Hubble Space TelescopeObservations of NGC 6240: A Case Study of an Ultraluminous Infrared Galaxy with Obscured Activity

We present results from an HST study of the morphology and kinematics of NGC6240. This merging galaxy with a double nucleus is one of the nearest andbest-studied ultraluminous infrared galaxies. HST resolves both nuclei intoseperate components. The distance between the northern and southernoptical/near-infrared components is greater than that observed in radio andX-ray studies, arguing that even in K-band we may not be seeing all the waythrough the dust to the true nuclei. The ionized gas does not display rotationaround either of the nuclei, or equilibrium motion in general. There is astrong velocity gradient between the nuclei, similar to what is seen in COdata. There is no such gradient in our stellar kinematics. The velocitydispersion of the gas is larger than expected for a cold disk. We also map andmodel the emission-line velocity field at an off-nuclear position where a steepvelocity gradient was previously detected in ground-based data. Overall, thedata indicate that line-of-sight projection effects, dust absorption,non-equilibrium merger dynamics, and the possible influence of a wind may beplaying an important role in the observed kinematics. Chandra observations ofhard X-rays have shown that both of the nuclei contain an Active GalacticNucleus (AGN). The HST data show no clear sign of the two AGNs: neithercontinuum nor narrow-band imaging shows evidence for unresolved components inthe nuclei, and there are no increased emission line widths or rapid rotationnear the nuclei. This underscores the importance of X-ray data for identifyingAGNs in highly dust-enshrouded environments.