Hubble Space Telescope Observations of NGC 6240: a Case Study of an Ultra-Luminous Infrared Galaxy with Obscured Activity

We present results from an HST study of the morphology and kinematics of NGC 6240. This merging galaxy with a double nucleus is one of the nearest and best-studied ultraluminous infrared galaxies. HST resolves both nuclei into seperate components. The distance between the northern and southern optical/near-infrared components is greater than that observed in radio and X-ray studies, arguing that even in K-band we may not be seeing all the way through the dust to the true nuclei. The ionized gas does not display rotation around either of the nuclei, or equilibrium motion in general. There is a strong velocity gradient between the nuclei, similar to what is seen in CO data. There is no such gradient in our stellar kinematics. The velocity dispersion of the gas is larger than expected for a cold disk. We also map and model the emission-line velocity field at an off-nuclear position where a steep velocity gradient was previously detected in ground-based data. Overall, the data indicate that line-of-sight projection effects, dust absorption, non-equilibrium merger dynamics, and the possible influence of a wind may be playing an important role in the observed kinematics. Chandra observations of hard X-rays have shown that both of the nuclei contain an Active Galactic Nucleus (AGN). The HST data show no clear sign of the two AGNs: neither continuum nor narrow-band imaging shows evidence for unresolved components in the nuclei, and there are no increased emission line widths or rapid rotation near the nuclei. This underscores the importance of X-ray data for identifying AGNs in highly dust-enshrouded environments.