Circumnuclear Star Clusters in the Galaxy Merger NGC 6240, Observed with Keck Adaptive Optics and the Hubble Space Telescope

We discuss images of the central ~10 kpc (in projection) of the galaxy merger NGC 6240 at H and K' bands, taken with the NIRC2 narrow camera on Keck II using natural guide star adaptive optics. We detect 28 star clusters in the NIRC2 images, of which only 7 can be seen in the similar spatial resolution, archival WFPC2 Planetary Camera data at either B or I band. Combining the NIRC2 narrow camera pointings with wider NICMOS NIC2 images taken with the F110W, F160W, and F222M filters, we identify a total of 32 clusters that are detected in at least one of these five infrared (λ_c > 1 μm) bandpasses. By comparing to instantaneous burst, stellar population synthesis models (Bruzual & Charlot), we estimate that most of the clusters are consistent with being ~15 Myr old and have photometric masses ranging from 7 × 10⁵ to 4 × 10⁷ M_☉. The total contribution to the star formation rate (SFR) from these clusters is approximately 10 M_☉ yr^-1, or ~10% of the total SFR in the nuclear region. We use these newly discovered clusters to estimate the extinction toward NGC 6240's double nuclei and find values of A_V as high as 14 mag along some sight lines, with an average extinction of A_V ~ 7 mag toward sight lines within ~3'' of the double nuclei.