Context : Spitzer Space Telescope observations revealed powerful mid-infrared ( mid-IR ) H _ { 2 } rotational line emission from the Stephan ’ s Quintet ( SQ ) X-ray emitting large scale shock ( \sim 15 \times 35 kpc ^ { 2 } ) associated with a collision between two galaxies .
Because H _ { 2 } forms on dust grains , the presence of H _ { 2 } is physically linked to the survival of dust , and we expect some dust emission to come from 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 H _ { 2 } gas .
It is either diffuse , penetrated by UV radiation , or fragmented into clouds optically thick to UV .

Results : Faint PAH and dust continuum emission are detected in the SQ shock , outside star-forming regions .
The 12 / 24 \mu m 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 PAH emission spectrum in the shock differ from that of the Galaxy , which may suggest an enhanced fraction of large and neutrals PAHs .
In both models ( diffuse or clumpy H _ { 2 } gas ) , the IR SED is consistent with the expected emission from dust associated with the warm ( > 150 K ) H _ { 2 } 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 \pm 0.2 [ Habing units ] .

Conclusions : The presence of PAHs and dust grains in the high-speed ( \sim 1000 km s ^ { -1 } ) galaxy collision suggests that dust survives .
We propose that the dust that survived destruction was in pre-shock gas at densites larger than a few 0.1 cm ^ { -3 } , which was not shocked at velocities larger than \sim 200 km s ^ { -1 } .
Our model assumes a Galactic dust-to-gas mass ratio and size distribution , and present data do not allow us to identify any significant deviations of the abundances and size distribution of dust grains from that of the Galaxy .
Our model calculations show that far-IR Herschel observations will help 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 .