Globular cluster systems of six shell galaxies

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: sikkema@astro.rug.nl
² Astrophysics Research Institute, Liverpool John Moores University, 12 Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
³ Instituto de Astrofísica de Canarias, vía Láctea s/n, La Laguna 38200, Spain

Received: 29 November 2005
Accepted: 14 June 2006

Abstract

Context.Shells in Elliptical Galaxies are faint, sharp-edged features, believed to provide evidence of a recent ( years ago) merger event. We analyse the Globular Cluster (GC) systems of six shell elliptical galaxies, to examine the effects of mergers upon the GC formation history.

Aims.We examine the colour distributions, and investigate differences between red and blue globular cluster populations. We present luminosity functions, spatial distributions and specific frequencies () at 50 kpc radius for our sample.

Methods.We present V and I magnitudes for cluster candidates measured with the HST Advanced Camera for Surveys (ACS). Galaxy background light is modelled and removed, and magnitudes are measured in 8 pixel (0.4 arcsec) diameter apertures. Background contamination is removed using counts from Hubble Deep Field South.

Results. We find that the colour distributions for NGC 3923 and NGC 5982 have a bimodal form typical of bright ellipticals, with peaks near and . In NGC 7626, we find in addition a population of abnormally luminous clusters at M_I=-12.5. In NGC 2865 we find an unusually blue population, which may also be young. In NGC 1344 and NGC 474 the red cluster population is marginally detected. The radial surface density profiles are more flattened than the galaxy light in the cores. As already known, in NGC 3923, which has a high of 5.6, the radial density distribution is more shallower than the diffuse galaxy light.

Conclusions.The clusters in NGC 2865 and NGC 7626 provide evidence for formation of a population associated with a recent merger. In the other galaxies, the properties of the clusters are similar to those observed in other, non-shell, elliptical galaxies.