EDP Sciences
Free access
Volume 419, Number 3, June I 2004
Page(s) 863 - 876
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034528

A&A 419, 863-876 (2004)
DOI: 10.1051/0004-6361:20034528

Extragalactic globular clusters in the near infrared

IV. Quantifying the age structure using Monte-Carlo simulations
M. Hempel and M. Kissler-Patig

European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
    e-mail: [mhempel,mkissler]@eso.org
(Received 17 October 2003 / Accepted 23 February 2004 )

In previous papers of the series, we used a combination of optical and near-infrared colours to derive constrains on the relative age structure in globular cluster systems. Here, we present the details, strength and limitations of our method based on Monte-Carlo simulations of colour-colour diagrams and cumulative age distributions. The simulations are based on general informations about the globular cluster systems (e.g. colour-ranges, the number ratios between sub-populations) and the different single stellar population models (SSP's) which are used to derive relative ages. For both the modeled systems and the observed globular cluster systems we derive the cumulative age distribution and introduce two parameters to define it, the so-called 50% age and the result of the reduced  $\chi$ 2 test of the comparison between models and observations. The method was tested successfully on several systems and allowed to reveal significant intermediate age populations in two of them.

Key words: galaxies: photometry -- galaxies: star clusters

Offprint request: M. Hempel, mhempel@eso.org

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