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We present the first detections of the 3um water ice and 3.4um amorphoushydrocarbon (HAC) absorption features in z~2 ULIRGs. These are based on deeprest-frame 2-8um Spitzer IRS spectra of 11 sources selected for theirappreciable silicate absorption. The HAC-to-silicate ratio for our z~2 sourcesis typically higher by a factor of 2-5 than that observed in the Milky Way.This HAC `excess' suggests compact nuclei with steep temperature gradients asopposed to predominantly host obscuration. Beside the above molecularabsorption features, we detect the 3.3um PAH emission feature in one of oursources with three more individual spectra showing evidence for it. Stackinganalysis suggests that water ice, hydrocarbons, and PAH are likely present inthe bulk of this sample even when not individually detected. The mostunexpected result of our study is the lack of clear detections of the 4.67um COgas absorption feature. Only three of the sources show tentative signs of thisfeature and at significantly lower levels than has been observed in localULIRGs. Overall, we find that the closest local analogs to our sources, interms of 3-4um color, HAC-to-silicate and ice-to-silicate ratios, as well aslow PAH equivalent widths are sources dominated by deeply obscured nuclei. Suchsources form only a small fraction of ULIRGs locally and are commonly believedto be dominated by buried AGN. Our sample suggests that, in absolute number,such buried AGN are at least an order of magnitude more common at z~2 thantoday. The presence of PAH suggests that significant levels of star-formationare present even if the obscured AGN typically dominate the power budget.

Detections of water ice, hydrocarbons, and 3.3um PAH in z~2 ULIRGs