The Core of NGC 6240 from Keck Adaptive Optics and Hubble Space Telescope NICMOS Observations

Abstract

We present results of near-infrared imaging of the disk-galaxy merger NGC 6240 using adaptive optics on the Keck II telescope and reprocessed archival data from NICMOS on the Hubble Space Telescope. Both the northern and southern nuclei of NGC 6240 are clearly elongated, with considerable substructure within each nucleus. In K' band there are at least two point sources within the northern nucleus; we tentatively identify the southwestern point source within the northern nucleus as the position of one of the two active galactic nuclei (AGNs). Within the southern nucleus, the northern subnucleus is more highly reddened. Based on the nuclear separation measured at 5 GHz, we suggest that the AGN in the southern nucleus is still enshrouded in dust at K' band and is located slightly to the north of the brightest point in K' band. Within the southern nucleus there is strong H2 1-0 S(1) line emission from the northern subnucleus, contrary to the conclusions of previous seeing-limited observations. Narrowband H2 emission line images show that a streamer or ribbon of excited molecular hydrogen connects the northern and southern nuclei. We suggest that this linear feature corresponds to a bridge of gas connecting the two nuclei, as seen in computer simulations of mergers. Many pointlike regions are seen around the two nuclei. These are most prominent at 1.1 μm with NICMOS and in K' band with Keck adaptive optics. We suggest that these point sources represent young star clusters formed in the course of the merger.