The Event Horizon Telescope (EHT) project with representatives of ALMA and APEX radio telescope researchers held a press conference, one of many held in the world to present the first EHT results. A project that for two years engaged a series of radio telescopes from around the world to combine their observations had the aim of peering directly into the environment surrounding a black hole and in particular the supermassive black hole at the center of the Milky Way, known as Sagittarius A*, and the one at the center of the Virgo A galaxy.
The technique of interferometry used to combine the signals detected by a number of radio antennas to obtain a result similar to that of a single huge antenna is well known but for the EHT project it was pushed to the limits with the version of that technique that’s called Very Long Baseline Interferometry (VLBI). In April 2017 the first observation campaign involved Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT), Atacama Pathfinder EXperiment (APEX), IRAM 30-meter telescope, James Clerk Maxwell Telescope (JCMT), The Large Millimeter Telescope (LMT), The Submillimeter Array (SMA), Atacama Large Millimeter/Submillimeter Array (ALMA) and South Pole Telescope (SPT). From 2018, for the following campaigns, Greenland Telescope, IRAM NOEMA Observatory and Kitt Peak National Observatory (KPNO) were added.
To combine the data collected by different radio telescopes, two supercomputers were used, one at the Haystack Observatory at MIT, USA, and the other at the Max Planck Institut fur Radioastronomie, in Bonn, Germany. This process uses extremely high precision synchronization which required the use of atomic clocks that recorded the arrival times of the signals on each antenna involved in the observation campaign. All the recorded data were archived and the archives were taken to the labs where the two supercomputers performed the combining task obtaining the delayed result of a radio telescope almost as big as the Earth.
The 2017 observation Campaign collected around 4 petabytes of data that took two years to be analyzed. The first campaign is the most difficult because it’s the first time that the software produced for that task is used and that required a long time and the unprecedented data quality made the analysis times longer than expected. Initially it was expected that it would take about a year to show the first results, in the end the timeline was doubled.
The two targets are supermassive black holes but they are very different. Sagittarius A*, or simply Sgr A*, has a mass that’s about 4 million times the Sun’s with a distance of about 26,000 light years. The supermassive black hole at the center of the Virgo A galaxy, also known as M87, has a mass estimated at about 6.5 billion times the Sun’s with a distance of about 50 million light years.
The surroundings of supermassive black holes are really extreme environments, being able to grasp details of what happens in those areas could provide extremely interesting scientific information. They’re useful to test the theory of relativity and the ongoing processes can have a significant influence on the galaxies that host them.
The first results were published in a special issue of “The Astrophysical Journal Letters”. Someone already called them photos of the century but this is only the beginning because others will arrive and the accumulated experience will allow to obtain results even more detailed. It’s an important moment and it arrived thanks to an extraordinary international collaboration.