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

GRAVITY Collaboration; Amorim, A.; Bourdarot, G.; Brandner, W.; Cao, Y.; Clénet, Y.; Davies, R.; de Zeeuw, P. T.; Dexter, J.; Drescher, A.; Eckart, A.; Eisenhauer, F.; Fabricius, M.; Feuchtgruber, H.; Förster Schreiber, N. M.; Garcia, P. J. V.; Genzel, R.; Gillessen, S.; Gratadour, D.; Hönig, S.; Kishimoto, M.; Lacour, S.; Lutz, D.; Millour, F.; Netzer, H.; Ott, T.; Paumard, T.; Perraut, K.; Perrin, G.; Peterson, B. M.; Petrucci, P. O.; Pfuhl, O.; Prieto, M. A.; Rabien, S.; Rouan, D.; Santos, D. J. D.; Shangguan, J.; Shimizu, T.; Sternberg, A.; Straubmeier, C.; Sturm, E.; Tacconi, L. J.; Tristram, K. R. W.; Widmann, F.; Woillez, J.
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Astronomy and Astrophysics

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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 MBH-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.

GRAVITY is developed in a collaboration by the Max Planck Institute for Extraterrestrial Physics, LESIA of Observatoire de Paris/Université PSL/CNRS/Sorbonne Université/Université de Paris and IPAG of Université Grenoble Alpes /CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the CENTRA - Centro de Astrofisica e Gravitação, and the European Southern Observatory.