2010A&A...518A..59G

Spitzer Space Telescope observations have detected powerful mid-infrared (mid-IR) H₂ rotational line emission from the X-ray emitting large-scale shock (∼15x35kpc²) associated with a galaxy collision in Stephan's Quintet (SQ). Because H₂ forms on dust grains, the presence of H₂ is physically linked to the survival of dust, and we expect some dust emission to originate in the molecular gas. To test this interpretation, IR observations and dust modeling are used to identify and characterize the thermal dust emission from the shocked molecular gas. The spatial distribution of the IR emission allows us to isolate the faint PAH and dust continuum emission associated with the molecular gas in the SQ shock. We model the spectral energy distribution (SED) of this emission, and fit it to Spitzer observations. The radiation field is determined with GALEX UV, HST V-band, and ground-based near-IR observations. We consider two limiting cases for the structure of the H₂ gas: it is either diffuse and penetrated by UV radiation, or fragmented into clouds that are optically thick to UV. Faint PAH and dust continuum emission are detected in the SQ shock, outside star-forming regions. The 12/24µm flux ratio in the shock is remarkably close to that of the diffuse Galactic interstellar medium, leading to a Galactic PAH/VSG abundance ratio. However, the properties of the shock inferred from the PAH emission spectrum differ from those of the Galaxy, which may be indicative of an enhanced fraction of large and neutrals PAHs. In both models (diffuse or clumpy H₂ gas), the IR SED is consistent with the expected emission from dust associated with the warm (>150K) H₂ gas, heated by a UV radiation field of intensity comparable to that of the solar neighborhood. This is in agreement with GALEX UV observations that show that the intensity of the radiation field in the shock is G_UV=1.4±0.2[Habing units]. The presence of PAHs and dust grains in the high-speed (∼1000km/s) galaxy collision suggests that dust survives. We propose that the dust that survived destruction was in pre-shock gas at densites higher than a few 0.1cm^–3, which was not shocked at velocities larger than ∼200km/s. Our model assumes a Galactic dust-to-gas mass ratio and size distribution, and current data do not allow us to identify any significant deviations of the abundances and size distribution of dust grains from those of the Galaxy. Our model calculations show that far-IR Herschel observations will help in constraining the structure of the molecular gas, and the dust size distribution, and thereby to look for signatures of dust processing in the SQ shock.