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Warm Molecular Gas in M51: Mapping the Excitation Temperature and Mass of H_2 with the Spitzer Infrared Spectrograph | | G. Brunner
; K. Sheth
; L. Armus
; M. Wolfire
; S. Vogel
; E. Schinnerer
; G. Helou
; R. Dufour
; J. Smith
; D. Dale
; | | Date: |
30 Nov 2007 | | Abstract: | 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. | | Source: | arXiv, 0712.0022 | | Services: | Forum | Review | PDF | Favorites | |
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