Detection of molecular hydrogen in two merging galaxies

Abstract

NGC6240 and Arp 220 (IC4553) are two of the most luminous IR galaxies known1,2. They are also both thought to be examples of galaxy–galaxy mergers. As part of our study of interacting and merging galaxies1,3,4 we have obtained IR spectra in the 2-µm region of NGC6240 and Arp 220 and detected the ν = 1–0 S(1) quadrupole rotation–vibration emission line of H2 (rest wavelength 2.122 µm) in both galaxies. (This line in NGC6240 has also recently been detected by E. E. Becklin et al., personal communication.) These detections, which double the number of previously published measurements of extragalactic H2 (refs 5, 6) suggest that the merger of two galaxies results in the production of massive quantities of shocked molecular gas. This shocked gas must cool and collapse, leading to an enormous burst of star formation. These measurements of shocked H2 thus strengthen our earlier interpretation1 of the extremely large IR luminosity of NGC6240 in terms of a ‘super-starburst’. Arp 220 may be undergoing similar activity. We suggest that merging galaxies are a unique new class of ultra-luminous IR galaxies.