3.1 $μ$m H$_{2}$O Ice Absorption in LINER-Type Ultraluminous Infrared Galaxies with Cool Far-Infrared Colors: the Centrally-Concentrated Nature of Their Deeply Buried Energy Sources

Abstract

Ground-based 2.8--4.1 $μ$m slit spectra of the nuclei of seven ultraluminous infrared galaxies (ULIRGs) that are classified optically as LINERs and have cool far-infrared colors are presented. All the nuclei show 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) emission, with equivalent widths that are systematically lower than those in starburst galaxies. Strong 3.1 $μ$m H$_{2}$O ice absorption, with optical depth greater than 0.6, is also detected in five nuclei, and 3.4 $μ$m carbonaceous dust absorption is detected clearly in one of the five nuclei. It is quantitatively demonstrated that the large optical depths of the H$_{2}$O ice absorption in the five sources, and the 3.4 $μ$m absorption in one source, are incompatible with a geometry in which the energy sources are spatially mixed with dust and molecular gas, as is expected for a typical starburst, but instead require that a large amount of nuclear dust (including ice-covered grains) and molecular gas be distributed in a screen in front of the 3--4 $μ$m continuum-emitting sources. This geometrical requirement can naturally be met if the energy sources are more centrally concentrated than the nuclear dust and molecular gas. The low equivalent widths of the PAH emission compared to starbursts and the central concentration of the nuclear energy sources in these five ULIRGs are best explained by the presence of energetically important active galactic nuclei deeply buried in dust and molecular gas.