Warm Molecular Gas in M51: Mapping the Excitation Temperature and Mass of H₂ with the Spitzer Infrared Spectrograph

We have mapped the warm molecular gas traced by the H₂ S(0)-H₂ 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 H₂ lines have markedly different emission distributions. We obtained the H₂ 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_☉ pc⁻² in the northwest spiral arm, whereas the hot gas surface density peaks at 0.24 M_☉ pc⁻² at the nucleus. The spatial offset between the peaks in the different phases suggests that the warm phase is more efficiently heated by star formation activity and the hot phase is more efficiently heated by nuclear activity. The warm H₂ is found in the dust lanes of M51 and is generally spatially coincident with the cold molecular gas traced by CO emission, consistent with excitation of the warm phase in dense photodissociation regions. The hot H₂ is most prominent in the nuclear region. Here, the hot H₂ coincides with [O IV] (25.89 μm) and X-ray emission indicating that shocks and/or X-rays are responsible for exciting this phase.