Probing the cold and warm molecular gas in the Whirlpool Galaxy: Herschel SPIRE-FTS observations of the central region of M51 (NGC 5194)
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We present Herschel SPIRE-FTS intermediate-sampled mapping observations of
the central ~8 kpc (~150") of M51, with a spatial resolution of 40". We detect
4 12CO transitions (J=4-3 to J=7-6) and the CI 3P2-3P1 and 3P1-3P0 transitions.
We supplement these observations with ground based observations of 12CO J=1-0
to J=3-2 and perform a two-component non-LTE analysis. We find that the
molecular gas in the nucleus and centre regions has a cool component
(T_kin~10-20 K) with a moderate but poorly constrained density (n(H2)~10^3-10^6
cm^-3), as well as significant molecular gas in a warmer (T_kin~300-3000 K),
lower density (n(H2)~10^1.6-10^2.5 cm^-3) component. We compare our CO line
ratios and calculated densities along with ratios of CO to total infrared
luminosity to a grid of photon dominated region (PDR) models and find that the
cold molecular gas likely resides in PDRs with a field strength of G_o~10^2.
The warm component likely requires an additional source of mechanical heating,
from supernovae and stellar winds or possibly shocks produced in the strong
spiral density wave. When compared to similar two-component models of other
star-forming galaxies published as part of the Very Nearby Galaxies Survey (Arp
220, M82 and NGC 4038/39), M51 has the lowest density for the warm component,
while having a warm gas mass fraction that is comparable to those of Arp 220
and M82, and significantly higher than that of NGC 4038/39.
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