SCYON Abstract

Received on September 21 2004

Gemini/GMOS Imaging of Globular Clusters in the Virgo Galaxy NGC 4649 (M60)

AuthorsDuncan A. Forbes 1, Favio Raul Faifer2, Juan Carlos Forte2,Terry Bridges3, Michael A. Beasley4, Karl Gebhardt5, David A. Hanes3, Ray Sharples6, Stephen E. Zepf7
Affiliation1 Swinburne University
3 Queen's University
5 University of Texas
6 Durham University
7 Michigan State University
Accepted byMonthly Notices of the Royal Astronomical Society


We present Sloan g and i imaging from the GMOS instrument on the Gemini North telescope for the globular cluster (GC) system around the Virgo galaxy NGC 4649 (M60). Our three pointings, taken in good seeing conditions, cover an area of about 90 sq. arcmins. We detect 2,151 unresolved sources. Applying colour and magnitude selection criteria to this source list gives 995 candidate GCs that is greater than 90% complete to a magnitude of i = 23.6, with little contamination from background galaxies. We find fewer than half a dozen potential Ultra Compact Dwarf galaxies around NGC 4649. Foreground extinction from the nearby spiral NGC 4647 is limited to be A_V < 0.1. We confirm the bimodality in the GC colour distribution found by earlier work using HST/WFPC2 imaging. As is commonly seen in other galaxies, the red GCs are concentrated towards the centre of the galaxy, having a steeper number density profile than the blue GC subpopulation. The varying ratio of red-to-blue GCs with radius can largely explain the overall GC system colour gradient. The underlying galaxy starlight has a similar density profile slope and colour to the red GCs. This suggests a direct connection between the galaxy field stars and the red GC subpopulation. We estimate a total GC population of 3700 #&177; 900, with the uncertainty dominated by the extrapolation to larger radii than observed. This total number corresponds to a specific frequency S_N = 4.1 #&177; 1.0. Future work will present properties derived from GMOS spectra of the NGC 4649 GCs.