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The broadening of atomic emission lines by high-velocity motion of gas nearaccreting supermassive black holes is an observational hallmark of quasars.Observations of broad emission lines could potentially constrain the mechanismfor transporting gas inwards through accretion disks or outwards through winds.The size of this broad-line region has been estimated by measuring the lighttravel time delay between the variable nuclear continuum and the emission lines- a method known as reverberation mapping. In some models the emission linesarise from a continuous outflow, whereas in others they are produced byorbiting gas clouds. Directly imaging such regions has not hitherto beenpossible because of their small angular sizes (< 0.1 milli-arcseconds). Here wereport a spatial offset (with a spatial resolution of ten micro-arcseconds orabout 0.03 parsecs for a distance of 550 million parsecs) between the red andblue photo-centres of the broad Paschen-{\alpha} line of the quasar 3C 273perpendicular to the direction of its radio jet. This spatial offsetcorresponds to a gradient in the velocity of the gas and thus implies that thegas is orbiting the central supermassive black hole. The data are well fittedby a broad-line-region model of a thick disk of gravitationally bound materialorbiting a black hole of 300 million solar masses. We infer a disk radius of150 light days; a radius of 100-400 light days was found previously usingreverberation mapping. The rotation axis of the disk aligns in inclination andposition angle with the radio jet. Our results support the methods that areoften used to estimate the masses of accreting supermassive black holes and tostudy their evolution over cosmic time.

Spatially resolved rotation of the broad-line region of a quasar at sub-parsec scale