2008ApJ...675...96S

We present the results of infrared L-band (3-4 µm) and M-band (4-5 µm) Very Large Telescope (VLT) ISAAC spectroscopy of five bright ultraluminous infrared galaxies (ULIRGs) hosting an AGN. From our analysis we distinguish two types of sources: ULIRGs in which the AGN is unobscured (with a flat continuum and no absorption features at 3.4 and 4.6 µm), and those with highly obscured AGNs (with a steep, reddened continuum and absorption features due to hydrocarbons and CO). Starburst activity is also present in all of the sources, as inferred from the 3.3 µm PAH emission line. A strong correlation is found between continuum slope and CO optical depth, which suggests that deep carbon monoxide absorption is a common feature of highly obscured ULIRG AGNs. Finally, we show that the AGN dominates the 3-4 µm emission, even if its contribution to the bolometric luminosity is small.