FEEDING THE MONSTER: ACTIVE GALACTIC NUCLEI
Active galactic nuclei (AGN) are supermassive black holes (SMBHs) actively accreting material from its surrounding accretion disk. This central engine is surrounded by a torus of dust that constitutes the source of material used by the accretion disk to feed the SMBH. According to the Unified Model of AGN, the dusty torus is thought to be responsible for the AGN dichotomy of type 1 and type 2 sources: in type 1 AGN the torus is face-on and allows a direct sight of the central engine, while in type 2 AGN the torus is edge-on and obscures the central region at optical wavelengths.
At a distance of about 100 pc and extending out to 4 kpc from the SMBH, a region of ionised gas clouds is usually observed forming a conical shape. Whether this ionisation cone is collimated by the torus is still uncertain.
THE CENTRAL PARSECS OF ACTIVE GALACTIC NUCLEI: CHALLENGES TO THE TORUS
To probe which is the mechanism responsible for the optical nuclear obscuration in type 2 AGN and for the collimation of the ionised gas, we have unambiguously identified the precise location of the nucleus of a sample of nearby, type 2 AGN via accurate registration of infrared (IR) Very Large Telescope adaptive optics images with optical Hubble Space Telescope images. Dust extinction maps of the central few kpc of these galaxies are constructed from optical-IR color images, which allow tracing the dust morphology at scales of few pc. In almost all cases, the IR nucleus is shifted by several tens of pc from the optical peak and its location is behind a dust filament, prompting to this being a major, if not the only, cause of the nuclear obscuration. The extinctions of these nuclear dust lanes are sufficient to at least hide the low-luminosity AGN class, and in some cases are observed to connect with kpc-scale dust structures, suggesting that these are the nuclear fueling channels. A precise location of the ionised gas Halpha and [SI VII] 2.48microns coronal emission lines relative to those of the IR nucleus and dust is determined. The Halpha peak emission is often shifted from the nucleus location and its sometimes conical morphology appears not to be caused by a nuclear –torus– collimation but to be strictly defined by the morphology of the nuclear dust lanes. Conversely, the [SI VII] 2.48microns emission, less subjected to dust extinction, reflects the truly, rather isotropic, distribution of the ionised gas.
All together, the precise location of the dust, ionised gas and nucleus is found compelling enough to cast doubts on the universality of the pc-scale torus and supports its vanishing in low-luminosity AGN:
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NGC 1386. Top left: HST/F814W image with Ks-band continuum contours in white. The FoV is 24 arcsec x 24 arcsec. Blue circles mark the position of the point-like sources used for image alignment, which are also shown in the two inner panels at the top-right corner. The inner 4 arcsec x 4 arcsec region (blue square) is shown in detail in the middle and bottom panels. Top right: Ks/F814W ratio or dust map with the same FoV as the previous panel. Middle left: F814W image with Ks contours in white. Middle right: Ks/F814W ratio with Ks contours in white. The position of the nucleus and its error is marked with a cross in the middle panels. Bottom left: Ks/F814W ratio with Ks contours in white and Halpha contours in green. Bottom right: Ks/F814W ratio with Ks contours in white and [Si VII] coronal emission-line contours in green. The orientation of the radio jet (Nagar & Wilson 1999) is plotted with a dashed grey line. North is up and east is to the left (Prieto, Mezcua et al. 2014).
Some AGN show also a pair of highly collimated relativistic plasma outflows that emerge in opposite directions from the proximity of the SMBH. These jets are believed to form within 10-100 Schwarschild radii from the black hole, where the twisting of magnetic fields in the accretion disk and the rotating black hole collimates a mixture of electrons, positrons and protons, extracting angular momentum from the accreting matter. Jets can extend beyond the host galaxy, several kpc (or even Mpc) out from the core. The best way to observe them is at radio wavelengths:
THE RADIO JETS OF LOW-LUMINOSITY AGN
One of the puzzling questions in AGN studies is whether the Unified Model for AGN also holds for the most numerous class among them: the low-luminosity AGN (LLAGN; bolometric luminosity Lbol ≤ 10^42 erg/s). LLAGN outstand from the Unified Model as lacking the big blue bump in the optical-UV, footprint of the accretion disk, and being radiatively inefficient. A possible explanation is that the overall energy output in these faint nuclei is dominated by a jet. This scenario is supported by the finding that most LLAGN present extended jet radio emission when observed with sufficient angular resolution and sensitivity and that they are thus able to power, at least, parsec-scale radio jets:
To increase the detection rate of jets in LLAGNs, we analyzed subarcsecond resolution data of three low-ionization nuclear emission regions (LINERs). This yielded the detection of extended jet-like radio structures in NGC 1097 and NGC 2911 and the first resolved parsec-scale jet of NGC 4594 (Sombrero). The three sources belong to a sample of nearby LLAGN for which high-spatial-resolution SED of their core emission is available. This allowed us to investigate their energetic balance without drawing on (most) of the ad-hoc assumptions usually considered in large statistical surveys. We found that the kinematic jet luminosity is at least as large as the radiated bolometric luminosity for all LLAGN, which indicates that the jet kinematic output dominates the nuclear energetics of LLAGN disregard of the jet size. The Eddington ratios are highly sub-Eddington (< 10^-4) even when adding the jet power to the total emitted luminosity (radiated plus kinetic), which indicates that LLAGN are not only very inefficent radiators but that they also accrete inefficiently or are very efficient advectors.
VLBA image of NGC 4594 at 23.8 GHz. Contours start at three times the off-source rms noise of 0.3mJy/beam and increase with factors of √2. The beam size is 1.13 mas × 0.62 mas oriented along a P.A. of 2.1 deg. The peak flux density is 8.9mJy/beam (Mezcua & Prieto 2014).