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We have mapped the warm molecular gas traced by the H_2 S(0) - H_2 S(5) pure rotational mid-infrared emission lines over a radial strip across the nucleus and disk of M51 (NGC 5194) using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The six H_2 lines have markedly different emission distributions. We obtained the H_2 temperature and surface density distributions 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 - 1000 K) traced by the high J (S(2) - S(5)) lines. The lowest molecular gas temperatures are found within the spiral arms (T ~ 155 K), while the highest temperatures are found in the inter-arm regions (T > 700 K). The warm gas surface density reaches a maximum of 11 M_sun/pc^2 in the northwestern spiral arm, whereas the hot gas surface density peaks at 0.24 M_sun/pc^2 at the nucleus. The spatial offset between the peaks in the warm and hot phases and the differences in the distributions of the H_2 line emission suggest that the warm phase is mostly produced by UV photons in star forming regions while the hot phase is mostly produced by shocks or X-rays associated with nuclear activity. The warm H_2 is found in the dust lanes of M51, spatially offset from the brightest HII regions. The warm H_2 is generally spatially coincident with the cold molecular gas traced by CO (J = 1 - 0) emission, consistent with excitation of the warm phase in dense photodissociation regions (PDRs). In contrast, the hot H_2 is most prominent in the nuclear region. Here, over a 0.5 kpc radius around the nucleus of M51, the hot H_2 coincides with [O IV](25.89 micron) and X-ray emission indicating that shocks and/or X-rays are responsible for exciting this phase.
We present our initial results on the CO rotational spectral line energy distribution (SLED) of the $J$ to $J$$-$1 transitions from $J=4$ up to $13$ from Herschel SPIRE spectroscopic observations of 65 luminous infrared galaxies (LIRGs) in the Great
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(simplified) Results on the properties of warm H2 in 57 normal galaxies are derived from H2 rotational transitions, obtained as part of SINGS. This study extends previous extragalactic surveys of H2, the most abundant constituent of the molecular ISM
Observations of dense molecular gas lie at the basis of our understanding of the density and temperature structure of protostellar envelopes and molecular outflows. We aim to characterize the properties of the protostellar envelope, molecular outflow