The image was produced by a global research team named Event Horizon Telescope Collaboration (EHT), which used a worldwide radiotelescope network.

The scientific community had already studied stars orbiting around something invisible, compact and massive in the centre of our galaxy. These orbits allowed to hypothesise that this target, known as Sagittarius A* (or ‘Sgr A*’), is a black hole. The image was published today and it presents the first visual and direct evidence of it.

This is the second black hole to be direcly imaged. The first one that was photographed is located in the centre of the galaxy M87*, 50 million light years away, and its image was published in April 2019. This time, the image that the EHT published shows the heart of our galaxy, which is a black hole with a mass equivalent to 4 million suns and that is located 27,000 light years away from planet Earth.

‘A little piece of this image bears a Valencian imprint’, says Iván Martí Vidal, GenT researcher of the Generalitat Valenciana (Valencian Government) at the Universitat de València and author of the calibration algorithms that allowed the participation of the ALMA millimetre telescope (the most sensitive telescope in the world) in these observations. Moreover, the Valencian team has contributed to the analysis of the image reconstruction with one of the various algorithms developed by the EHT.

‘The apparent size of the ring of this black hole is similar to that of a tennis ball on the Moon’, says Alejandro Mus, GenT student at the Universitat de València and one of the researchers who made it possible to the correct the effects of interstellar plasma in the black hole image. In order to obtain the image of such a tiny target in the sky, the EHT team ceated a network of eight telescopes. These were combined to form one virtual telescope the size of planet Earth [1]. The EHT observed Sgr A* over several nights, collecting data for many hours, similar to how a traditional camera would take an image with a very long exposure time.

‘The high coincidence between the size of the light ring and the predictions in Einstein’s Theory of General Relativity is remarkable’, states the scientist in the Geoffrey Bower project, from the Astronomy and Astrophysics Institute of the Taipei Academia Sinica. ‘These unprecedented observations represent a big step forward in our knowledge of what occurs in the centre of our galaxy and they also offer new information on how these gigantic black holes interact with their surroundings’. The results of the EHT team are being published today on a special issue of The Astrophysical Journal Letters magazine.

This achievement was considerably more difficult than that of M87* (the black hole whose image was already published by the EHT in 2019), even though Sgr A* is a lot closer to us. ‘Although our galactic centre is much closer than that of M87*, its mass is also a lot smaller, which makes its event horizon much smaller than that of M87* (only a few light-minutes away)’ adds Iván Martí-Vidal. ‘Since it is such a small hole, its shine and shape can vary quite rapidly, which represents serious problems when it comes to generating an image with our telescopes’.

The EHT research team had to develop new and sophisticated tools that considered the gas movement around the Sgr A*. ‘Basically, we had to reinvent the astronomical interferiometry techniques, in which the EHT telescopes are based on’. explains alejandro Mus, whose GenT thesis project deals with the development of new algorithms that allow the collection of higher quality images from these type of observations.

The IRAM-30m telescope, from Sierra Nevada (Spain), played a very important role in the reconstruction of the image from the EHT interferometric observations. ‘The IRAM-30m radiotelescope was the only one in Europe that was able to participate in these observations. This telescope has provided valuable information for the reconstruction of the SgrA* image’, affirms Rebecca Azulay, who participated in the observations from the IRAM-30m telescope.

This job was possible thanks to the talent and effort of a research team consisting of more than 300 people in more than 80 institutions around the world.  The EHT team is particularly pleased to finally have images of two black holes of different sizes, which offers an opportunity to understand how they compare and contrast with each other. They have also started to use the new data to prove theories and models about the behaviour of the gas surrounding super massive blak holes. This process is not yet completely understood, but it seems that it plays an important role in the formation and evolution of new galaxies.

‘We will progressively obtain new and better images (and even films) of super massive black holes near the Earth’ concludes Martí-Vidal, ‘which would make it possible to study how nature bahaves in these extreme regions, so close to the frontiers of time and space’.

Apart from Iván Martí-Vidal and Alejandro Mus, other Valencians participate in the EHT Collaboration, such as Juan Carlos Algaba (University of Malaya), Rebecca Azulay and Eduardo Ros (both from the Universitat de València and the Max-Planck Radioastronomy Institute, Germany, when the observations were conducted).

Notes

[1] The individual telescopes that participated in the EHT on the 17th of April, 2017 when the observations were conduceted were: the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX), the IRAM 30 metre telescope, the James Clerk Maxwell Telescope (JCMT), the Gran Telescopio Milimétrico Alfonso Serrano (GTM), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT) of the University of Arizona, and the South Pole Telescope (SPT). From then onwards, the EHT has added to its network the Greenland Telescope (GLT), the Northern Extended Millimeter Array (NOEMA) University of Arizona 12 metre telescope in Kitt Peak.

ALMA is a joint project from the European Southern Observatory (ESO; Europe, on behalf of its member states) and the US National Science Foundation. (NSF) and the National Institutes of Natural Sciences (NINS) in Japan, together with the National Research Council (Canada), the Minister of Science and Technology (MOST; Taiwan), the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA; Taiwan) and the Korean Astronomy and Space Sciences Institute (KASI; Republic of Korea), in cooperation with the Republic of Chile. The joint observatory ALMA is operated by ESO, the Associated Universities Inc./ National Radio Astronomy Observatory (AUI/NRAO) and the National Astronomic Observatory of Japan (NAOJ). APEX is a collaboration among the Max Planck Radioastronomy Institute (Germany), the Onsala Space Observatory (Sweden) and ESO, who is also the operator. IRAM operates the 30 metre telescope and the MPG, from Germany; the CNS, form France; and the IGN, from Spain, are the organizations associated to the IRAM. The East Asian Observatory operates the JCMT on behalf of the Center for Astronomical Mega-Science of the Chinese Science Academy, the NAOJ, ASIAA KASI and the National Astronomical Research Institute of Thailand and organisations from the United Kingdom and Canada. The INAOE (Mexico) and the UMass operate the GTM. The Center for Astrophysics | Harvard and Smithsonian and ASIAA operate the SMA. The University of Arizona operates the SMT. The University of Chicago operates the SPT utilising specialised instrumentation for the EHT, which is provided by the University of Arizona.

ASIAA and the Smithsonian astrophysical Observatory (SAO) operate the Greenlaand Telescope. The GLT is the part of the ALMA-Taiwan project and is partially funded by the Sinica Academy (SA) and MOST. The IRAM operates NOEMA and the University of Arizona operates the 12 metre telescope in Kitt Peak.

More information

The EHT consortium is comprised of 13 institutes: the Astronomy and Astrophysics Institute of the Sinica Academy; the University of Arizona; the Harvard Astrophysics Centre and the Smithsonian; the University of Chicago; the East Asian Observatory; the Goethe University in Frankfurt; the Instituto de Radioastronomía Milimétrica (IRAM); the Gran Telescopio Milimétrico Alfonso Serrano (Event Horizon Telescope); the Max Planck Institute for Radio Astronomy; the MIT Haystack Observatory; the National Astronomical Observatory of Japan; the Perimeter Institute for Theoretical Physics; and the Radboud University.

Credits: EHT Collaboration

Caption of the ALMA footage

Footage of the SgrA* black hole surroundings, taken with the ALMA telescope, many light-years away. Some ‘mini-spirals’ surrounding the galactic centre, formed mainly by dust and gas can be appreciated. At the centre, the emission of the black hole can be appreciated and its changes in shine are quick and intense. The changeability of the SgrA* is on of the reasons why it has been so difficult to obtain an image of then SgrA* with the EHT.