Astronomers spot ‘missing link’ black hole – not too big and not too small

WASHINGTON: Scientists have detected a mid-size black hole – considered the “missing link” in the understanding of these celestial brutes – eviscerating an unfortunate star that strayed too close.

Using data from the Hubble Space Telescope and two X-ray observatories, the researchers determined that this black hole is more than 50,000 times the mass of our sun and located 740 million light years from Earth in a dwarf galaxy, one containing far fewer stars than our Milky Way.

Black holes are extraordinarily dense objects possessing gravitational pulls so powerful that not even light can escape.

This is one of the few “intermediate-mass” black holes ever identified, being far smaller than the supermassive black holes that reside at the centre of large galaxies but far larger than so-called stellar-mass black holes formed by the collapse of massive individual stars.

“We confirmed that an object that we discovered originally back in 2010 is indeed an intermediate-mass black hole that ripped apart and swallowed a passing star,” said University of Toulouse astrophysicist Natalie Webb, a co-author of the study published this week in Astrophysical Journal Letters.

The star was probably roughly a third the mass of the sun, Webb said.

Webb said scientists have searched for intermediate-mass black holes for four decades and fewer than 10 good examples are known, though large numbers may exist.

“So finding a new one is very significant. Also, a black hole swallowing a star happens on average only once every 10,000 years or so in any particular galaxy so these are rare occurrences,” Webb added.

The supermassive black hole at the centre of the Milky Way is 4 million times the mass of the sun and located 26,000 light-years from Earth. The closest stellar-mass black star is about 6,000 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

Webb called intermediate-mass black holes the “missing link” in understanding the range of black holes. Scientists know how stellar-mass black holes – roughly three to 100 times the mass of our sun – form. They do not know how intermediate-mass black holes form but suspect that supermassive black holes arise from their mid-size brethren.

“Without finding such objects, it was impossible to validate this theory,” Webb said.

Intermediate-mass black holes have remained elusive.

“The best explanation is that they are mostly in an environment that is devoid of gas, leaving the black holes with no material to consume and thus little radiation to emit – which in turn makes them extremely difficult to spot,” said University of New Hampshire astronomer and study lead author Dacheng Lin.

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