Colour–colour diagrams and extragalactic globular cluster ages

Systematic uncertainties using the (V – K) – (V – I) diagram

¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead CH41 1LD, UK e-mail: ms@astro.livjm.ac.uk
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, Garching 85748, Germany
³ INAF - Osservatorio Astronomico Collurania, via Mentore Maggini, Teramo 64100, Italy e-mail: cassisi@oa-teramo.inaf.it

Received: 14 July 2006
Accepted: 29 September 2006

Abstract

Context.Age and metallicity estimates for extragalactic globular clusters, from integrated colour–colour diagrams, are examined.

Aims.We investigate biases in cluster ages and [Fe/H] estimated from the  diagram, arising from inconsistent Horizontal Branch morphology, metal mixture, treatment of core convection between observed clusters and the theoretical colour grid employed for age and metallicity determinations. We also study the role played by statistical fluctuations of the observed colours, caused by the low total mass of typical globulars.

Methods.Synthetic samples of globular cluster systems are created, by means of Monte-Carlo techniques. Each sample accounts for a different possible source of bias, among the ones addressed in this investigation. Cumulative age and [Fe/H] distributions are then retrieved by comparisons with a reference theoretical colour–colour grid, and analyzed.

Results.Horizontal Branch morphology is potentially the largest source of uncertainty. A single-age system harbouring a large fraction of clusters with an HB morphology systematically bluer than the one accounted for in the theoretical colour grid, can simulate a bimodal population with an age difference as large as ~8 Gyr. When only the redder clusters are considered, this uncertainty is almost negligible, unless there is an extreme mass loss along the Red Giant Branch phase. The metal mixture affects mainly the redder clusters; the effect of colour fluctuations becomes negligible for the redder clusters, or when the integrated M_V is brighter than ~-8.5 mag. The treatment of core convection is relevant for ages below ~4 Gyr. The retrieved cumulative [Fe/H] distributions are overall only mildly affected. Colour fluctuations and convective core extension have the largest effect. When 1σ photometric errors reach 0.10 mag, all biases found in our analysis are erased, and bimodal age populations with age differences of up to ~8 Gyr go undetected. The use of both and  diagrams may help disclosing the presence of blue HB stars unaccounted for in the theoretical colour calibration.