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NGC 6240 is a pair of colliding disk galaxies, each with a black hole in itscore. We have used laser guide star adaptive optics on the Keck II telescope toobtain high-resolution ($\sim 0.06$") near-infrared integral-field spectra ofthe region surrounding the supermassive black hole in the south nucleus of thisgalaxy merger. We use the K-band CO absorption bandheads to trace stellarkinematics. We obtain a spatial resolution of about 20 pc and thus directlyresolve the sphere of gravitational influence of the massive black hole. Weexplore two different methods to measure the black hole mass. Using a JeansAxisymmetric Multi-Gaussian mass model, we investigate the limit that a relaxedmass distribution produces all of the measured velocity dispersion, and find anupper limit on the black hole mass at $2.0 \pm 0.2 \times 10^9 M_{\sun}$. Whenassuming the young stars whose spectra we observe remain in a thin disk, wecompare Keplerian velocity fields to the measured two-dimensional velocityfield measured and fit for a mass profile containing a black hole point massplus a radially-varying spherical component, which suggests a lower limit forthe black hole mass of $8.7 \pm 0.3 \times 10^8 M_{\sun}$. Our measurements ofthe stellar velocity dispersion place this AGN within the scatter of the$M_{BH}$-$\sigma_{*}$ relation. As NGC 6240 is a merging system, this mayindicate that the relation is preserved during a merger at least until thefinal coalescence of the two nuclei.

Mass of the Southern Black Hole in NGC 6240 from Laser Guide Star Adaptive Optics