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

Abstract

The Stephan's Quintet (hereafter SQ) is a template source to study the impact of galaxies interaction on the physical state and energetics of their gas. We report on IRAM single-dish CO observations of the SQ compact group of galaxies. These observations follow up the Spitzer discovery of bright mid-IR H_2 rotational line emission (L(H_2) ≈ 10^(35) W) from warm (10^(2–3) K) molecular gas, associated with a 30 kpc long shock between a galaxy, NGC 7318b, and NGC 7319's tidal arm. We detect CO(1-0), (2-1) and (3-2) line emission in the inter-galactic medium (IGM) with complex profiles, spanning a velocity range of ≈1000 km s^(–1). 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 much of the molecular gas has formed out of diffuse gas accelerated by the galaxy-tidal arm collision. 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. Assuming a Galactic CO(1-0) emission to H_2 mass conversion factor, a total H_2 mass of ≈5 × 10^9 M_☉ is detected in the shock. The ratio between the warm H_2 mass derived from Spitzer spectroscopy, and the H_2 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 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 H_2 gas derived from CO observations is comparable to that of the warm H_2 gas from Spitzer spectroscopy, and a factor ≈5 greater than the thermal energy of the hot plasma heated by the collision. In the shock and bridge regions, the ratio of the PAH-to-CO surface luminosities, commonly used to measure the star formation efficiency of the H_2 gas, is lower (up to a factor 75) than the observed values in star-forming galaxies. We suggest that turbulence fed by the galaxy-tidal arm collision maintains a high heating rate within the H_2 gas. This interpretation implies that the velocity dispersion on the scale of giant molecular clouds in SQ is one order of magnitude larger than the Galactic value. The high amplitude of turbulence may explain why this gas is not forming stars efficiently.