The size-luminosity relation of local active galactic nuclei from interferometric observations of the broad-line region^⋆

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
e-mail: dsantos@mpe.mpg.de
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 Physics, Kyoto Sangyo University, Kita-ku, 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 Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
²¹ Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave., New York, NY 10010, USA
²² c/o Tracy L. Turner, 205 South Prospect Street, Granville, OH 43023, USA

Received: 5 October 2023
Accepted: 11 January 2024

Abstract

By using the GRAVITY instrument with the near-infrared (NIR) Very Large Telescope Interferometer (VLTI), the structure of the broad (emission-)line region (BLR) in active galactic nuclei (AGNs) can be spatially resolved, allowing the central black hole (BH) mass to be determined. This work reports new NIR VLTI/GRAVITY interferometric spectra for four type 1 AGNs (Mrk 509, PDS 456, Mrk 1239, and IC 4329A) with resolved broad-line emission. Dynamical modelling of interferometric data constrains the BLR radius and central BH mass measurements for our targets and reveals outflow-dominated BLRs for Mrk 509 and PDS 456. We present an updated radius-luminosity (R-L) relation independent of that derived with reverberation mapping (RM) measurements using all the GRAVITY-observed AGNs. We find our R-L relation to be largely consistent with that derived from RM measurements except at high luminosity, where BLR radii seem to be smaller than predicted. This is consistent with RM-based claims that high Eddington ratio AGNs show consistently smaller BLR sizes. The BH masses of our targets are also consistent with the standard M_BH-σ_* relation. Model-independent photocentre fitting shows spatial offsets between the hot dust continuum and the BLR photocentres (ranging from ∼17 μas to 140 μas) that are generally perpendicular to the alignment of the red- and blueshifted BLR photocentres. These offsets are found to be related to the AGN luminosity and could be caused by asymmetric K-band emission of the hot dust, shifting the dust photocentre. We discuss various possible scenarios that can explain this phenomenon.