The spatially resolved broad line region of IRAS 09149−6206

¹ Max Planck Institute for Extraterrestrial Physics (MPE), Giessenbachstr. 1, 85748 Garching, Germany
² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
³ I. Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
⁴ Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
⁵ Department of Physics and Astronomy, University of Southampton, Southampton, UK
⁶ Department of Physics, Kyoto Sangyo University, Kita-ku, Japan
⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
⁸ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
⁹ Department of Astronomy, The Ohio State University, Columbus, OH, USA
¹⁰ Center for Cosmology and AstroParticle Physics, The Ohio State University, Columbus, OH, USA
¹¹ Space Telescope Science Institute, Baltimore, MD, USA
¹² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
¹⁴ Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave., New York, NY 10010, USA
¹⁵ European Southern Observatory, Casilla, 19001 Santiago 19, Chile
¹⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁷ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹⁸ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁹ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
²⁰ Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
²¹ CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
²² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²³ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
²⁴ Department of Astrophysical & Planetary Sciences, JILA, University of Colorado, Duane Physics Bldg., 2000 Colorado Ave, Boulder, CO 80309, USA
²⁵ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia

Received: 30 July 2020
Accepted: 16 September 2020

Abstract

We present new near-infrared VLTI/GRAVITY interferometric spectra that spatially resolve the broad Brγ emission line in the nucleus of the active galaxy IRAS 09149−6206. We use these data to measure the size of the broad line region (BLR) and estimate the mass of the central black hole. Using an improved phase calibration method that reduces the differential phase uncertainty to 0.05° per baseline across the spectrum, we detect a differential phase signal that reaches a maximum of ∼0.5° between the line and continuum. This represents an offset of ∼120 μas (0.14 pc) between the BLR and the centroid of the hot dust distribution traced by the 2.3 μm continuum. The offset is well within the dust sublimation region, which matches the measured ∼0.6 mas (0.7 pc) diameter of the continuum. A clear velocity gradient, almost perpendicular to the offset, is traced by the reconstructed photocentres of the spectral channels of the Brγ line. We infer the radius of the BLR to be ∼65 μas (0.075 pc), which is consistent with the radius–luminosity relation of nearby active galactic nuclei derived based on the time lag of the Hβ line from reverberation mapping campaigns. Our dynamical modelling indicates the black hole mass is ∼1 × 10⁸ M_⊙, which is a little below, but consistent with, the standard M_BH–σ_* relation.