Near-Infrared Line Imaging of NGC 6240: Collision Shock and Nuclear Starburst
Abstract
Images of the merging luminous IR galaxy NGC 6240 in the H_2_ ν = 1 - 0 S(1)2.12 micron line and the [Fe II] 1.64 micron line are presented, together with velocity-resolved slit spectra of these lines. The images have an angular resolution of < ~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 ~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 ~ 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]/Brγ ratio indicates a high gas-phase iron abundance, resulting from grain destruction in the SNR shocks. The high [Fe II]/Brγ, radio/Brγ and far-IR/Brγ ratios in NGC 6240 indicate a deficiency in stars more massive than ~25 M_sun_, 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 1993
- DOI:
- 10.1086/172384
- Bibcode:
- 1993ApJ...405..522V
- Keywords:
-
- Galactic Nuclei;
- Infrared Imagery;
- Interacting Galaxies;
- Near Infrared Radiation;
- Shock Wave Propagation;
- Star Formation;
- Starburst Galaxies;
- Hydrogen;
- Hydrogen Clouds;
- Iron;
- Line Spectra;
- Supernova Remnants;
- Supernovae;
- Astrophysics;
- GALAXIES: INDIVIDUAL NGC NUMBER: NGC 6240;
- GALAXIES: INTERACTIONS;
- GALAXIES: NUCLEI;
- GALAXIES: STARBURST;
- INFRARED: GALAXIES