Warm molecular hydrogen in the Spitzer SINGS galaxy sample - AstrophysicsReport as inadecuate

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Abstract: simplified Results on the properties of warm H2 in 57 normal galaxies arederived from H2 rotational transitions, obtained as part of SINGS. This studyextends previous extragalactic surveys of H2, the most abundant constituent ofthe molecular ISM, to more common systems L FIR = e7 to 6e10 L sun of allmorphological and nuclear types. The S1 transition is securely detected inthe nuclear regions of 86% of SINGS galaxies with stellar masses above 10^9.5M sun. The derived column densities of warm H2 T > ~100 K, even thoughaveraged over kiloparsec-scale areas, are commensurate with those of resolvedPDRs; the median of the sample is 3e20 cm-2. They amount to between 1% and >30%of the total H2. The power emitted in the sum of the S0 to S2 transitionsis on average 30% of the SiII line power, and ~4e-4 of the total infraredpower TIR within the same area for star-forming galaxies, which is consistentwith excitation in PDRs. The fact that H2 emission scales tightly with PAHemission, even though the average radiation field intensity varies by a factorten, can also be understood if both tracers originate predominantly in PDRs,either dense or diffuse. A large fraction of the 25 LINER-Sy targets, however,strongly depart from the rest of the sample, in having warmer H2 in the excitedstates, and an excess of H2 emission with respect to PAHs, TIR and SiII. Wepropose a threshold in H2 to PAH power ratios, allowing the identification oflow-luminosity AGNs by an excess H2 excitation. A dominant contribution fromshock heating is favored in these objects. Finally, we detect, in nearly halfthe star-forming targets, non-equilibrium ortho to para ratios, consistent withFUV pumping combined with incomplete ortho-para thermalization by collisions,or possibly non-equilibrium PDR fronts advancing into cold gas.

Author: H. Roussel 1, G. Helou 2, D.J. Hollenbach 3, B.T. Draine 4, J.D. Smith 5, L. Armus 2, E. Schinnerer 1, F. Walter 1, C.W. Engelbra

Source: https://arxiv.org/

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