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

The highly luminous galaxy-merger system NGC 6240 exhibits strong emission lines of vibrationally excited molecular hydrogen in the near-infrared. In disagreement with previous analyses, which have attributed these H₂ lines to shock excitation or X-ray irradiation, we show here that the published line ratios imply ultraviolet excitation followed by infrared fluorescence. We apply the method of analysis developed for galactic sources to the central ∼2 kpc of NGC 6240 and show that the line ratios are well reproduced as a mixture of fluorescent and thermal components. Although the fluorescent component contributes only 30% to the υ = 1-0 S(1) intensity, fluorescent lines from higher vibrational levels makes up a large luminosity which overwhelms the thermal component; the total H₂ luminosity amounts to 3 × 10⁹ L_⊙, of which 70% is due to fluorescence. This luminous fluorescent H₂ emission is excited by ultraviolet (λ ∼ 1000 Å) photons most probably radiated by many (∼1 × 10⁸) early B stars. The abundance of early B stars and the well-known deficit of O stars indicate a sharp upper mass cutoff at ∼20 M_⊙ in the present-day mass spectrum in the central region of NGC 6240. We present a starburst model in which the present-day stellar population and other observed features can be reproduced if (1) the central region of NGC 6240 experienced a cataclysmic starburst which stopped suddenly a few times 10⁷ yr ago and if (2) the initial mass function there had a lower mass cutoff at ∼5 M_⊙. The high mass edge of the present-day mass spectrum can be explained as a result of evolution of massive stars; no artificial high mass cutoff in the initial mass function is necessary, although we do not exclude a possibility of its presence. An important parameter in the present starburst model is the efficiency to convert ultraviolet photons to H₂ infrared fluorescence. The above explanation for NGC 6240 requires the efficiency as high as that in Galactic reflection nebulae (e.g., NGC 2023), while other luminous starburst galaxies may be less efficient by an order of magnitude.