • Evolution of Star Cluster Systems : color distributions, luminosity functions, mass functions, ... in normal, starburst, interacting galaxies, mergers & merger remnants: age sequence.
  • Universality vs. environmental dependence, secular evolution;
  • nature of young star clusters: open clusters or globulars;
  • young star clusters as tracers for the dynamical evolution of starbursts in interacting/merging galaxies (with P. Papaderos, Goettingen, B. Cunow, Pretoria).

Origin of Elliptical and S0 Galaxies

  • monolithic initial collapse vs. one major spiral-spiral merger vs. hierarchical accretion: clues from color and metallicity distributions of their globular cluster populations (with T. Lilly, Goettingen, P. Anders, S. Larsen, H. Lamers, Utrecht, M. Kissler - Patig, ESO Garching, D. Geisler, T. Richtler, Univ. Concepcion).

Evolution and Transformation of Galaxies in Clusters

over the redshift range from z = 0 to z > 1:
  • galaxy - galaxy and galaxy - cluster gas interactions,
  • starbursts and post-starburst populations, Butcher-Oemler galaxies
  • origin of S0 and dwarf galaxy populations in clusters (with J. Bicker, B. Ziegler, Goettingen, P. Woudt, R. Kraan -- Korteweg, Cape Town).

Dwarf Galaxies

  • spectral, chemical, and dynamical evolution (with P.Papaderos, K. Noeske, in collaboration with G. Hensler, Kiel, J. Gallagher, Madison),
  • possible evolutionary connections among various types of dwarf galaxies.
  • Formation and Evolution of Tidal Dwarf Galaxies (= TDGs) = dwarf galaxies forming in tidal tails of gas-rich interacting/merging galaxies: selection of promising TDG candidates against background, foreground and physically unrelated objects,
    • analysis of multi-band photometry and spectra to determine masses, evolutionary state,
    • relative contributions of inherited and burst stars, possible future evolution, dynamical independence ?
    • Cosmological implications: possible contribution of TDGs to faint blue galaxy excess, deep counts, galaxy group/cluster populations (with P. Weilbacher, Potsdam, P.-A. Duc, Saclay, J. Hibbard, Charlottesville)

Chemical Evolution of Galaxies and Protogalaxies

  • chemically consistent chemical evolution models for various galaxy types - accounting for metallicity-dependent stellar yields - in comparison with QSO absorption line observations (CIV-, MgII- and Damped Lyman Alpha Absorber); redshift evolution of abundances and abundance ratios (with M. Pettini, Cambridge);
  • possible insights into early nucleosynthesis (with N. Langer, Utrecht).
  • Optical identifications of QSO absorbers: spectrophototmetric redshift evolution of MgII- and CIV- absorber galaxies, prediction of spectral properties of Damped Lyman Alpha Galaxies (with U. Lindner).

Formation and Dynamical Evolution of Galaxies in Cosmological Scenarios

  • Combining spectrophotometric evolutionary synthesis models for single burst populations with numerical GRAPE-SPH & N-body models for the formation and dynamical evolution of galaxies in cosmological structure formation scenarios (with M. Steinmetz, Potsdam).

Morphological Evolution of Galaxies

  • Correspondence between spectral and morphological galaxy types
  • evolutionary synthesis models for bulge and disk components vs. multi-band photometry.
  • Wavelength dependence of bulge-to-disk light ratios of local galaxies
  • Modelling and interpretation of the redshift evolution of HST-determined galaxy morphologies: clues to bulge formation scenarios.

Lyman Break Galaxies

  • Star formation rates, age and evolutionary stage, metallicity and dust content,
  • spectral properties from UV through IR,
  • evolutionary links between Lyman Break and local galaxy types.
  • Effects of attenuation (with T. Tepper Garcia, P. Richter, Potsdam) 

Resolved Stellar Populations vs. Integrated Light

  • Reconstruction of the star formation histories of galaxies - from CMDs and from integrated spectra
    • a methodological comparison: advantages and limitations of both approaches.
    • Evolutionary synthesis construction of color - magnitude diagrams for evolving stellar populations with arbitrary star formation histories (with T. Lilly, Goettingen, C. Gallart, IAC Tenerife, D. Alloin, ESO Santiago, P. Demarque, Yale)