VLTI/GRAVITY interferometric measurements of the innermost dust structure sizes around active galactic nuclei^⋆

¹ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
² CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
³ Max Planck Institute for Extraterrestrial Physics (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
⁶ Leiden University, 2311 EZ Leiden, The Netherlands
⁷ Department of Astrophysical & Planetary Sciences, JILA, University of Colorado, Duane Physics Bldg. 2000 Colorado Ave, Boulder, CO 80309, USA
⁸ I. Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
⁹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Vitacura, Santiago, Chile
¹¹ Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
¹² Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹³ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁴ Department of Physics and Astronomy, University of Southampton, Southampton, UK
¹⁵ Department of Astrophysics & Atmospheric Sciences, Kyoto Sangyo University, Kamigamo-motoyama, Kita-ku, Kyoto 603-8555, Japan
¹⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
¹⁸ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
¹⁹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²⁰ Retired – c/o T.L. Turner, 205 South Prospect Street, Granville, OH 43023, USA
²¹ Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain
²² Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave., New York, NY 10010, USA

Received: 16 May 2024
Accepted: 18 July 2024

Abstract

We present new Very Large Telescope Interferometer (VLTI)/GRAVITY near-infrared interferometric measurements of the angular size of the innermost hot dust continuum for 14 type 1 active galactic nuclei (AGNs). The angular sizes are resolved on scales of ∼0.7 mas and the inferred ring radii range from 0.028 to 1.33 pc, comparable to those reported previously and a factor of 10−20 smaller than the mid-infrared sizes in the literature. Combining our new data with previously published values, we compiled a sample of 25 AGNs with bolometric luminosity ranging from 10⁴² to 10⁴⁷ erg s⁻¹, with which we studied the radius-luminosity (R − L) relation for the hot dust structure. Our interferometric measurements of radius are offset by a factor of 2 from the equivalent relation derived through reverberation mapping. Using a simple model to explore the dust structure’s geometry, we conclude that this offset can be explained if the 2 μm emitting surface has a concave shape. Our data show that the slope of the relation is in line with the canonical R ∝ L^0.5 when using an appropriately non-linear correction for bolometric luminosity. In contrast, using optical luminosity or applying a constant bolometric correction to it results in a significant deviation in the slope, suggesting a potential luminosity dependence on the spectral energy distribution. Over four orders of magnitude in luminosity, the intrinsic scatter around the R − L relation is 0.2 dex, suggesting a tight correlation between the innermost hot dust structure size and the AGN luminosity.