Colour-colour diagrams and extragalactic globular cluster ages. Systematic uncertainties using the (V-K)-(V-I) diagram

TL;DR

This study examines biases in estimating globular cluster ages and metallicities using the (V-K)-(V-I) diagram, highlighting the impact of Horizontal Branch morphology, metal mixture, and statistical colour fluctuations on the accuracy of these estimates.

Contribution

It identifies key sources of systematic uncertainties in age and metallicity determinations from colour diagrams, and evaluates how observational errors influence these biases.

Findings

Horizontal Branch morphology causes significant age estimation biases.

Colour fluctuations can mask true age differences up to 8 Gyr.

Using multiple colour diagrams can help detect unaccounted blue HB stars.

Abstract

(abridged) We investigate biases in cluster ages and [Fe/H] estimated from the (V-K)-(V-I) 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. 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 Mv is brighter than -8.5 mag. The treatment of core convection is relevant for ages below ~4 Gyr. The retrieved [Fe/H] distributions are overall only mildly affected. Colour fluctuations and convective core extension have the largest effect. When 1sigma 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 (U-I)-(V-K) and (V-I)-(V-K) diagrams may help disclosing the presence of blue HB stars unaccounted for in the theoretical colour calibration.