2003A&A...398..101M

Near-infrared 3-5 micron spectroscopic and imaging observations of NGC 1068 have been obtained with the VLT at an angular resolution of ∼0.4arcsec. The [L-M] color index has been derived as a function of distance to the central source of the AGN and found to show significant variations. In particular, there is a blue excess of 0.2mag in the vicinity of the dust/molecular torus. The 3.28µm polycyclic aromatic hydrocarbon (PAH) emission feature has been tentatively detected toward the central 70pc of the AGN, with a luminosity of 7.4x10³⁹erg/s. PAH emission is thought to be a strong indicator of nuclear starburst activity. Using the measured PAH 3.28µm flux we estimate that the nuclear starburst activity in NGC 1068 may contribute up to 1 percent of the total infrared luminosity. Such a very small contribution is supported by the measured equivalent width of the PAH 3.28µm emission feature, 1.2nm, which is one hundred times smaller than for starburst-dominated galaxies. Absorption features at 3.4µm, corresponding to carbonaceous dust have also been clearly identified yielding an optical depth value of τ_3.4=0.14. From this we derive an extinction of A_V=28mag toward the central source. The temperature and mass of the hot dust present in the inner 200 parsecs of the central source are estimated from the L- and M-band photometry. For an average temperature of T_grain∼475K, we find M_hot dust_∼0.6M☉_. Barely detected and at L band only, the contribution of the kilo-parsec-scale ring of star formation is found to be negligible in the 3-5µm domain.