Observations and modeling of the dust emission from the H 2-bright galaxy-wide shock in Stephan's Quintet

Abstract

Context. Spitzer Space Telescope observations have detected powerful mid-infrared (mid-IR) H2 rotational line emission from the X-ray emitting large-scale shock (∼15 × 35kpc2) 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. Aims. To test this interpretation, IR observations and dust modeling are used to identify and characterize the thermal dust emission from the shocked molecular gas. Methods. 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. Results. Faint PAH and dust continuum emission are detected in the SQ shock, outside star-forming regions. The 12/24 μm 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 (> 150