EDP Sciences
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Volume 398, Number 1, January IV 2003
Page(s) 101 - 106
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021619

A&A 398, 101-106 (2003)
DOI: 10.1051/0004-6361:20021619

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

O. Marco and K. J. Brooks

European Southern Observatory, Casilla 19001, Santiago 19, Chile
(Received 8 October 2002 / Accepted 5 November 2002 )

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 torus. The 3.28  ${\,\mu {\rm m}}$ polycyclic aromatic hydrocarbon (PAH) emission feature has been tentatively detected toward the central 70 pc of the AGN, with a luminosity of $7.4\times
10^{39}$  erg s -1. PAH emission is thought to be a strong indicator of nuclear starburst activity. Using the measured PAH 3.28  ${\,\mu {\rm m}}$ 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  ${\,\mu {\rm m}}$ emission feature, 1.2 nm, which is one hundred times smaller than for starburst-dominated galaxies. Absorption features at 3.4  ${\,\mu {\rm m}}$, corresponding to carbonaceous dust have also been clearly identified yielding an optical depth value of $\tau_{3.4}=0.14$. 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 $T_{\rm grain} \sim475$ K, we find $M_{\rm hot
~dust}\sim 0.6~M_{\odot}$ . 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  ${\,\mu {\rm m}}$ domain.

Key words: galaxies: individual: NGC 1068 -- galaxies: Seyfert -- galaxies: nuclei -- infrared: galaxies -- galaxies: starburst

Offprint request: O. Marco, omarco@eso.org

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