Scientists find an exotic black hole deemed a ‘needle in a haystack’

Astronomers have spotted in a galaxy adjacent to our Milky Way what they are calling a cosmic “needle in a haystack” – a black hole that not only is classified as dormant but appears to have been born without the explosion of a dying star.

Researchers said on Monday this one differs from all other known black holes in that it is “X-ray quiet” – not emitting powerful X-ray radiation indicative of gobbling up nearby material with its strong gravitational pull – and that it was not born in a stellar blast called a supernova.

This artist’s impression shows what the binary system VFTS 243 might look like if we were observing it up close. The system, which is located in the Tarantula Nebula in the Large Magellanic Cloud, is composed of a hot, blue star with 25 times the Sun’s mass and a black hole, which is at least nine times the mass of the Sun. The sizes of the two binary components are not to scale: in reality, the blue star is about 200 000 times larger than the black hole. Note that the ‘lensing’ effect around the black hole is shown for illustration purposes only, to make this dark object more noticeable in the image. The inclination of the system means that, when looking at it from Earth, we cannot observe the black hole eclipsing the star.

Black holes are extraordinarily dense objects with gravity so intense not even light can escape.

This one, with a mass at least nine times greater than our sun, was detected in the Tarantula Nebula region of the Large Magellanic Cloud galaxy and is located about 160,000 light years from Earth. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

An extremely luminous and hot blue star with a mass about 25 times that of the sun orbits with this black hole in a stellar marriage. This so-called binary system is named VFTS 243. The researchers believe the companion star eventually also will become a black hole and could merge with the other one.

Dormant black holes, thought to be relatively common, are hard to detect because they interact very little with their surroundings. Numerous prior proposed candidates have been debunked with further study, including by members of the team that uncovered this one.

“The challenge is finding those objects,” said Tomer Shenar, a research fellow in astronomy at Amsterdam University, lead author of the study published in the journal Nature Astronomy. “We identified a needle in a haystack.”

“It’s the first object of its kind discovered after astronomers have been searching for decades,” said astronomer and study co-author Kareem El-Badry of the Harvard & Smithsonian Center for Astrophysics.

Glowing brightly about 160 000 light-years away, the Tarantula Nebula is the most spectacular feature of the Large Magellanic Cloud, a satellite galaxy to our Milky Way. This image from VLT Survey Telescope at ESO’s Paranal Observatory in Chile shows the region and its rich surroundings in great detail. It reveals a cosmic landscape of star clusters, glowing gas clouds and the scattered remains of supernova explosions.

The researchers used six years of observations from the European Southern Observatory’s Chile-based Very Large Telescope.

There are different categories of black holes. The smallest, like the newly detected one, are so-called stellar-mass black holes formed by the collapse of massive individual stars at the ends of their life cycles. There also are intermediate-mass black holes as well as the enormous supermassive black holes residing at the center of most galaxies.

“Black holes are intrinsically dark objects. They do not emit any light. Therefore, in order to detect a black hole, we usually look at binary systems in which we see one luminous star moving around a second, not-detected object,” said study co-author Julia Bodensteiner, a postdoctoral research fellow at the European Southern Observatory in Munich.

It is typically assumed that the collapse of massive stars into black holes is associated with a powerful supernova explosion. In this case, a star perhaps 20 times our sun’s mass blew some of its material into space in its death throes, then collapsed in on itself without an explosion.

The shape of its orbit with its companion offers evidence for the lack of an explosion.

“The orbit of the system is almost perfectly circular,” Shenar said.

This composite image shows the star-forming region 30 Doradus, also known as the Tarantula Nebula. The background image, taken in the infrared, is itself a composite: it was captured by the HAWK-I instrument on ESO’s Very Large Telescope (VLT) and the Visible and Infrared Survey Telescope for Astronomy (VISTA), shows bright stars and light, pinkish clouds of hot gas. The bright red-yellow streaks that have been superimposed on the image come from radio observations taken by the Atacama Large Millimeter/submillimeter Array (ALMA), revealing regions of cold, dense gas which have the potential to collapse and form stars. The unique web-like structure of the gas clouds led astronomers to the nebula’s spidery nickname.

Had a supernova occurred, the blast’s force would have kicked the newly formed black hole in a random direction and yielded an elliptical rather than circular orbit, Shenar added.

Black holes can be mercilessly ravenous, guzzling any material – gas, dust and stars – wandering within their gravitational pull.

“Black holes can only be mercilessly ravenous if there is something close enough to them that they can devour. Usually, we detect them if they are receiving material from a companion star, a process we call accretion,” Bodensteiner said.

Shenar added, “In so-called dormant black hole systems, the companion is far enough away that the material does not accumulate around the black hole to heat up and emit X-rays. Instead, it is immediately swallowed by the black hole.”