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

Abstract

We investigate the initial mass function and mass segregation in super star cluster M82-F with high-resolution Keck NIRSPEC echelle spectroscopy. Cross-correlation with template supergiant spectra provides the velocity dispersion of the cluster, enabling measurement of the kinematic (virial) mass of the cluster when combined with sizes from NICMOS and Advanced Camera for Surveys (ACS) images. We find a mass of 6:6 AE 0:9 ; 10 5 M based on near-IR light and 7:0 AE 1:2 ; 10 5 M based on optical light. Using PSF-fitting photometry, we derive the cluster's light-to-mass (L/M ) ratio in both near-IR and optical light and compare to population-synthesis models. The ratios are inconsistent with a normal 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 images of M82-F, in combination with fits to archival near-IR images, indicate mass segregation in the cluster. As a result, the virial mass represents a lower limit on the mass of the cluster.

A Universal Stellar Initial Mass Function? A Critical Look at Variations

“…This measurement suggests M82F is highly deficient in low mass stars. McCrady, Graham and Vacca (2005) measured the velocity dispersion of the cluster in the near-IR and re-measured its size on HST High Resolution Camera imaging, and came to the same conclusion. Based on a filter dependent size (i.e.…”

“…More worrisome is that some of these results have been derived within the same galaxy (e.g. M82, McCrady et al 2003). noted that there was a trend in how well cluster L V /M dyn ratios were fit by SSP models, in the sense that older clusters (> 20 Myr) were well fit (with one exception to be discussed below) with a canonical IMF, while there exists a significant scatter in young clusters.…”

The M / L * Ratio of Young Star Clusters in Galactic Mergers

“…This larger value of h would go some way toward solving the apparent overluminosity of young M82 clusters with respect to a standard IMF. Recent work on M82-F has highlighted the possibility of strong mass segregation in that cluster (McCrady et al 2005).…”

Feedback in the Antennae Galaxies (NGC 4038/9). I. High‐Resolution Infrared Spectroscopy of Winds from Super Star Clusters

“…The IMF of SSCs is especially important for this question because the light of SSCs is dominated by massive stars, while old GCs are comprised entirely of subsolar mass stars (and stellar remnants). Observations of SSC dynamical masses to date suggest that some SSCs do have normal IMFs and could become GCs, while others cannot (e.g., Sternberg 1998; Smith & Gallagher 2001;Mengel et al 2002;Gilbert & Graham 2001;McCrady et al 2003McCrady et al , 2005, although these results could be subject to systematic errors in extinction corrections or neglect of effects like spatial mass segregation, and they always rely on the assumption of cluster virialization.…”