The old globular cluster system of NGC 4365: new VLT/FORS2 spectra

TL;DR

This study provides new spectroscopic data on globular clusters in NGC 4365, showing they are mostly old (~8-14 Gyrs) with metallicities from -1.3 to 0.3 dex, challenging previous claims of intermediate-age clusters.

Contribution

The paper offers new VLT/FORS2 spectra and analysis that clarify the ages and metallicities of GCs in NGC 4365, resolving contradictions from earlier studies.

Findings

Most GCs are old (~8-14 Gyrs) with large uncertainties.

GC metallicities range from [Fe/H]~-1.3 to 0.3 dex.

Data is consistent with 14 Gyr models for optical colours.

Abstract

We present new spectroscopic observations of a sample of globular clusters (GCs) in the Virgo Cluster elliptical NGC4365. The great majority of the objects analysed here, overlap on purpose with objects presented in two previous studies that came to contradictory conclusions. We measure Lick indices for the GCs and infer ages, abundances and alpha-enhancement values by comparing to population synthesis models. Our results do not support the existence of numerous intermediate-age GCs in this galaxy as suggested by some of the past studies. Instead, we find ages, consistent with the range ~8-14 Gyrs (model dependent and with large uncertainties). We also find that the newly observed GCs have [Fe/H]\sim-1.3 to 0.3 dex and the majority are consistent with [alpha/Fe]\sim0.3 dex, independent of metallicity. By comparing the metal sensitive index [MgFe] with optical colours, we find that our data is consistent with 14Gyr models. However for optical/near-infrared colours we find that the same models would have to be redder by ~0.2 mag in order to be consistent with our data. This offset is not found to be consistent with an age effect as it is even larger for younger models.