Authors: Isbell, Jacob W.; Ertel, Steve; Kishimoto, Makoto; Weigelt, Gerd; Pott, Jörg-Uwe; Carlson, Jared; Duan, Qixiang; Gámez Rosas, Violeta; Jaffe, Walter; Leftley, James; May, Daniel; Petrov, Romain. G.; Power, Jennifer; Rousseau, Hélène; Rupert, Justin
First Author’s Institution: Department of Astronomy and Steward Observatory, The University of Arizona, 933 N Cherry Ave, Tucson, AZ 85719, USA
Status: Accepted in The Astrophysical Journal [open access]
At the centre of every galaxy lies a supermassive black hole (SMBH) which dominates this innermost region. Not only is there this millions of solar masses dark beast, but usually too a whole bunch of stuff – stars, gas, dust, and more – which is often quite bright! When there is plenty of this material quite close to the black hole, we believe physics takes control to flatten it into a family of disk and toroidal structures in what we call the Unified Model of Active Galactic Nuclei (AGN – Figure 1).
Figure 1: The Unified Model of AGN asserts that there is an inner accretion disk, surrounded by a dusty torus which cohabitates with clouds moving at different velocities (the so-called broad and narrow line regions), and sometimes even relativistic jets extending from the accretion disk to galactic scales; you can read more about AGN structure in this Astrobites guide. Source: Emma Alexander
When astronomers look at different active galaxies (read: galaxies with active SMBHs at their core), we see a range of phenomenologies related to their brightness, spectra, morphologies, and more. The Unified Model of AGN seeks to explain these different AGN appearances simultaneously by positing that they all have the same physical structure and we are just viewing them from different angles, hence seeing different features.
The cores of AGN are often imaged at the smallest scales (e.g. the accretion disk with interferometers like the VLTI) and the largest scales (e.g. the relativistic jets with radio interferometers), but comparatively less effort has gone to directly observing the predicted dusty tori in the mid-infrared (MIR). That is exactly what today’s authors set out to do using the Large Binocular Telescope Interferometer (LBTI) – a pair of 8.4 metre aperture mirrors separated by just over 14 metres which combines the light from each to simulate a telescope effectively 29 metres wide. This lets astronomers take direct images at a much higher resolution than a smaller aperture telescope, and hence directly peer into the region around AGN where this dusty torus should lie.
Today’s authors turn the LBTI towards NGC 4151 – a medium-luminosity AGN. With the large effective aperture of the LBTI, they were able to resolve scales in the AGN region as small as 4.4 to 9.1 pc, about 14 to 30 light years depending on the wavelength (see Figure 2), in the mid-infrared, revealing warm dust emission in a complex structure around the central SMBH. The authors note a central bar at all wavelengths, with a significant extension of cool dust arcing to the west (right in the image) and warmer dust localised to the centre (as evident by 3.7 and 4.8 micron emission only nearest to the SMBH and its accretion disk).
Figure 2: The deconvolved images of the AGN core of NGC 4151 show a very bright central source (the innermost region around the SMBH), as well as some complex surrounding structure particularly at large wavelengths. The interpreted morphology is described in Figure 3. These images are deconvolved, meaning that known optical effects are corrected for on the raw data to produce a sharper image. Source: Adapted from Fig. 3 in today’s paper.
To explain the observed morphology in Figure 1, the authors compare three different interpretations based on this new high resolution imagery together with the results of previous studies looking at other scales and the spectrum of the AGN core. Each interpretation is illustrated and briefed in Figure 3.
Figure 3: Three interpretations of the observed morphology are presented by the authors. The left panel illustrates the different regions surrounding the SMBH (together with the results of other studies cited in the paper). The right panel shows the suggested interpretations explaining the Source: Fig. 6 in today’s paper.
The first interpretation is in keeping with the Unified Model of AGN: a geometrically and optically thick torus of dust surrounds the inner region, and a slightly misaligned jet interacts with a polar outflow producing MIR radiation. They immediately disfavour this explanation, as other studies have shown bright ionised emission at the location of the would-be torus, which is not expected if an optically thick torus were to be present. Hence, these new observations somewhat challenge the one ‘flavour’ of the Unified Model of AGN.
The second interpretation of the observations aligns with a different version of the Unified Model: one in which a geometrically thin disk replaces a thick torus around the AGN core. Provided the disk is optically thin too, the authors favour this approach as it is consistent with the geometry of ionised emission that worked against the first interpretation. While other studies suggest that this thin disk should be optically thick, this morphology is at least better aligned with what we see in other AGN.
The third interpretation suggests that the emission comes from only the radiation pressure driven wind emanating from the AGN core. The authors disfavour this, citing that previous ray-tracing simulations show that the flux should fall off with distance from the core too quickly to be consistent with the observations.
No matter the interpretation, these LBTI observations are an important glimpse into the future of mid-infrared AGN studies that will be done with 30 metre class telescopes (such as ESO’s ELT). This paper, together with the group’s similar study on NGC 1068, is challenging and refining an accepted view of AGN morphology – the consequences of which apply to galaxies near and far, and poses new questions perfect for sophisticated hydrodynamic and radiative simulations.
Astrobite edited by Margaret Verrico
Featured image credit: [NGC 4151 Background:] NASA, ESA, Joseph DePasquale (STScI), [Binoculars:] AbsurdWordPreferred on DeviantArt, [Doughnut:] OpenClipart on freesvg, edited by Ryan White
