Mass Segregation and the Initial Mass Function of Super Star Cluster M82‐F

Abstract

We investigate the initial mass function and mass segregation in superstar cluster M82-F with high-resolution Keck NIRSPEC echellespectroscopy. Cross-correlation with template supergiant spectraprovides the velocity dispersion of the cluster, enabling measurement ofthe kinematic (virial) mass of the cluster when combined with sizes fromNICMOS and Advanced Camera for Surveys (ACS) images. We find a mass of6.6+/-0.9{\times}10^{5}M_{solar} based on near-IR lightand 7.0+/-1.2{\times}10^{5}M_{solar} based on opticallight. Using PSF-fitting photometry, we derive the cluster'slight-to-mass (L/M) ratio in both near-IR and optical light and compareto population-synthesis models. The ratios are inconsistent with anormal stellar initial mass function for the adopted age of 40-60 Myr,suggesting a deficiency of low-mass stars within the volume sampled.King model light profile fits to new Hubble Space Telescope ACS imagesof M82-F, in combination with fits to archival near-IR images, indicatemass segregation in the cluster. As a result, the virial mass representsa lower limit on the mass of the cluster.Based on observations made at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.