Turbulent Molecular Gas and Star Formation in the Shocked Intergalactic Medium of Stephan’s Quintet

We report on single-dish radio CO observations towards the inter-galactic medium (IGM) of the Stephan’s Quintet (hereafter SQ) compact group of galaxies. Extremely bright mid-IR H2 rotational line emission (L(H2) ≈ 1035 W) from warm (102−3 K) molecular gas has been detected by the Spitzer satellite in the kpcscale shock created by a collision between a galaxy, NGC 7318b, and NGC 7319’s tidal arm. 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−1. Assuming a Galactic CO(1-0) emission to H2 mass conversion factor, a total H2 mass of ≈ 5× 109 M⊙ 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−1), 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. CO emission is also detected in a bridge feature that connects the shock to the Seyfert member of the group, NGC 7319, and in the northern star forming region, SQ-A, where a new velocity component is identified at 6900 km s−1, in addition to the two velocity components already known. Spitzer IRS mid-IR spectral mapping is used to estimate the warm H2 masses and excitation at the positions observed in radio. The ratio between the warm H2 mass and the H2 mass derived from CO fluxes is 0.23± 0.07 in the IGM of SQ, which is 10 − 100 times higher than in star-forming galaxies. We suggest that the dissipation of turbulent kinetic energy maintain 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 gas is not forming stars efficiently. Subject headings: Galaxies: clusters: individual: Stephan’s Quintet – galaxies: interactions – galaxies: ISM – intergalactic medium