Do black holes get different when they grow larger, asks S.Ananthanarayanan.

A spectacular discovery of modern astronomy is the black hole, a collapsed star that grows so dense that it holds back the light that it should emit. But, does the nature of black holes change when they grow, to be gigantic even at their own scale?

The idea of the black hole, as we understand it, arose from Einstein’s General Theory of relativity, and the notion that a large mass should crush itself, by gravity, so that close packed matter bends the surrounding space, and even light cannot escape. The idea has been developed over the last century, and observatories have detected a large number of these objects. And the effort is to view their features, to see if they are in keeping with what theory says.

A team of scientists from the Netherlands, Germany, Poland, Italy, Spain, different parts of the USA, Canada, Chile, Japan, Taiwan, China and Australia, writing in the journal, Nature Astronomy, draw on current work, to find that the nature of black holes remains basically unchanged even when they grow to very large dimensions. This is further to a breakthrough of 2017, when a world-wide collaboration of radio telescopes created a visual rendition of the region around one of the largest black holes known. The team writing in the journal describes the latest findings, of 2021, that the appearance of an intermediate-size black hole is largely the same.

The idea of a black hole was first proposed in the 18th century, when light was considered to consist of material particles, the corpuscular theory. This, of course, was discarded when light was recognised as an electromagnetic wave. Thinking about the nature of the e.m. wave, however, led Albert Einstein to find that energy, mass and gravitation were properties of space itself – a conclusion dramatically verified when the rays of light from distant stars were seen to be bent by gravity when they grazed past the sun.

The result was the return of the black hole, now understood as the fate of a star that starts with a mass more than certain times that of our sun, finally to collapse into itself. The force of gravity at the surface creates a boundary, the event horizon, beyond which nothing that happens within the star can be detected. The effect of the high gravitational force, however, is that matter in the vicinity of the collapsed star is powerfully attracted, to stream in, to feed the black hole, which grows, to become more massive.

The way a black hole can be detected, in fact is with the help of radiation from this matter whirling around and crashing into the black hole, which, on its own, did not emit any light. With this method, and another, of the gravitational effect of bending starlight that came from behind the black hole, a great many black holes have been discovered. While many are modest sized, as a star that is 1.4 times the mass of the sun could become a black hole, a great many are thousands, even billions of times the mass of the sun. The largest black holes are known as supermassive black holes (SMBH) and it is believed that there is an SMBH at the centre of all galaxies. An object, called Sagittarius A, so called because it is in the Sagittarius constellation, is considered to be the one at the centre of the Milky way.

Although the surroundings of black holes can be made out in optical telescopes, details are not possible. Because of the great distances, any detail survives only in long wavelength radiation, radio waves. Images in radio waves cannot be formed with ordinary telescopes, but need radio antennas. And to capture images in long wavelengths we need telescopes with very large apertures. Radio telescopes hence usually consist of an array of antennas located over an area of many kilometres, and the arrangement is called Very Long Baseline Interferometry (VLBI).

Now, whether in radio waves or in visible light, it is radiation of shorter wavelengths that yield the sharpest images. The sharpest images formed by radio telescopes are hence the ones formed by the radiation of wavelength of the order of centimetres, as opposed to waves with wavelength in meters. Obtaining such images, however, calls for radio telescopes with even larger apertures, and for this, there are collaborations of telescopes spread countrywide, or over groups of countries. And the largest is the Event Horizon Telescope (EHT), a worldwide network of synchronised radio telescopes. This network of telescope arrays works like a telescope with a lens (or reflector) as large as the earth.

It was the EHT that announced in 2019 the imaging, which was in 2017, of the SMBH at the centre of Messier 87 (also known as Virgo A ), a supergiant galaxy with many trillion stars, found in the constellation, Virgo. This was possible because EHT has the capacity to detect radiation of wavelength down to millimetres. At this wavelength, the resolution of images can be as fine as a lightday, or 30 billion kilometres (which is excellent resolution for an object, M87, 53 million light years away).

The images of the ‘jets’ of ionised particles around M87 were highly consistent with the predictions based on the General Theory of Relativity. M87, however, is one of the most massive black holes known, and it does not follow that what has been seen would hold for less massive black holes, where the accretion of matter is less vigorous. The team writing in Nature Astronomy hence turned their attention to Centaurus A, a black hole of intermediate mass, in the constellation Centaurus. At only 55 million solar masses, compared to 6.5 billion of M87, Cen A accumulates matter only at a fraction of the rate of M87.

Cen A, as one of the nearest active radio galaxies, has been extensively studied. The galaxy, however, lies to the south of the galactic plane and is outside what most VLBI arrays, which are in the Northern Hemisphere, can reach. And the few that can have been able to work only at longer, centimetre wavelengths, explains Dr Michael Janssen, the leader of the research team. It is thanks to EHT that radio telescope arrays in the south, like Atacama Large Millimetre Array and Atacama Pathfinder Experimental Telescope, in Chile, and the South Pole Telescope, in Amundsen–Scott South Pole Station, Antarctica, became available.

Now, with the help of these resources, the team had data from Centaurus A at ten times shorter wavelengths and sixteen times sharper resolution, to enable imaging of structures at the lightday scale. The team reports that characteristic jet structures, or streams of matter streaming into the Cen A resemble the jets imaged with M 87 “remarkably well”. And further, the team is able to locate where the SMBH should lie, with respect to the structures seen, and it appears that the boundary of the core should be visible at about four times shorter wavelengths.

The main conclusion, however, is that the environment of charged particles in motion around the largest black holes is similar to that around intermediate-size black holes, and modest black holes, like the one at the centre of our own galaxy.

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