Scientists from the Event Horizon Telescope(EHT) facility, a collaboration of over 300 researchers, revealed the first image of the black hole at the centre of the Milky Way in press conferences held around the world. This image of the black hole referred to as Sagittarius A* (SgrA*) gave further support to the idea that the compact object at the centre of our galaxy is indeed a black hole. This strengthens Einstein’s general theory of relativity.
In 2019, the Event Horizon Telescope made history by releasing the first ever image of a black hole M87* – the black hole at the centre of a galaxy Messier 87, which is a supergiant elliptic galaxy.
The ring-shaped image of SgrA*, which looked a lot similar to the one of M87*, occupied 52 micro arcseconds in the field of view, which is as big a span of our view as a doughnut on the moon! The whole exercise was possible because of the enormous power of the Event Horizon Telescope, an ensemble of several telescopes around the world, which together were like a giant eye on the earth with a sight that is 3 million times sharper than the human eye. Sagittarius A* is 27,000 light years from us.
At the press conference held simultaneously at several centres around the world, the researchers said that imaging Sagittarius A* (SgrA*), the black hole at the centre of the Milky Way, was much more difficult than imaging M87* for the following reasons: firstly, SgrA* is only one-thousandth the size of M87*; secondly, the line of sight is obscured by a lot of matter; and as SgrA* is much smaller than M87*, the gas swirling around it takes mere minutes to complete an orbit around SgrA* as opposed to taking weeks to go around M87*. The last gives a variability that makes it difficult to image. A clear imaging requires long exposure of about 8-10 hours, where ideally the object should not change much.
In this relation, Venkatessh Ramakrishnan, who is a postdoctoral researcher at Aalto University Metsahovi Radio Observatory, a key member of the calibration and imaging team, says “Since the physics of plasma flows around SgrA* changes on an hourly time-scale, getting a coherent image with all relevant information from photons corresponding to one orbit is difficult. This requires higher sensitive observations which comes with the addition of more telescopes to the EHT and with advanced image reconstruction algorithms.” He is an author of the paper on this work that is published today in The Astrophysical Journal Letters.
The collaboration hopes to improve their capacity so that they can not only image black holes but construct movies and study the magnetic field further.
