Pierre Guillard, François Boulanger, Guillaume Pineau des Forêts, Edith Falgarone, Antoine Gusdorf, Michelle Cluver, Philip Appleton, Ute Lisenfeld, Pierre-Alain Duc, Patrick Ogle, Kevin Xu
We report on single-dish radio CO observations towards the inter-galactic
medium (IGM) of the Stephan's Quintet (SQ) group of galaxies. Extremely bright
mid-IR H2 rotational line emission from warm molecular gas has been detected by
Spitzer in the kpc-scale shock created by a galaxy collision. We detect in the
IGM CO(1-0), (2-1) and (3-2) line emission with complex profiles, spanning a
velocity range of 1000 km/s. A total H2 mass of 5x10^9 solar masses is detected
in the shock. Note that this mass could be lower by a factor of a few because
of the large uncertainties on the CO to H2 conversion factor. The molecular gas
carries a large fraction of the gas kinetic energy involved in the collision,
meaning that this energy has not been thermalized yet. The kinetic energy of
the H2 gas derived from CO observations is comparable to that of the warm H2
gas derived from Spitzer IRS observations. The turbulent kinetic energy of the
H2 gas is at least a factor of 5 greater than the thermal energy of the hot
plasma heated by the collision. The spectra exhibit the pre-shock recession
velocities of the two colliding gas systems (5700 and 6700 km/s), but also
intermediate velocities. This shows that some of the molecular gas originates
from the cooling of post-shock gas, which had time to cool and be accelerated
by the shock. The ratio between the warm H2 mass derived from Spitzer IRS
spectroscopy and the H2 mass derived from CO fluxes is ~0.3 in the IGM of SQ,
which is 10-100 times higher than in star-forming galaxies. The dissipation of
turbulent kinetic energy maintains a high heating rate within the H2 gas. This
interpretation implies that the velocity dispersion on the scale of giant
molecular clouds in SQ is an order of magnitude larger than the Galactic value.
This may explain why this molecular gas is not forming stars efficiently.
View original:
http://arxiv.org/abs/1202.2862
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