Parsec-Scale Dust Emission From the Polar Region in the Type 2 Nucleus of NGC 424

Abstract

Advancements in infrared (IR) interferometry open up the possibility to spatially resolve active galactic nuclei (AGN) on the parsec-scale level and study the circumnuclear dust distribution, commonly referred to as the "dust torus", that is held responsible for the type 1/type 2 dichotomy of AGN. We used the mid-IR beam combiner MIDI together with the 8 m telescopes at the Very Large Telescope Interferometer (VLTI) to observe the nucleus of the Seyfert 2 galaxy NGC 424, achieving an almost complete coverage of the uv-plane accessible by the available telescope configurations. We detect extended mid-IR emission with a relatively baseline-and model-independent mid-IR half-light radius of (2.0 ± 0.2) pc × (1.5 ± 0.3) pc (averaged over the 8 − 13 µm wavelength range). The extended mid-IR source shows an increasing size with wavelength. These properties are in agreement with the idea of dust heated in thermal equilibrium with the AGN. The orientation of the major axis in position angle ∼ −27 • is closely aligned with the system axis as set by optical polarization observations. Torus models typically favor extension along the mid-plane at mid-IR wavelengths instead. Therefore, we conclude that the majority of the pc-scale mid-IR emission ( 60%) in this type 2 AGN originates from optically-thin dust in the polar region of the AGN, a scenario consistent with the near-to far-IR SED. We suggest that a radiatively-driven dusty wind, possibly launched in a puffed-up region of the inner hot part of the torus, is responsible for the polar dust. In this picture, the torus dominates the near-IR emission up to about 5 µm, while the polar dust is the main contributor to the mid-IR flux. Our results of NGC 424 are consistent with recent observations of the AGN in the Circinus galaxy and resemble large-scale characteristics of other objects. If our results reflect a general property of the AGN population, the current paradigm for interpreting and modeling the IR emission of AGN have to be revised.

X-ray spectral variability of Seyfert 2 galaxies

“…At radio frequencies, it was observed with VLA at 6 and 20 cm, showing an extended structure (Ulvestad & Wilson 1989). A possible mid-IR variability was reported by Hönig et al (2012Hönig et al ( ) between 2007Hönig et al ( and 2009, but it could also be due to an "observational inaccuracy". In X-rays, it is a Comptonthick source (Baloković et al 2014).…”

“…Flux variations are indeed a property of AGN, and they have been reported at different frequencies for Seyfert 2s, such as in radio (Nagar et al 2002;Neff & de Bruyn 1983) or infrared (Sharples et al 1984;Hönig et al 2012). In the present work we used data from the OM onboard XMM-Newton to study UV variability.…”

Modeling optical and UV polarization of AGNs

“…The clouds are optically thin in order to represent the NLR clouds detected farther away from the A66, page 7 of 14 irradiating source, where dust can form and survive (Hönig et al 2012). We consider that the dusty double-cone sustains the same half-opening angle as the ionized outflows, θ 0 = 30 • .…”

“…We consider that the dusty double-cone sustains the same half-opening angle as the ionized outflows, θ 0 = 30 • . According to the observations of Hönig et al (2012) and our simulations in , the inner and outer boundaries of the reprocessing region were fixed to 10 pc and 100 pc above the source, respectively.…”

On the Anisotropy of Nuclei Mid-Infrared Radiation in Nearby Active Galactic Nuclei

“…Infrared interferometry observations of four nearby AGNs (Circinus, NGC 1068, NGC 424, NGC 3783) at subparsec resolution suggested that, instead of the torus, the major contributor of MIR emission is dust in the polar region (Tristram et al 2007(Tristram et al , 2014Raban et al 2009;Hönig et al 2012Hönig et al , 2013. Assuming that all AGNs contain polar region dust, the dust emission in polar regions will likely dilute the anisotropic emission from the torus.…”