Supermassive black hole and beyond

Print edition : July 05, 2019

The image of the disk of cool hydrogen gas flowing around the supermassive black hole at the centre of the Milky Way, as produced by the ALMA. Photo: ALMA (ESO/NAOJ/NRAO), E.M. Murchikova ET AL

ASTRONOMERS now have a fairly good idea of the chaotic and crowded neighbourhood surrounding the supermassive black hole at the centre of the Milky Way, that is about 26,000 light years from the earth (one light year is about 9.5 billion km). Sagittarius A, the supermassive black hole, is 4 million times the mass of the sun. The region around the black hole is brimming with roving stars, interstellar dust clouds, and a large reservoir of both phenomenally hot and comparatively colder gases. These gases accumulate in a huge “accretion disk”, which orbits the black hole, the centre of very large gravitational attraction. As the gas spirals in, it becomes hot and emits energy at a variety of wavelengths, including X-ray and radio waves. These emissions extend a few tenths of a light year from the black hole’s event horizon, the boundary surface of a black hole beyond which nothing can escape from within the black hole.

Until now, however, astronomers have been able to image only the tenuous, hot portion of this flow of accreting gas, whose temperature is estimated to be 10 million°C (about two-thirds the temperature found at the core of the sun). At this temperature, the gas glows fiercely in X-ray light, allowing it to be studied by space-based X-ray telescopes, down to scale of about a tenth of a light year from the black hole. However, images captured so far of this accreting gas, which forms a roughly spherical flow, showed no obvious rotation.

In addition to this hot glowing gas, previous observations have detected a vast store of comparatively cooler hydrogen gas (about 10,000°C) within a few light years of the black hole.

With its high sensitivity and powerful ability to see fine details, the Atacama Large Millimetre/submillimetre Array (ALMA), Chile, was able to detect this faint radio signal and produce the first-ever image of the cooler gas disk at only about a hundredth of a light year away (or about 1,000 times the distance from the earth to the sun) from the supermassive black hole. It has been estimated that the amount of hydrogen in this cool disk is about one-tenth the mass of Jupiter, or one ten-thousandth of the mass of the sun.

“We were the first to image this elusive disk and study its rotation,” said Elena Murchikova of the Institute for Advanced Study, Princeton, and lead author of the paper, published in the latest issue of “Nature”. “We are also probing accretion onto the black hole. This is important because this is our closest supermassive black hole. Even so, we still have no good understanding of how its accretion works. We hope these new ALMA observations will help the black hole give up some of its secrets.”

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