[Paper Review] A diversity of dusty AGN tori: Data release for the VLTI/MIDI AGN Large Program and first results for 23 galaxies

L. Burtscher, K. Meisenheimer+10 more|arXiv (Cornell University)|Jul 8, 2013

Astrophysical Phenomena and Observations66 references117 citations

TL;DR

This study presents high-resolution mid-infrared interferometric observations of 23 active galactic nuclei (AGNs) using the VLTI/MIDI instrument, revealing a diverse range of dusty torus structures on parsec scales. The key finding is that 70% of type 1 AGNs and 47% of type 2 AGNs exhibit unresolved flux components, indicating compact, unresolved emission on scales <5 mas (0.1–10 pc), challenging the existence of a universal size-luminosity relation for AGN tori.

ABSTRACT

The AGN-heated dust distribution (the "torus") is increasingly recognized not only as the absorber required in unifying models, but as a tracer for the reservoir that feeds the nuclear Super-Massive Black Hole. Yet, even its most basic structural properties (such as its extent, geometry and elongation) are unknown for all but a few archetypal objects. Since most AGNs are unresolved in the mid-infrared, we utilize the MID-infrared interferometric Instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) that is sensitive to structures as small as a few milli-arcseconds (mas). We present here an extensive amount of new interferometric observations from the MIDI AGN Large Program (2009 - 2011) and add data from the archive to give a complete view of the existing MIDI observations of AGNs. Additionally, we have obtained high-quality mid-infrared spectra from VLT/VISIR. We present correlated and total flux spectra for 23 AGNs and derive flux and size estimates at 12 micron using simple axisymmetric geometrical models. Perhaps the most surprising result is the relatively high level of unresolved flux and its large scatter: The median "point source fraction" is 70 % for type 1 and 47 % for type 2 AGNs meaning that a large part of the flux is concentrated on scales smaller than about 5 mas (0.1 - 10 pc). Among sources observed with similar spatial resolution, it varies from 20 % - 100 %. For 18 of the sources, two nuclear components can be distinguished in the radial fits. While these models provide good fits to all but the brightest sources, significant elongations are detected in eight sources. The half-light radii of the fainter sources are smaller than expected from the size ~ L^0.5 scaling of the bright sources and show a large scatter, especially when compared to the relatively tight size--luminosity relation in the near-infrared.

Motivation & Objective

• To resolve the parsec-scale dust distribution in AGNs and determine its structural properties, including size, geometry, and luminosity scaling.
• To investigate the connection between the torus structure, obscuration, and feeding mechanisms of super-massive black holes.
• To determine whether a universal size-luminosity relation exists for AGN tori, based on high-angular-resolution interferometric data.
• To disentangle contributions from AGN-heated dust, star formation, and narrow-line region emission in the mid-infrared flux.
• To release a comprehensive data set of 23 AGNs, including new and archival MIDI interferometric and VISIR spectroscopic data, for the broader community.

Proposed method

• Utilized mid-infrared interferometry with the VLTI/MIDI instrument to achieve milli-arcsecond resolution (≤5 mas), enabling direct imaging of dust structures on parsec scales.
• Combined new and archival MIDI observations with high-spectral-resolution VLT/VISIR mid-infrared spectra to provide accurate total flux calibration for interferometric data.
• Fitted observed correlated fluxes with axisymmetric geometric models (e.g., Gaussian, ring-like) to estimate half-light radii and flux fractions.
• Quantified the unresolved (point-like) flux fraction as a function of luminosity and AGN type to assess compact emission dominance.
• Identified significant elongations in eight sources through radial profile fitting and visibility amplitude analysis.
• Used a multi-epoch, multi-baseline observing strategy to ensure robustness and minimize systematic errors in visibility measurements.

Experimental results

Research questions

• RQ1 What is the typical size and geometry of the dusty torus in AGNs on parsec scales, and how does it vary across different AGN types?
• RQ2 To what extent is the mid-infrared flux in AGNs dominated by compact, unresolved emission, and how does this depend on AGN luminosity and classification?
• RQ3 Does a universal size-luminosity relation exist for AGN tori, or are structural properties fundamentally diverse?
• RQ4 What are the relative contributions of AGN-heated dust, star formation, and narrow-line region emission to the observed mid-infrared flux?
• RQ5 How do structural properties of the torus correlate with obscuration and feeding mechanisms in AGNs?

Key findings

• The median unresolved flux fraction is 70% for type 1 AGNs and 47% for type 2 AGNs, indicating a significant compact component on scales <5 mas (0.1–10 pc).
• Among sources with similar spatial resolution, the unresolved flux fraction varies widely, from 20% to 100%, revealing strong intrinsic diversity in torus structure.
• For 18 sources, two distinct nuclear components (compact and extended) were identified in radial profile fits, suggesting complex dust distributions.
• Significant elongations were detected in eight sources, indicating non-symmetric or anisotropic dust distributions in the torus.
• The half-light radii of fainter AGNs show large scatter and do not follow the expected L^0.5 scaling seen in brighter sources, suggesting no universal size-luminosity relation.
• The results imply that torus structure is dominated by intrinsic differences in dust geometry, with contributions from non-thermal emission and extended dust in the narrow-line region or from star formation.