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First images of the centre of a galaxy

7 May 2019

On 10 April, researchers working on the Event Horizon Telescope – a network of eight radio dishes that creates an Earth-sized interferometer – released the first direct image of a black hole. The landmark result, which shows the radiation emitted by superheated gas orbiting the event horizon of a super massive black hole in a nearby galaxy, opens a brand new window on these incredible objects.

Super massive black holes (SMBHs) are thought to occupy the centre of most galaxies, including our own, with masses up to billions of solar masses and sizes up to 10 times larger than our solar system. Discovered in the 1960s via radio and optical measurements, their origin, as well as their nature and surrounding environments, remain important open issues within astrophysics. Spatially resolved images of an SMBH and the potential accretion disks around them form vital input, but producing such images is extremely challenging.

SMBHs are relatively bright in radio wavelengths. However, since the imaging resolution achievable with a telescope scales with the wavelength (which is long in the radio range) and scales inversely with the telescope diameter, it is difficult to obtain useful images in the radio region. For example, producing an image with the same resolution as the optical Hubble Space Telescope would require a km-wide telescope, while obtaining a resolution that would allow an SMBH to be imaged, would require a telescope diameter of thousands of kilometres. One way around this is to use interferometry to turn many telescopes dishes at different locations into one large telescope. Such an interferometer measures the differences in arrival time of one radio wave at different locations on Earth (induced by the difference in travel path), from which it is possible to reconstruct an image on the sky. This does not only require a large coordination between many telescopes around the world, but also very precise timing, vast amounts of collected data and enormous computing power.

Despite the considerable difficulties, the Event Horizon Telescope project used this technique to produce the first image of an SMBH using an observation time of only tens of minutes. The imaged SMBH lies at the centre of the supergiant elliptical galaxy Messier 87, which is located in the Virgo constellation at a distance of around 50 million light years. Although relatively close in astronomical terms, its very large mass makes its size on the sky comparable to that of the much lighter SMBH in the centre of our galaxy. Furthermore, its accretion rate (brightness) is variable on longer time scales, making it easier to image. The resulting image (above) shows the clear shadow of the black hole in the centre surrounded by an asymmetric ring caused by radio waves that are bent around the SMBH by its strong gravitational field. The asymmetry is likely a result of relativistic beaming of part of the disk of matter which moves towards Earth.

The team compared the image to a range of detailed simulations in which parameters such as the black hole’s mass, spin and orientation were varied. Additionally, the characteristics of the matter around the SMBH, mainly hot electrons and ions, as well as the magnetic field properties were varied. While the image alone does not allow researchers to constrain many of these parameters, combining it with X-ray data taken by the Chandra and NuSTAR telescopes enables a deeper understanding. For example, the combined data constrain the SMBH mass to 6.5 billion solar masses and appears to exclude a non-spinning black hole. Whether the matter orbiting the SMBH rotates in the same direction or opposite to the black hole, as well as details on the environment around it, will require additional studies. Such studies can also potentially exclude alternative interpretations of this object; currently, exotic objects like boson stars, gravastars and wormholes cannot be fully excluded.

The work of the Event Horizon Telescope collaboration, which involves more than 200 researchers worldwide, was published in six consecutive papers in The Astrophysical Journal Letters. While more images at shorter wavelengths are foreseen in the future, the collaboration also points out that much can be learned by combining the data with that from other wavelengths, such as gamma-rays. Despite this first image being groundbreaking, it is likely only the start of a revolution in our understanding of black holes and, with it, the universe.

Further reading

Event Horizon Telescope Collaboration 2019 ApJL 875 L1—L6.

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