We have studied the molecular hydrogen energetics of the edge-on spiral galaxy NGC?891, using a 34 position map in the lowest three pure rotational H2 lines observed with the Spitzer Infrared Spectrograph. The S(0), S(1), and S(2) lines are bright with an extinction-corrected total luminosity of ~2.8 × 107 L ?, or 0.09% of the total-infrared luminosity of NGC?891. The H2 line ratios are nearly constant along the plane of the galaxy?we do not observe the previously reported strong drop-off in the S(1)/S(0) line intensity ratio in the outer regions of the galaxy, so we find no evidence for the very massive cold CO-free molecular clouds invoked to explain the past observations. The H2 level excitation temperatures increase monotonically indicating that there is more than one component to the emitting gas. More than 99% of the mass is in the lowest excitation (T ex ~ 125 K) "warm" component. In the inner galaxy, the warm H2 emitting gas is ~16% of the CO(1-0)-traced cool molecular gas, while in the outer regions the fraction is twice as high. This large mass of warm gas is heated by a combination of the far-UV photons from stars in photodissociation regions (PDRs) and the dissipation of turbulent kinetic energy. Including the observed far-infrared [O I] and [C II] fine-structure line emission and far-infrared continuum emission in a self-consistent manner to constrain the PDR models, we find essentially all of theS(0) and most (70%) of the S(1) line arise from low excitation PDRs, while most (80%) of the S(2) and the remainder of the S(1) line emission arise from low-velocity microturbulent dissipation.
Available at: http://works.bepress.com/sarah_higdon/48/