Infrared fluorescence of H2 in NGC 6240 - A starburst origin for the H2 luminosity

It is shown here that the published emission line ratios for vibrationally excited molecular hydrogen in the near-IR for the highly luminous galaxy merger system NGC 6240 imply UV excitation followed by IR fluorescence. The line ratios for the central 2 kpc or so of the system are well-reproduced as a mixture of fluorescent and thermal components. Of the total H2 luminosity of 3 {times} 10 to the 9th solar, 70 percent is due to fluorescence. This emission is excited by UV photons probably radiated by many early B stars. The abundance of such stars and the deficit of O stars indicate a sharp upper mass cutoff at about 20 solar masses in the present-day mass spectrum in the central region. A starburst model is presented in which the observations can be reproduced if the central region experienced a cataclysmic starburst which stopped suddenly a few ten billion yr ago and if the initial mass function there had a lower mass cutoff at about 5 solar. 31 refs.