The property of star-forming regions in ULIRGs probed with NIR absorption bands

The 4th AKARI International Conference: The Cosmic Wheel and the Legacy of the AKARI archive: from galaxies and stars to planets and life (October 17-20, 2017. The University of Tokyo), Bunkyo-ku, Tokyo, JapanWe observed near-infrared (NIR) absorption bands in 48 ultraluminous infrared galaxies (ULIRGs) to study physical conditions in star-forming regions. We focus on two absorption features in this study: the H2O ice absorption at 3.0 micrometers, which traces dark clouds (A(sub V) greater than a few mag), and the aliphatic carbon absorption at 3.4 micrometers, which traces diffuse clouds (A(sub V) less than 1 mag). Spectral analysis shows that optical depths of H2O ice and aliphatic carbon in most of the ULIRGs are similar to those in diffuse clouds in the Galaxy when normalized by silicate optical depth, and ULIRGs do NOT consist of dark clouds. This suggests that the star-forming regions in ULIRGs have more intense radiation field than typical dark clouds in the Galaxy. We also examined the profile of H2O ice for objects showing relatively deep absorption. The observed profiles of H2O ice in ULIRGs sometimes show a sign of saturated absorption, while the continuum emission is not completely absorbed by the feature. This suggests that the dark clouds, where H2O ice resides, do not cover the background sources entirely. These results imply that the dark clouds are sparsely distributed in ULIRGs.Physical characteristics: Original contains color illustrations