Cosmic survey: 100 million black holes in the Milky Way galaxy; says study

The celestial census began few years ago after the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected ripples inthe space-time continuum created by the distant collision of two black holes. Its size is as huge as 30 suns.

There may be as many as 100 million black holes in the Milky Way galaxy, according to scientists, including one of Indian origin, who conducted a cosmic survey to calculate and categorise the enigmatic, dark objects.

"Fundamentally, the detection of gravitational waves wasa huge deal, as it was a confirmation of a key prediction of Einstein's general theory of relativity," said James Bullock, University of California, Irvine (UCI) in the US.

"But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are blackholes of this size, and how often do they merge?'" Bullocksaid.

Scientists assume most stellar-remnant black holes -which result from the collapse of massive stars at the end oftheir lives - will be about the same mass as our Sun. To see the evidence of two black holes of such epic proportions coming together in a cataclysmic collision left some astronomers puzzled.

The new research was an attempt to interpret the gravitational wave detections through the lens of what is known about galaxy formation and to form a framework for understanding future occurrences.

 According to Manoj Kaplinghat, professor at UCI, the number of black holes of a given mass per galaxy will depend on the size of the galaxy. The reason is that larger galaxies have many metal-richstars, and smaller dwarf galaxies are dominated by big starsof low metallicity.

 Stars that contain a lot of heavier elements, like our sun, shed a lot of that mass over their lives. When it comes time for one to end it all in a super nova, there is not as much matter left to collapse in on itself,resulting in a lower-mass black hole.

Big stars with low metal content do not shed as much of their mass over time, so when one of them dies, almost all of its mass will wind up in the black hole.

"We have a pretty good understanding of the overall population of stars in the universe and their mass distribution as they're born, so we can tell how many blackholes should have formed with 100 solar masses versus 10 solarmasses," Bullock said.

"We were able to work out how many big black holes should exist, and it ended up being in the millions - way more than I anticipated," he said.

In addition, to shed light on subsequent phenomena,researchers sought to determine how often black holes occur in pairs, how often they merge, and how long it takes. They wondered whether the 30-solar-mass black holes detected by LIGO were born billions of years ago and took along time to merge or came into being more recently (within the past 100 million years) and merged soon after.

"We show that only 0.1 to one per cent of the black holes formed have to merge to explain what LIGO saw," Kaplinghat said. "If the current ideas about stellar evolution are right,then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years," he said.

Earlier, a new galaxy was discovered by the scientists, which is around 10 thousand million light years away and 1,000 times brighter than the Milky Way galaxy.

The researchers said that it is the brightest of all the sub-millimetre galaxies and also have a very strong emission in the far infrared.

Gravitational lensing was used by the scientists led by Anastasio Diaz-Sanches from Polytechnic University of Cartagena (UPCT) in Spain. The lensing acts as a sort of magnifier, changing the size and intensity of the apparent image of the original object.

Diaz-Sanchez said, "Thanks to the gravitational lens produced by a cluster of galaxies between ourselves and the source, which acts as if it was a telescope, the galaxy appears 11 times bigger and brighter than it really is, and appears as several images on an arc centred on the densest part of the cluster, which is known as an Einstein Ring."

With PTI Input