2007ApJ...669..959R

Result. on the properties of warm molecular hydrogen in 57 normal galaxies are derived from measurements of H₂ rotational transitions, obtained as part of SINGS. This study extends previous extragalactic surveys of emission lines of H₂ to fainter and more common systems (L_FIR=10⁷-6x10¹⁰ L_☉). The 17 µm S(1) transition is securely detected in the nuclear regions of 86% of galaxies with stellar masses above 10^9.5 M_☉. The derived column densities of warm H₂(T≥100 K), although averaged over kiloparsec-scale areas, are commensurate with values observed in resolved photodissociation regions. They amount to between 1% and >30% of the total H₂. The power emitted in the three lowest energy transitions is on average 30% of the power of the bright [Si II] cooling line (34.8 µm) and about 4x10^–4 of the total infrared power for star-forming galaxies, which is consistent with excitation in PDRs. The fact that the H₂ line intensities scale tightly with the aromatic band emission, even though the average radiation field intensity varies by a factor of 10, can also be understood if both tracers originate predominantly in PDRs, either dense or diffuse. Many of the 25 LINER/Seyfert targets strongly depart from the rest of the sample, in having warmer excited H₂ and excess H₂rotational power with respect to the dust emission. We propose a threshold in H₂-to-aromatic band power ratios, allowing the identification of low-luminosity AGNs by an excess H₂ excitation. A dominant contribution from shock heating is favored in these objects. Finally, we detect in nearly half the star-forming targets nonequilibrium ortho-to-para ratios, consistent with the effects of FUV pumping combined with incomplete ortho-para thermalization, or possibly nonequilibrium photodissociation fronts.