Warm Molecular Hydrogen in the<i>Spitzer</i>SINGS Galaxy Sample

Abstract

Results on the properties of warm molecular hydrogen in 57 normal galaxies are derived from measurements of H 2 rotational transitions, obtained as part of SINGS. This study extends previous extragalactic surveys of emission lines of H 2 to fainter and more common systems (L FIR 10 7 6 ; 10 10 L ). The 17 m 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 H 2 (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 H 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 m) and about 4 ; 10 4 of the total infrared power for star-forming galaxies, which is consistent with excitation in PDRs. The fact that the H 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 2 and excess H 2 rotational power with respect to the dust emission. We propose a threshold in H 2 -to-aromatic band power ratios, allowing the identification of low-luminosity AGNs by an excess H 2 excitation. A dominant contribution from shock heating is favored in these objects. Finally, we detect in nearly half the star-forming targets nonequilibrium orthoto-para ratios, consistent with the effects of FUV pumping combined with incomplete ortho-para thermalization, or possibly nonequilibrium photodissociation fronts.