Detalles de publicación
PP 014048
The central parsecs of active galactic nuclei: challenges to the torus
(1) Instituto de Astrofísica de Canarias (IAC), E-38200 La Laguna, Tenerife, Spain
(2) Universidad de La Laguna, Dept. Astrofísica, E-38206 La Laguna, Tenerife, Spain
(3) Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, D-53121 Bonn, Germany
(4) Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany
(5) Universitäts-Sternwarte München, Scheinerstrasse 1, D-81679 München, Germany
Type 2 active galactic nuclei (AGN) are by definition nuclei in which the broad-line region and continuum light are hidden at optical and ultraviolet (UV) wavelengths by dust. Using infrared (IR) Very Large Telescope adaptive optics images and via accurate registration with optical Hubble Space Telescope images we are able to unambiguously identify and determine the precise location of the nucleus of a reference sample of nearby, prototype 2 AGN. The nucleus is found in most cases shifted by several tens of parsec (pc) from the optical peak location, in general coinciding with a featureless region in the galaxy. The optical peak, on the other hand, is found associated with either a young stellar cluster, a cloud in the extended narrow-line region, or is just the maximum of the stellar brightness profile. Only in the galaxies with the lowest nuclear dust extinction, AV ∼ 1 mag, the optical peak is the IR nucleus itself.
Dust extinction maps of the central few kpc of the studied galaxies are constructed on the basis of optical-IR colour images. The high angular scale and astrometry precision of the combined images allow us to trace the dust morphology at scales of few pc. In almost all cases, the IR nucleus is found behind a dust filament or lane, prompting to these being a major, if not the only, cause of the nucleus obscuration. Estimated extinctions –lower limits– are AV ∼ 3 to 6 mag, an order of magnitude less than those inferred by X-rays, however sufficient to at least hide the low-luminosity AGN class.
The length scale of the nuclear dust lane or filaments is in the few 100 pc range, in some cases, seen connecting with kpc-scale dust structures. Their morphology and strategic location suggest these are the nuclear fueling channels.
A precise location of the ionised gas Hα and/or [SI VII] 2.48 μm coronal emission lines relative to those of the IR nucleus and dust is determined. The morphology of the Hα gas is strictly defined by that of the nuclear dust filaments and lanes. Its peak emission is often shifted from the nucleus location, its sometimes conical morphology appears not to be caused by a nuclear –torus– collimation but by the overall location of the dust. Conversely, [SI VII] 2.48 μm coronal emission, less subjected to dust extinction, reflects the truly, rather isotropic, distribution of the ionised gas.
All together, the precise location of dust, ionised gas and the nucleus, is found compelling enough to cast doubts on the universality of the pc-scale torus and supports its vanishing in low-luminosity AGN.
Finally, this work provides the most accurate positioning of NGC 1068 nucleus. It is placed at the South vertex of the so-called cloud B, consistent within the errors with the centre of the UV polarisation pattern.
Dust extinction maps of the central few kpc of the studied galaxies are constructed on the basis of optical-IR colour images. The high angular scale and astrometry precision of the combined images allow us to trace the dust morphology at scales of few pc. In almost all cases, the IR nucleus is found behind a dust filament or lane, prompting to these being a major, if not the only, cause of the nucleus obscuration. Estimated extinctions –lower limits– are AV ∼ 3 to 6 mag, an order of magnitude less than those inferred by X-rays, however sufficient to at least hide the low-luminosity AGN class.
The length scale of the nuclear dust lane or filaments is in the few 100 pc range, in some cases, seen connecting with kpc-scale dust structures. Their morphology and strategic location suggest these are the nuclear fueling channels.
A precise location of the ionised gas Hα and/or [SI VII] 2.48 μm coronal emission lines relative to those of the IR nucleus and dust is determined. The morphology of the Hα gas is strictly defined by that of the nuclear dust filaments and lanes. Its peak emission is often shifted from the nucleus location, its sometimes conical morphology appears not to be caused by a nuclear –torus– collimation but by the overall location of the dust. Conversely, [SI VII] 2.48 μm coronal emission, less subjected to dust extinction, reflects the truly, rather isotropic, distribution of the ionised gas.
All together, the precise location of dust, ionised gas and the nucleus, is found compelling enough to cast doubts on the universality of the pc-scale torus and supports its vanishing in low-luminosity AGN.
Finally, this work provides the most accurate positioning of NGC 1068 nucleus. It is placed at the South vertex of the so-called cloud B, consistent within the errors with the centre of the UV polarisation pattern.