We report the detection of strong absorption and weak emission features in the 4–27 \mu m Spitzer-IRS spectrum of the distant ultraluminous infrared galaxy ( ULIRG ) IRAS F 00183–7111 ( z=0.327 ) .
The absorption features of CO _ { 2 } and CO gas , water ice , hydrocarbons and silicates are indicative of a strongly obscured ( A _ { 9.6 } \geq 5.4 ; A _ { V } \geq 90 ) and complex line of sight through both hot diffuse ISM and shielded cold molecular clouds towards the nuclear power source .
From the profile of the 4.67 \mu m CO fundamental vibration mode we deduce that the absorbing gas is dense ( n \sim 10 ^ { 6 } cm ^ { -3 } ) and warm ( 720 K ) and has a CO column density of \sim 10 ^ { 19.5 } cm ^ { -2 } , equivalent to N _ { H } \sim 10 ^ { 23.5 } cm ^ { -2 } .
The high temperature and density , as well as the small infered size ( < 0.03 pc ) , locates this absorbing gas close to the power source of this region .
Weak emission features of molecular hydrogen , PAHs and Ne ^ { + } , likely associated with star formation , are detected against the 9.7 \mu m silicate feature , indicating an origin away from the absorbing region .
Based on the 11.2 \mu m PAH flux , we estimate the star formation component to be responsible for up to 30 % of the IR luminosity of the system .
While our mid-infrared spectrum shows no tell-tale signs of AGN activity , the similarities to the mid-infrared spectra of deeply obscured sources ( e.g .
NGC 4418 ) and AGN hot dust ( e.g .
NGC 1068 ) , as well as evidence from other wavelength regions , suggest that the power source hiding behind the optically thick dust screen may well be a buried AGN .