Mathematics – Logic
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009eimw.confe..27g&link_type=abstract
"The Evolving ISM in the Milky Way and Nearby Galaxies, The Fourth Spitzer Science Center Conference, Proceedings of the confere
Mathematics
Logic
Ism: General, Ism: Dust, Extinction, Ism: Molecules, Atomic Processes, Molecular Processes, Shock Waves, Galaxies: Interactions
Scientific paper
Spitzer has detected extremely powerful (LH_2 > 1041 erg s-1), shock-powered, mid-IR H2 emission towards the Stephan's Quintet (SQ) galaxy group (Appleton et al. 06). This is the first time an almost pure H2 line spectrum has been seen in an extragalactic object. The luminosity in the H2 lines exceeds the X-ray luminosity. How can we explain the formation and excitation of such a huge amount of warm (T ≥185 K) molecular gas (MH_{2} ˜ 4 × 107 M&sun;) coexisting with a X-ray emitting plasma? We present a scenario where the molecular gas is formed in the shock. The pre-shock gas is assumed to be inhomogeneous. Schematically, low density volume filling pre-shock gas (nH ≲ 0.01 cm3) is shocked at the collision speed (˜ 1 000 km s-1 for SQ) and becomes a dust-free X-ray emitting plasma. The hot gas pressure induces slower shocks in diffuse gas clouds. This gas (nH ≳ 0.1 cm3) is heated at lower temperatures and dust survives. In the context of the SQ shock, this gas had time to cool and become molecular. Then, because all the mechanical energy is not still dissipated, the relative motions between the gas phases lead to energetic exchanges through the plasma-clouds interface. The observations show that this energy dissipation is very efficient to excite H2 in its rotational lines. We phenomenologically model this turbulence by secondary low velocity shocks driven into these clouds. These shocks are able to heat the newly-formed H2 and produce the observed unusual strong H2 to X-ray luminosity ratio. This contribution is a preliminary presentation of a model to be fully described in Guillard et al. 2008, in preparation.
Appleton Ph.
Boulanger Francois
Guillard Pierre
Pineau des Forêts Guillaume
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