WCA member Avery Broderick is a founding member of the Event Horizon Telescope (EHT) Collaboration, which is responsible for constructing, operating, and interpreting observations from a global array of high-frequency radio telescopes that together comprise the highest resolution telescope in history. The EHT is an international collaboration that formed to improve the capability of Very Long Baseline Interferometry (VLBI) at short wavelengths by linking radio dishes across the globe to create an Earth-sized interferometer. The aim of the EHT was to achieve a long-standing goal in astrophysics to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon.
The EHT has been used to measure the size of the emission regions of the two supermassive black holes with the largest apparent event horizons: SgrA* at the center of the Milky Way and M87 in the center of the Virgo A galaxy. Broderickparticipated in the creation and interpretation of the first horizon-resolving images of astronomical black holes in the history of astronomy. Using large-scale computer simulations his group explored model images, looking for signatures of deviations from general relativity and the high-energy astrophysical processes responsible for the growth of black holes and the launching of outflows that extend their influence on intergalactic distances.
On April 10th 2019, the EHT collaboration reported the first ever image of a black hole. They used eight interconnected ground-based radio telescopes spread over four continents. These telescopes work together using a technique called very-long-baseline interferometry (VLBI). It synchronises facilities around the world and exploits the rotation of our planet to form one huge, Earth-size telescope. VLBI allowed the EHT to achieve an astounding resolution of 10 to 20 micro-arcseconds — equivalent to reading a newspaper in New York from a sidewalk café in Paris. With these new cluster of powerful instruments, they have obtained images of the supermassive black hole M87, the first ever image of a black hole.
The European Southern Observatory estimated that the EHT’s image of M87 has been seen by more than half of humanity, emphasizing the profound societal impact that science can have. It also provided a critical quantitative confirmation of the predictions of Einstein’s general theory of relativity, with the size of the shadow consistent with that anticipated by observations of stars 100,000 times farther from the black hole. “This is a landmark in astronomy, an unprecedented scientific feat accomplished by more than 200 scientists”, said EHT project director Sheperd S. Doeleman of the Center for Astrophysics | Harvard & Smithsonian. “This remarkable result has given humanity its first glimpse of the shadow of a supermassive black hole.” The EHT observations revealed a ring-like structure with a dark central region — the black hole’s shadow. This ring appears in several observations using different imaging methods, making the scientists involved confident that they have indeed captured the shadow.
Following the release of the first images of M87, the EHT Collaboration has overseen the development of ground-breaking advances in imaging techniques, retrospective studies of M87’s “wobble” in light of the historic first image of a black hole horizon, and the surprising revelation that the quasar 3C 279 is much more complex than previously realized. Like M87, 3C 279 is home to a relativistic jet, a narrowly beamed collection of magnetic fields and plasma streaming away from a central supermassive black hole at very near lightspeed. Unlike M87, 3C 279 is much farther away and thus even the EHT cannot see the shadow cast by its horizon. Nevertheless, surprises abound.
In April 2019 the EHT published the first images of 3C 279 along with the first images of M87. In April 2020 the EHT published the first science results derived from the 3C 279 data. What all other instruments perceived as a sequence of bright blobs, the EHT resolved into two structure features, indicating conclusively that the journey of the luminous plasma was not nearly as straightforward as previously believed, and must “kink” along the way. Moreover, even over the single week of the EHT’s observing campaign, 3C 279 moved, betraying motions that appear nearly 20 times that of light speed -- this is a trick of the light, an effect called “super-luminal motion” and closely related to the Doppler effect. Ultimately it demonstrates that on smaller scales the ever before probed in 3C 279 the jet must be moving at speed larger than 99.5% of light speed!
The EHT stations also record the polarization of the incident astronomical radio waves, which carry information about magnetic fields in EHT sources. In March 2021, the EHT published the first horizon-resolving polarization maps of M87, proving the existence of large-scale ordered magnetic fields wrapped about the black hole. These fields have long been implicated in the launching and acceleration of jets, the lightspeed outflows observed to emanate from active galactic nuclei.
Achieving this milestone required the development of new Bayesian imaging techniques that could quantify the significance of image features and address the complex instrumental systematics presented by EHT. In August 2022, these techniques enabled a reanalysis of the M87 data that identified and isolated the photon ring, a purely gravitational feature arising from photons that execute a full half-orbit about the black hole before streaming toward Earth.
In May 2022, the EHT reported the first images of the second horizon-science target: the supermassive black hole at the centre of the Milky Way, Sagittarius A* (Sgr A*). Doing so was the culmination of many fundamental developments in radio astronomy, the most important of which was addressing Sgr A*’s extreme variability; Sgr A* would evolve on timescales of minutes while EHT images are synthesized from observations that extend over many hours. Overcoming the structural variability in Sgr A* produced only the second image of a black hole event horizon, in a very different context than M87, and confirmed the implications for gravity. Characterizing the structural variability in Sgr A* probed a crucial process for driving accretion, turbulence, and placed our best theoretical models on a firm empirical foundation.
The image releases of the black holes in the centre of M87 and SgrA* were reported in the UWaterloo news articles, “First image of black hole captured” and “Finding our galactic centre”.
Over the prior two decades, Broderick laid much of the scientific groundwork for the historic observations. He now sits of the Board of the EHT and plays a central role in the science extraction.