Pinpointing the Origins of Far-Infrared Emission in Quasars with ALMA
 

a) Left: ALMA Band 4 (red) and Hubble Space Telescope F814W (white) imaging of RXJ1131-1231 showing the four lensed images (A,B,C,D) and the lensing galaxy (G). Center: ALMA imaging from July 2015. Right: ALMA imaging from March 2020.
b) A schematic showing the relative sizes of mm-wave (orange), X-ray (corona, purple) and UV emission (accretion disc, blue) from the lensed quasar RXJ1131-1231.

Understanding how quasars produce their emission is key to studying the environments around supermassive black holes. For radio-faint quasars, the source of centimetre- to millimetre-wave radiation remains uncertain, with possible origins including star formation, a dusty torus, weak jets, or the accretion-disc corona. Resolving these possibilities requires extremely fine spatial detail (<0.1 pc) - well beyond ALMA’s capabilities.

In a recent study, Matus Rybak and colleagues report the serendipitous discovery of microlensing of millimetre emission from the gravitationally lensed quasar RXJ1131-1231 (z = 0.65). ALMA Band 4 continuum observations over multiple epochs from 2015 and 2020 showed changes in the relative brightness quasar’s three lensed images. These changes in flux ratios (rather than total flux) are due to microlensing by stars in the intervening galaxy. This effect implies that the millimetre emission originates from a very compact region, less than 100 AU across, ruling out jets or a dusty torus as the source. The inferred size matches expectations for emission from the accretion disc corona, providing the first direct measurement of the size of a quasar’s millimetre-wave corona.

Contributed by Matus Rybak
 

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