Near-Infrared Line Imaging Of Ngc 6240 - Collision Shock And Nuclear Starburst

Images of the merging luminous IR galaxy NGC 6240 in the H-2 upsilon = 1 --> 0 S(1) 2.12 mum line and the [Fe II] 1.64 mum line are presented, together with velocity-resolved slit spectra of these lines. The images have an angular resolution of less than or similar 1'', and for the H-2 line, images in three adjacent 320 km s-1 wide velocity intervals are presented. The H-2 emission does not follow the stellar light and shows no trace of the two nuclei of the galaxy. Instead, it peaks between these nuclei. The H-2 emission extends over approximately 5 kpc and shows a complex morphology and velocity structure in its outer parts. It is concluded that the bright H-2 emission near the nuclei is generated in shocks resulting from the collision of the interstellar media of the merging galaxies. Line ratios indicate that the shock velocity is at most 40 km s-1. It is argued that a fast shock propagating in a low-density medium generates slower shocks in dense clouds, giving rise to the observed H-2 emission. It is shown that the shocks in NGC 6240 cannot account for the observed far-IR emission. High-velocity wings on the H-2 line profile (FWZI approximately 1600 km s-1) are interpreted as evidence for molecular material entrained in and shocked by the ''superwind'' in NGC 6240, created by multiple supernova explosions in a nuclear starburst. In contrast to the H-2 emission, the [Fe II] emission comes from the nuclei of the galaxy and is generated in fast supernova remnant (SNR) shocks in the nuclear starbursts. The high [Fe II]/Brgamma ratio indicates a high gas-phase iron abundance, resulting from grain destruction in the SNR shocks. The high [Fe II]/Brgamma, radio/Brgamma and far-IR/Brgamma ratios in NGC 6240 indicate a deficiency in stars more massive than approximately 25 M., which can be accounted for by a decrease with time in nuclear star formation activity. This decrease may be due to the expulsion of molecular gas from the nuclei by the ''superwind'' created by the nuclear starburst, which is confirmed by the detection of high-velocity H-2 emission from material entrained in the superwind.