Warm Molecular Hydrogen in the Spitzer 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 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 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 μm) 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 ortho-to-para ratios, consistent with the effects of FUV pumping combined with incomplete ortho-para thermalization, or possibly nonequilibrium photodissociation fronts.

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