Carbonaceous dust grains in luminous infrared galaxies^*

Spitzer/IRS reveals a-C:H as an abundant and ubiquitous ISM component

¹ Institut d'Astrophysique Spatiale (IAS), Université Paris-Sud 11, CNRS (UMR 8617), Bâtiment 121, 91405 Orsay, France e-mail: emmanuel.dartois@ias.u-psud.fr
² Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir, km. 4, Torrejón de Ardoz, 28850 Madrid, Spain

Received: 5 May 2007
Accepted: 18 July 2007

Abstract

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 ~6.85 and 7.25 μm 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 ~10¹⁸ 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 re-processing 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 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.