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We have mapped the warm molecular gas traced by the H_2 S(0) - H_2 S(5) purerotational mid-infrared emission lines over a radial strip across the nucleusand disk of M51 (NGC 5194) using the Infrared Spectrograph (IRS) on the SpitzerSpace Telescope. The six H_2 lines have markedly different emissiondistributions. We obtained the H_2 temperature and surface densitydistributions by assuming a two temperature model: a warm (T = 100 - 300 K)phase traced by the low J (S(0) - S(2)) lines and a hot phase (T = 400 - 1000K) traced by the high J (S(2) - S(5)) lines. The lowest molecular gastemperatures are found within the spiral arms (T ~ 155 K), while the highesttemperatures are found in the inter-arm regions (T > 700 K). The warm gassurface density reaches a maximum of 11 M_sun/pc^2 in the northwestern spiralarm, whereas the hot gas surface density peaks at 0.24 M_sun/pc^2 at thenucleus. The spatial offset between the peaks in the warm and hot phases andthe differences in the distributions of the H_2 line emission suggest that thewarm phase is mostly produced by UV photons in star forming regions while thehot phase is mostly produced by shocks or X-rays associated with nuclearactivity. The warm H_2 is found in the dust lanes of M51, spatially offset fromthe brightest HII regions. The warm H_2 is generally spatially coincident withthe cold molecular gas traced by CO (J = 1 - 0) emission, consistent withexcitation of the warm phase in dense photodissociation regions (PDRs). Incontrast, the hot H_2 is most prominent in the nuclear region. Here, over a 0.5kpc radius around the nucleus of M51, the hot H_2 coincides with [O IV](25.89micron) and X-ray emission indicating that shocks and/or X-rays are responsiblefor exciting this phase.

Warm Molecular Gas in M51: Mapping the Excitation Temperature and Mass of H_2 with the Spitzer Infrared Spectrograph