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

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 bands. 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 5 infrared (λc> 1 µm) bandpasses. By comparing to instantaneous burst, stellar population synthesis models (Bruzual & Charlot 2003), we estimate that most of the clusters are consistent with being ∼15 Myr old and have photometric masses ranging from 7×10 5 M ⊙ to 4×10 7 M⊙. The total contribution to the star formation rate (SFR) from these clusters is approximately 10M ⊙ 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 AV as high as 14 magnitudes along some sightlines, with an average extinction of AV ∼ 7 mag toward sightlines within ∼ 3 ′ ′ of the double nuclei