14 research outputs found
Accounting for Stochastic Fluctuations when Analysing Integrated Light of Star Clusters. I: First Systematics
Star clusters are studied widely both as benchmarks for stellar evolution
models and in their own right. Cluster age and mass distributions within
galaxies are probes of star formation histories, and of cluster formation and
disruption processes. The vast majority of clusters in the Universe is small,
and it is well known that the integrated fluxes and colors have broad
probability distributions, due to small numbers of bright stars. This paper
goes beyond the description of predicted probability distributions, and
presents results of the analysis of cluster energy distributions in an
explicitly stochastic context. The method developed is Bayesian. It provides
posterior probability distributions in the age-mass-extinction space, using
multi-wavelength photometric observations and a large collection of Monte-Carlo
simulations of clusters of finite stellar masses. Both UBVI and UBVIK datasets
are considered, and the study conducted in this paper is restricted to the
solar metallicity. We first reassess and explain errors arising from the use of
standard analysis methods, which are based on continuous population synthesis
models: systematic errors on ages and random errors on masses are large, while
systematic errors on masses tend to be smaller. The age-mass distributions
obtained after analysis of a synthetic sample are very similar to those found
for real galaxies in the literature. The Bayesian approach on the other hand,
is very successful in recovering the input ages and masses. Taking stochastic
effects into account is important, more important for instance than the choice
of adding or removing near-IR data in many cases. We found no immediately
obvious reason to reject priors inspired by previous (standard) analyses of
cluster populations in galaxies, i.e. cluster distributions that scale with
mass as M^-2 and are uniform on a logarithmic age scale.Comment: 17 pages, 13 figures, Accepted for publication in A&A