K‐Band Spectroscopy of Compact Star Clusters in NGC 4038/4039

Abstract

Integral field spectroscopy in the K band (1.9-2.4 μm) was performed on four IR-bright star clusters and the two nuclei in NGC 4038/4039 (the Antennae). Two of the clusters are located in the overlap region of the two galaxies and together comprise ≈25% of the total 15 μm emission of the total 4.8 GHz emission from this pair of merging galaxies. The other two clusters, each of them spatially resolved into two components, are located in the northern galaxy, one in the western and one in the eastern loop of blue clusters. Comparing our analysis of Brγ, CO band heads, He I 2.058 μm, Hα (from archival Hubble Space Telescope data), and V-K colors to stellar population synthesis models indicates that the clusters are extincted (AV ≈ 0.7-4.3 mag) and young, displaying a significant age spread (4-13 Myr). The starbursts in the nuclei are much older (65 Myr), with the nucleus of NGC 4038 displaying a region of recent star formation northward of its K-band peak. Using our derived age estimates and assuming the parameters of the initial mass function (Salpeter slope, upper mass cutoff of 100 M☉, Miller-Scalo between 1 and 0.1 M☉), we find that the clusters have masses between 0.4 × 106 and 3 × 106 M☉.