| Abstract
| - Abstract. We present the results of L-band spectroscopical observations of local bright Ultraluminous Infrared Galaxies (ULIRGs), performed with the Infrared Spectrometer And Array Camera (ISAAC) at the Very Large Telescope. The excellent sensitivity of the telescope and of the instrument provided spectra of unprecedented quality for this class of objects, which allowed a detailed study of the active galactic nucleus (AGN)/starburst contribution to the energy output, and of the composition of the circumnuclear absorber. We discuss the L-band spectral features of seven single sources, and the statistical properties of a complete sample of 15 sources obtained combining our observations with other published 3-4 μm spectra. Our main results are as follows. (i) When a spectral indicator suggesting the presence of an AGN (low equivalent width of the 3.3-μm emission line, steep λ-fλ spectrum, presence of an absorption feature at 3.4 μm) is found, the AGN is always confirmed by independent analysis at other wavelengths. Conversely, in all known AGNs at least one of the above indicators is present. (ii) Two new diagnostic diagrams are proposed combining the above indicators, in which starbursts and AGNs are clearly and completely separated. (iii) The above diagnostic techniques are possible with spectra of relatively low quality, which can be obtained for several tens of ULIRGs with currently available telescopes. This makes L-band spectroscopy the current best tool to disentangle AGN and starburst contributions in ULIRGs. (iv) The L-band properties of ULIRGs are heterogeneous. However, we show that all the spectral differences among ULIRGs can be reproduced starting from pure intrinsic AGN and starburst spectra and two varying parameters: the amount of dust extinction of the AGN component, and the relative AGN/starburst contribution to the bolometric luminosity. (v) Using the above decomposition model, we show that AGNs in ULIRGs have a low dust-to-gas ratio and a dust extinction curve different from Galactic. (vi) The estimate of the presence and contribution of AGNs in a complete sample shows that AGNs are hosted by approximately two-thirds of ULIRGs, but their energetic contribution is relevant (>30 per cent of the bolometric luminosity) only in ∼20 per cent of the sample.
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