Extragalactic Globular Clusters in the Near Infrared I: A comparison between M87 and NGC 4478

TL;DR

This study compares optical and near-infrared colours of globular clusters in M87 and NGC 4478, revealing differences in their sub-populations and providing insights into their formation histories and metallicity distributions.

Contribution

It provides a detailed comparison of globular cluster populations in two Virgo galaxies using multi-band photometry, highlighting the presence of diverse sub-populations and their properties.

Findings

NGC 4478 hosts only a blue, metal-poor globular cluster sub-population.

M87 contains both blue and red, potentially intermediate-age clusters.

Red clusters in M87 show complex metallicity and age structures.

Abstract

We compare optical and near infrared colours of globular clusters in M87, the central giant elliptical in Virgo, and NGC 4478, an intermediate luminosity galaxy in Virgo, close in projection to M87. Combining V and I photometry obtained with the WFPC2 on HST and Ks photometry obtained with the NIRC on Keck 1, we find the broad range in colour and previously detected bi-modality in M87. We confirm that NGC 4478 only hosts a blue sub-population of globular clusters and now show that these clusters' V-I and V-K colours are very similar to those of the halo globular clusters in Milky Way and M31. Most likely, a metal-rich sub-population never formed around this galaxy (rather than having formed and been destroyed later), probably because its metal-rich gas was stripped during its passage through the centre of the Virgo cluster. The V-I, V-K colours are close to the predicted colours from SSP models for old populations. However, M87 hosts a few red clusters that are best explained by intermediate ages (a few Gyr). Generally, there is evidence that the red, metal-rich sub-population has a complex colour structure and is itself composed of clusters spanning a large metallicity and, potentially, age range. This contrasts with the blue, metal-poor population which appears very homogeneous in all galaxies observed so far.