Selections from 2021: Event Horizon Telescope Targets Seen by ALMA

Editor’s note: In these last two weeks of 2021, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Polarimetric Properties of Event Horizon Telescope Targets from ALMA

Published March 2021

Main takeaway:

A team led by Ciriaco Goddi (Radboud University and Leiden Observatory–Allegro, the Netherlands) used the Atacama Large Millimeter/submillimeter Array (ALMA) to measure the polarization — how orderly or randomly the electric and magnetic fields of light waves are oriented — of the two supermassive black holes targeted by the Event Horizon Telescope as well as that of 12 active galactic nuclei. The team’s observations will help us calibrate, analyze, and interpret future very long-baseline interferometry (VLBI) data from the Event Horizon Telescope and the Global mm-VLBI Array.

Why it’s interesting:

infographic of the Event Horizon Telescope and Global mm-VLBI Array facilities locations

This infographic details the locations of the participating telescopes of the Event Horizon Telescope (cyan) and the Global mm-VLBI Array (yellow). Click to enlarge. [ESO/O. Furtak]

The Event Horizon Telescope collaboration combined data from telescopes across the globe to study supermassive black holes in unprecedented detail, resulting in an image of the black hole at the center of one of the most massive nearby galaxies, Messier 87, in 2019. Now, astronomers have used ALMA — one of the world’s largest arrays of radio telescopes — to study the polarization of the galaxies imaged by the Event Horizon Telescope, as well as a dozen galaxies that host active galactic nuclei. Polarization data allows astronomers to measure the strength and orientation of magnetic fields in distant sources, which likely play a key role in the accretion of material onto black holes as well as the acceleration of relativistic jets in active galactic nuclei.

What the survey showed:

Goddi and collaborators find that the radio emission from active galactic nuclei is strongly polarized, with the highest degree of polarization seen in blazars — active galactic nuclei emitting relativistic jets that are pointed directly at us. In the case of Event Horizon Telescope target Messier 87, the direction of polarization is highly variable on the timescale of a few days, which the team explains with a new model that combines an event-horizon-scale variable source with a larger static source. With more observatories being added to the Event Horizon Telescope, we should soon be able to study the magnetic field near Messier 87’s central black hole in even more detail!


Ciriaco Goddi et al 2021 ApJL 910 L14. doi:10.3847/2041-8213/abee6a