Rare mid-sized black hole discovered at core of devoured galaxy

Astronomers have scrutinized a cluster of stars that is the apparent remnant core of a relatively small galaxy that was swallowed by the sprawling Milky Way 8 to 10 billion years ago. What lurks at the center of this cluster has them excited.

The researchers said on Wednesday the unusual motion of seven stars in this cluster provides compelling evidence for the presence of an elusive mid-sized black hole at its heart. These are bigger than the class of ordinary black holes formed in the implosion of a single star but smaller than the behemoths residing at the nucleus of most galaxies.

The cluster, called Omega Centauri, contains about 10 million stars. The black hole within it is at least 8,200 times as massive as our sun, the researchers said.

The supermassive black hole called Sagittarius A* at the center of the Milky Way possesses 4 million times the mass of the sun. And that is dwarfed by supermassive black holes billions of times the mass of the sun in other galaxies.

"There has been a long debate whether intermediate-mass black holes exist in general, and specifically in Omega Centauri, and our detection might help to resolve that debate," said astronomer Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, lead author of the study published in the journal Nature.

This black hole is located about 17,700 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). The Milky Way's only larger-known black hole is Sagittarius A*, located about 26,700 light years from Earth.

Black holes are extraordinarily dense objects with gravity so strong not even light can escape, making it difficult to spot them. This one was detected based on how its gravitational pull influences the velocity of seven fast-moving stars in its vicinity, documented in two decades of Hubble Space Telescope observations.

The researchers believe the smaller galaxy, which had been perhaps 10% the Milky Way's size, harbored a black hole that, if left undisturbed, would have become supermassive as it fed off gas and other nearby material drawn by its gravitational pull. But the galactic merger, which occurred when the Milky Way was about a quarter or third its current age, left the black hole frozen in time.

"In this merger process, the galaxy lost all of its gas, and hence the growth of its central black hole got interrupted, leaving it in an intermediate-mass state," Max Planck Institute for Astronomy astronomer and study co-author Nadine Neumayer said.

Astronomers have discovered a black hole with a mass about 33 times greater than that of our sun, the biggest one known in the Milky Way aside from the supermassive black hole lurking at the centre of our galaxy.

The newly identified black hole is located about 2,000 light-years from Earth - relatively close in cosmic terms - in the constellation Aquila, and has a companion star orbiting it, researchers said on Tuesday. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

Black holes are extraordinarily dense objects with gravity so strong that not even light can escape, making it difficult to spot them. This one was identified through observations made in the European Space Agency's Gaia mission, which is creating a huge stellar census, because it caused a wobbling motion in its companion star. Data from the European Southern Observatory's Chile-based Very Large Telescope and other ground-based observatories were used to verify the black hole's mass.

"This black hole is not only very massive, it is also very peculiar in many aspects. It is really something we never expected to see," said Pasquale Panuzzo, a research engineer at the French research agency CNRS working at the Observatoire de Paris and lead author of the study published in the journal Astronomy & Astrophysics.

For instance, the black hole, called Gaia BH3, and its companion are traveling within the galaxy in the opposite direction of how stars typically orbit in the Milky Way.

Gaia BH3 probably formed after the death of a star that was more than 40 as massive as the sun, the researchers said.

Black holes that result from the collapse of a single star are called stellar black holes. Gaia BH3 is the largest-known stellar black hole, according to astronomer and study co-author Tsevi Mazeh of the Tel Aviv University in Israel.

Stellar black holes are dwarfed in size by the supermassive black holes inhabiting the centre of most galaxies. One such black hole called Sagittarius A*, or Sgr A*, is located at the heart of the Milky Way. It possesses 4 million times the mass of our sun and is located about 26,000 light-years from Earth.

Gaia BH3's progenitor star was composed almost entirely of hydrogen and helium. Stars in the early universe had such a chemical composition, known as low metallicity. This star had formed relatively early in the universe's history - perhaps 2 billion years after the Big Bang event.

When that star exploded at the end of its lifespan - called a supernova - it blasted some material into space while the remnant violently collapsed to form a black hole

An artist's impression compares side-by-side three stellar black holes in our galaxy: Gaia BH1, Cygnus X-1 and Gaia BH3, whose masses are 10, 21 and 33 times that of the Sun respectively. PHOTO: REUTERS

The discovery of Gaia BH3, according to Panuzzo, supports stellar evolution models showing that massive stellar black holes can be produced only by a low metallicity star like this one's progenitor star.

Gaia BH3's companion star, just as old as the other one was, is about 76% of the mass of the sun and a bit colder, but around 10 times more luminous. It orbits the black hole on an elliptical path at a distance varying between about 4.5 times the distance between Earth and the sun - a measure called an astronomical unit (AU) - and 29 AU. By way of comparison, Jupiter orbits around five AU from the sun and Neptune around 30 AU.

"The surprising result for me was the fact that the chemical composition of this companion star does not show anything special, so it was not affected by the supernova explosion of the black hole," Observatoire de Paris astronomer and study co-author Elisabetta Caffau said

Scientists are not sure just how big stellar black holes can be.

"The maximum mass for a stellar black hole is a matter of active scientific debate," Panuzzo said.

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