The broadening of atomic emission lines by high-velocity motion of gas near accreting supermassive black holes is an observational hallmark of quasars1. Observations of broad emission lines could potentially constrain the mechanism for transporting gas inwards through accretion disks or outwards through winds2. The size of regions for which broad emission lines are observed (broad-line regions) has been estimated by measuring the delay in light travel time between the variable brightness of the accretion disk continuum and the emission lines3—a method known as reverberation mapping. In some models the emission lines arise from a continuous outflow4, whereas in others they arise from orbiting gas clouds5. Directly imaging such regions has not hitherto been possible because of their small angular size (less than 10−4 arcseconds3,6). Here we report a spatial offset (with a spatial resolution of 10−5 arcseconds, or about 0.03 parsecs for a distance of 550 million parsecs) between the red and blue photo-centres of the broad Paschen-α line of the quasar 3C 273 perpendicular to the direction of its radio jet. This spatial offset corresponds to a gradient in the velocity of the gas and thus implies that the gas is orbiting the central supermassive black hole. The data are well fitted by a broad-line-region model of a thick disk of gravitationally bound material orbiting a black hole of 3 × 108 solar masses. We infer a disk radius of 150 light days; a radius of 100–400 light days was found previously using reverberation mapping7,8,9. The rotation axis of the disk aligns in inclination and position angle with the radio jet. Our results support the methods that are often used to estimate the masses of accreting supermassive black holes and to study their evolution over cosmic time.
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The data were obtained at the VLTI of the European Southern Observatory (ESO), Paranal, Chile, and are available on the ESO archive (http://archive.eso.org/eso/eso_archive_main.html) under programme IDs 099.B-0606, 0100.B-0582 and 0101.B-0255.
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We are grateful to the ESO and the ESO/Paranal staff, and to the many scientific and technical staff members in our institutions who helped to make GRAVITY a reality. GRAVITY was developed in a collaboration by the Max Planck Institute for Extraterrestrial Physics, LESIA of Paris Observatory/CNRS/Sorbonne Université/Université Paris Diderot and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the CENTRA (Center for Astrophysics and Gravitation of Lisbon and Porto) and the ESO. J.D. and M.R.S. were supported by a Sofja Kovalevskaja award from the Alexander von Humboldt foundation. S.F.H. acknowledges support from the EU Horizon 2020 ERC Starting Grant DUST-IN-THE-WIND (ERC-2015-StG-677117). M.K. acknowledges support from JSPS (16H05731). P.O.P. acknowledges financial support from the French PNHE. A.A. and P.J.V.G. acknowledge support from FCT-Portugal, reference UID/FIS/00099/2013. J.D. thanks A. Pancoast for discussions related to velocity-resolved reverberation mapping models and observations. E.S. thanks L. Burtscher for discussions in the preparatory phase of the project.
Nature thanks E. Kara and Y. Shen for their contribution to the peer review of this work.
Extended data figures and tables
a, Differential phase curves (coloured points with 1σ error bars) on the six VLTI baselines (rows) at four epochs (columns) and time-averaged 3C 273 Paα line profile (black points, identical in each panel). The best-fitting BLR model to all data are shown as solid lines. b, u–v coverage for all of our data (in units of millions of λ, Mλ), with observed points (in colour) mirrored in grey. Note the close alignment between all baselines, particularly those without UT4, with the position angle of the large-scale radio jet of 3C 273.
Best-fitting centroids to the differential phase data in each wavelength channel are shown as in Fig. 1, but with contour ellipses containing 68% of the probability density. In addition, the extremal points to the blue (on the jet axis) and red are not shown in Fig. 1, because of their larger errors. Given the relatively low signal-to-noise ratio in each channel, we cannot measure the radial velocity as a function of position (for example, the rotation curve). ΔDec, declination offset; ΔRA, right ascension offset.
One- and two-dimensional probability density distributions from fitting the seven-parameter BLR model to the spectral line profile and differential phase data for 3C 273 obtained from GRAVITY. The blue points show the parameter values at which the highest likelihood was obtained, the dashed lines in the one-dimensional distributions are the 5% and 95% quantiles, and contours are at 1σ, 2σ and 3σ.
We averaged the differential visibility amplitude (blue, ‘visamp’) for 3C 273 over all epochs and between the two longest baselines (UT4-1 and UT3-1). Error bars are 1σ. The amplitude increases at the spectral line (black), demonstrating that the hot dust continuum (which has a radius of about 150 μas) is much larger in size than the BLR. This result rules out a previous claim of a large region size in 3C 27312, and is consistent with the compact size (RBLR ≈ 50 μas) that we find independently from modelling interferometric phase and spectral-line data.
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