Carbonaceous dust grains in luminous infrared galaxies -: Spitzer/IRS reveals a-C:H as an abundant and ubiquitous ISM component

Aims. The available ground-and space-based spectroscopic capabilities of observatories now allow us to extend Galactic interstellar medium composition studies to extragalactic cases. Absorptions in the mid-infrared shows evidence for silicate and carbonaceous grains in other galaxies. Methods. A set of extragalactic spectra of luminous infrared galaxies (LIRGs) has been extracted from the Spitzer database and compared to the spectra of laboratory-produced interstellar carbon dust analogues. Results. These highly obscured lines-of-sight display the characteristic absorptions at similar to 6.85 and 7.25 mu m of the CH3/CH2 deformation modes of hydrogenated amorphous carbon (a-C:H) grains. They are compared to laboratory-produced a-C:H and imply carbon atom column densities in the solid phase exceeding similar to 10(18) cm(-2). Conclusions. These observations further demonstrate the ubiquitousness of a-C:H in the diffuse interstellar medium (DISM) of galaxies, for a long time almost only observed in the Milky-Way ISM lines-of-sights. Whereas PAH emission lines trace the reprocessing of energetic young stellar radiation, the observed a-C:H features underline the existence of large masses of amorphous carbon dust in (extra-) galactic dust budgets. The difficulty in observing such an interstellar component in the mid-infrared is linked to its low absorption contrast (A(V)/tau( 6.85) approximate to 625 +/- 40) for the strongest band, which therefore requires high column densities to detect a-C:H grains. Such carbon grains might be present but spectroscopically hidden in many other galactic environments.