Extragalactic globular clusters in the near infrared*
I. A comparison between M87 and NGC 4478
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: email@example.com
2 UCO/Lick observatory, University of California, Santa Cruz, CA 95064, USA e-mail: firstname.lastname@example.org
3 Departamento de Astronomía y Astrofísica, P. Universidad Católica, Casilla 104, Santiago 22, Chile e-mail: email@example.com
Corresponding author: M. Kissler-Patig, firstname.lastname@example.org
Accepted: 4 June 2002
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' and 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 , 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.
Key words: galaxies clusters: general / galaxies: star clusters / galaxies: individual: M87, NGC 4478
Data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.
© ESO, 2002