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

We discuss images of the central ~10 kpc (in projection) of the galaxy mergerNGC 6240 at H and K' bands, taken with the NIRC2 narrow camera on Keck II usingnatural guide star adaptive optics. We detect 28 star clusters in the NIRC2images, of which only 7 can be seen in the similar-spatial-resolution, archivalWFPC2 Planetary Camera data at either B or I bands. Combining the NIRC2 narrowcamera pointings with wider NICMOS NIC2 images taken with the F110W, F160W, andF222M filters, we identify a total of 32 clusters that are detected in at leastone of these 5 infrared (lambda > 1 micron) bandpasses. By comparing toinstantaneous burst, stellar population synthesis models (Bruzual & Charlot2003), we estimate that most of the clusters are consistent with being ~15 Myrold and have photometric masses ranging from 7E5 M_sun to 4E7 M_sun. The totalcontribution to the star formation rate (SFR) from these clusters isapproximately 10 M_sun/year, or ~10% of the total SFR in the nuclear region. Weuse these newly discovered clusters to estimate the extinction toward NGC6240's double nuclei, and find values of A_V as high as 14 magnitudes alongsome sightlines, with an average extinction of A_V~7 mag toward sightlineswithin ~3 arcsec of the double nuclei.Comment: 37 pages, 9 figures; Accepted for publication by Ap