VLT 3-5 micron spectroscopy and imaging of NGC 1068: Does the AGN hide nuclear starburst activity?

Abstract

Near-infrared 3-5 micron spectroscopic and imaging observations of NGC 1068 have been obtained with the VLT at an angular resolution of ∼0.4 arcsec. 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 0.2 mag in the vicinity of the dust/molecular toms. The 3.28 μm polycyclic aromatic hydrocarbon (PAH) emission feature has been tentatively detected toward the central 70 pc of the AGN, with a luminosity of 7.4 × 1039 erg s-1. PAH emission is thought to be a strong indicator of nuclear starburst activity. Using the measured PAH 3.28 μm 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 3.28 μm emission feature, 1.2 nm, which is one hundred times smaller than for starburst-dominated galaxies. Absorption features at 3.4 μm, corresponding to carbonaceous dust have also been clearly identified yielding an optical depth value of τ3.4 = 0.14. From this we derive an extinction of Av = 28 mag 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 Tgrain ∼ 475 K, we find Mhot dust ∼ 0.6 M⊙. 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 3-5 μm domain.