H₂ emission as a tracer of molecular hydrogen: Large-scale observations of orion

We have detected extremely extended (>1°.5, or 12 pc) near-infrared H₂ line emission from the Orion A molecular cloud. We have mapped emission in the 1.601 μm v = 6-4 Q(1) and 2.121 μm v = 1-0 S(1) lines of H₂ along a ∼ 2° R.A. cut and from a 6′ × 6′ region near θ¹ Ori C. The surface brightness of the extended H₂ line emission is 10^-6 to 10^-5 ergs s^-1 cm^-2 sr^-1. Based on the distribution and relative strengths of the H₂ lines, we conclude that UV fluorescence is most likely the dominant H₂ emission mechanism in the outer parts of the Orion cloud. Shock-heated gas does not make a major contribution to the H₂ emission in this region. The fluorescent component of the total H₂ v = 1-0 S(1) luminosity from Orion is 30-40 L_⊙. Molecular hydrogen excited by UV radiation from nearby OB stars contributes 98%-99% of the global H₂ line emission from the Orion molecular cloud, even though this cloud has a powerful shock-excited H₂ source in its core. The ability to detect large-scale H₂ directly opens up new possibilities for the study of molecular clouds.