A systematic study of ULIRGs using near-infrared absorption bands reveals a strong UV environment in their star-forming regions

We present a systematic study of the 3.0 mu m H2O ice and the 3.4 mu m aliphatic carbon absorption features toward 48 local ultraluminous infrared galaxies (ULIRGs) using spectra obtained by the AKARI Infrared Camera to investigate the UV environment in their star-forming regions. All the ULIRGs in our sample exhibit a ratio of optical depth of H2O ice to silicate dust (tau(3.0)/tau(9.7)) that is lower than that in the Taurus dark cloud. This implies that ULIRGs cannot be described as an ensemble of low-mass star-forming regions and that a significant amount of high-mass star-forming regions contribute to star-forming clouds in local ULIRGs. The results also show that the ratios of optical depth of aliphatic carbon to silicate dust, tau(3.4)/tau(9.7), exhibit diverse values. We investigate two effects that can affect this ratio: the geometric temperature gradient (which increases the ratio) and the intense UV environment (which decreases it). The geometric temperature gradient is typically considered as a sign of active galactic nuclei (AGN). ULIRGs with AGN signs (optical classification, near-infrared color, and a polycyclic aromatic hydrocarbon emission strength of 3.3 mu m) indeed tend to exhibit a large tau(3.4)/tau(9.7) ratio. However, we find that the presence of buried AGN is not the only cause of the geometric temperature gradient, because the enhancement of the ratio is also evident in pure starburst-like ULIRGs without these AGN signs. Regarding the intense UV environment in star-forming regions, the correlation between the aliphatic carbon ratio and the ratio of the [C II] 158 mu m line luminosity to the far-infrared luminosity (L-[C II]/L-FIR), which represents the UV environment in photodissociation regions, implies that the intense UV environment causes the decrease of the aliphatic carbon ratio. We find that an intense UV environment (G/n(H) > 3) in star-forming regions is needed for the aliphatic carbon ratio to be suppressed.