Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope: Early Results on Markarian 1014, Markarian 463, and UGC 5101

International audienceWe present spectra taken with the Infrared Spectrograph on the Spitzer Space Telescope covering the 5-38 mum region of three ultraluminous infrared galaxies (ULIRGs): Mrk 1014 (z=0.163), Mrk 463 (z=0.051), and UGC 5101 (z=0.039). The continua of UGC 5101 and Mrk 463 show strong silicate absorption suggesting significant optical depths to the nuclei at 10 mum. UGC 5101 also shows the clear presence of water ice in absorption. Polycyclic aromatic hydrocarbon (PAH) emission features are seen in both Mrk 1014 and UGC 5101, including the 16.4 mum line in UGC 5101. The fine-structure lines are consistent with dominant active galactic nucleus (AGN) power sources in both Mrk 1014 and Mrk 463. In UGC 5101 we detect the [Ne V] 14.3 mum emission line, providing the first direct evidence for a buried AGN in the mid-infrared. The detection of the 9.66 mum and 17.03 mum H2 emission lines in both UGC 5101 and Mrk 463 suggest that the warm molecular gas accounts for 22% and 48% of the total molecular gas masses in these galaxies