The first image of a black hole was released today at a press conference in Washington D.C. held by the Event Horizon Telescope (EHT) team and the National Science Foundation (NSF). The subject of the image is the supermassive black hole located near the center of the Messier 87 (M87) galaxy. This black hole is about 55 million light years from Earth and is estimated to have a mass 6.5 billion times greater than our Sun. The image shows a glowing circular emission ring surrounding the dark region (shadow) containing the black hole. The brightest part of the image also may have captured a bright relativistic jet of plasma that appears to be streaming away from the black hole at nearly the speed of light, beaming generally in the direction of Earth.
The EHT is not one physical telescope. Rather, it an array of millimeter and sub-millimeter wavelength radio telescopes located around the world. The following map shows the eight telescopes that participated in the 2017 observations of M87. Three additional telescopes joined the EHT array in 2018 and later.
All of the EHT telescopes are used on a non-dedicated basis by an EHT team of more than 200 researchers during a limited annual observing cycle. The image of the M87 black hole was created from observations made during a one week period in April 2017.
The long baselines between the individual radio telescopes give the “synthetic” EHT the resolving power of a physical radio telescope with a diameter that is approximately equal to the diameter of the Earth. A technique called very long-baseline interferometry (VLBI) is used to combine the data from the individual telescopes to synthesize the image of a black hole. EHT Director, Shep Doeleman, referred to VLBI as “the ultimate in delayed gratification among astronomers.” The magnifying power of the EHT becomes real only when the data from all of the telescopes are brought together and the data are properly combined and processed. This takes time.
At a nominal operating wavelength of about 1.3 mm (frequency of 230 GHz), EHT angular resolution is about 25 microarcseconds (μas), which is sufficient to resolve nearby supermassive black hole candidates on scales that correspond to their event horizons. The EHT team reports that the M87 bright emission disk subtends an angle of 42 ± 3 microarcseconds.
For comparison, the resolution of a human eye in visible light is about 60 arcseconds (1/60thof a degree; there are 3,600 arcseconds in one degree) and the 2.4-meter diameter Hubble Space Telescope has a resolution of about 0.05 arcseconds (50,000 microarcseconds).
You can read five open access papers on the first M87 Event Horizon Telescope results written by the EHT team and published on 10 April 2019 in the Astrophysical Journal Letters here:
- First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole; https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7
- First M87 Event Horizon Telescope Results. II. Array and Instrumentation; https://iopscience.iop.org/article/10.3847/2041-8213/ab0c96
- First M87 Event Horizon Telescope Results. III. Data Processing and Calibration; https://iopscience.iop.org/article/10.3847/2041-8213/ab0c57
- First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole; https://iopscience.iop.org/article/10.3847/2041-8213/ab0e85
- First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring; https://iopscience.iop.org/article/10.3847/2041-8213/ab0f43
You’ll find more information on the Event Horizon Telescope website, here: https://eventhorizontelescope.org/about
There are ongoing efforts to improve the performance of the EHT. Key areas for improvement include:
- Operating the telescopes in the array at a shorter wavelength of 0.87 mm (frequency of 345 GHz) will improve angular resolution by about 40%. This is expected to start after 2020 and take 3 – 5 years to deploy.
- Adding terrestrial telescopes to the array will improve image fidelity and lengthen some baselines to improve angular resolution.
- Collecting data over wider bandwidths (several frequencies)
- Enabling dynamic imaging of time-variable phenomena, which will be very useful for observing the dynamic black hole known as Sgr A* at the center of our Milky Way galaxy.
- In the longer term, adding orbital radio telescopes to EHT will greatly expand the observation baselines and significantly improve angular resolution.
Congratulations to the EHT Collaboration for their extraordinary success in creating the first-ever image of a black hole shadow.
For more background information on the Event Horizon Telescope, see the following Lyncean post:
3 March 2017, The Event Horizon Telescope, https://lynceans.org/all-posts/the-event-horizon-telescope/