Warm Molecular Hydrogen in the Spitzer SINGS Galaxy Sample

Results on the properties of warm molecular hydrogen in 57 normal galaxies are derived from measurements of H sub(2) rotational transitions, obtained as part of SINGS. This study extends previous extragalactic surveys of emission lines of H sub(2) to fainter and more common systems ( [unk]=10 super(7)-6 x 10 super(10) L [unk],). The 17 mu m S(1) transition is securely detected in the nuclear regions of 86% of galaxies with stellar masses above 10 super(9.5) M [unk]. The derived column densities of warm H2 (T greater than or equal to 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 H sub(2). 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 mu m) and about 4 x 10 super(-4) of the total infrared power for star-forming galaxies, which is consistent with excitation in PDRs. The fact that the H sub(2) 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 H sub(2) and excess H sub(2) rotational power with respect to the dust emission. We propose a threshold in H sub(2)-to-aromatic band power ratios, allowing the identification of low-luminosity AGNs by an excess H sub(2) 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.