Hubble finds an unexpected collection of black holes

By Georgina Torbet February 14, 2021

Hubble's view of dazzling globular cluster NGC 6397 — Hubble’s view of dazzling globular cluster NGC 6397. NASA, ESA, and T. Brown and S. Casertano (STScI) Acknowledgement: NASA, ESA, and J. Anderson (STScI)

When scientists using the Hubble Space Telescope turned their attention to a globular cluster called NGC 6397, they were expecting to find one, medium-sized black hole at its center. But instead, they found something rather more strange. They found evidence of a collection of smaller black holes, in a cosmic oddity that could teach us about how black holes evolve.

There’s a “missing link” in the world of black holes, as we see regular-sized black holes caused by the collapse of a star and very large black holes at the heart of galaxies called supermassive black holes. But we almost never see black holes in between these two sizes. This means we don’t really understand how black holes merge or grow.

Recommended Videos

The hunt for medium-sized black holes, or intermediate-mass black holes (IMBH) as they are known, is what drew the researchers’ attention to NGC 6397. They hoped to find one of these missing link black holes at the center of the cluster. But that’s not what they found.

Artist’s Impression of the Black Hole Concentration in NGC 6397 — This is an artist’s impression created to visualize the concentration of black holes at the center of NGC 6397. In reality, the small black holes here are far too small for the direct observing capacities of any existing or planned future telescope, including Hubble. It is predicted that this core-collapsed globular cluster could be host to more than 20 black holes. ESA/Hubble, N. Bartmann

The orbits of the stars in the cluster indicated that there wasn’t a single point of mass around which they orbited. They had seemingly random orbits that were best explained by the presence of multiple points of mass.

“We found very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster,” explained lead researcher Eduardo Vitral in a statement.

That made them think that what they were looking at was a collection of smaller black holes, each too small to be imaged directly. But their presence can be inferred by looking at the movements of the stars.

This unusual finding fits in with other recent work suggesting that the central regions of globular clusters could be full of smaller black holes. “Our study is the first finding to provide both the mass and the extent of what appears to be a collection of mostly black holes in a core-collapsed globular cluster,” Vitral said.

Scientists believe these black holes formed from the remnants of the massive stars which once populated the cluster before they ran out of fuel and collapsed in on themselves. These stars sank to the center of the cluster due to their mass, forming the concentration of smaller black holes.

Topics

Georgina Torbet

Space Writer

Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…

Space

The universe has a cosmic ‘hum’ caused by merging black holes

This artist’s concept shows stars, black holes, and nebula laid over a grid representing the fabric of space-time.

In the last decade, astronomers made a major discovery, confirming the existence of gravitational waves. These long-theorized ripples in spacetime are created when extremely massive bodies such as two black holes collide, creating shocks that spread out across the universe and can be detected from millions of light-years away.

Now, a 15-year study has provided more evidence of these gravitational waves, including those at very low frequencies. A large international team in the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration used three ground-based telescopes, the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, and the Very Large Array in New Mexico, to observe pulsars. These rotating neutron stars give off regular pulses of energy, and these pulses can be affected by gravitational waves. By looking for small deviations in the pulses, the researchers were able to see how spacetime was being rippled.

Space

Hubble image of the week shows an unusual jellyfish galaxy

$The jellyfish galaxy JO206 trails across this image from the NASA/ESA Hubble Space Telescope, showcasing a colorful star-forming disk surrounded by a pale, luminous cloud of dust. A handful of foreground bright stars with crisscross diffraction spikes stands out against an inky black backdrop at the bottom of the image. JO206 lies over 700 million light-years from Earth in the constellation Aquarius.$

This week's image from the Hubble Space Telescope shows an unusual type of galaxy named for its aquatic look-alike: a jellyfish.

The jellyfish galaxy JO206 is shown below in an image taken using Hubble's Wide Field Camera 3 instrument. Located 700 million light-years away, in the constellation of Aquarius, this image of the galaxy shows both the bright center of the galaxy and its long tendrils reaching out toward the bottom right. It is these tendrils that give jellyfish galaxies their names, and they are formed through a process called ram pressure stripping.

Space

Hubble scientists create tool for erasing satellite trails from images

This image captures the streak of an Earth-orbiting artificial satellite crossing Hubble's field of view during an observation of "The Mice" interacting galaxies (NGC 4676). A typical satellite trail is very thin and will affect less than 0.5% of a single Hubble exposure. Though in this case the satellite overlaps a portion of the target galaxy, the observation quality is not affected. That's because multiple exposures are taken of the same target. And the satellite trail is not in other frames. Developers at the Space Telescope Science Institute in Baltimore, Maryland, have software that identifies the bad pixels from the satellite photobombing, the extent to which they affect the image, and then flags them. When flagged, scientists can recover the full field of view. Even as the number of satellites increases over the decade, these tools for cleaning the images will still be applicable.

With ever-increasing numbers of satellites in the sky, astronomers have repeatedly expressed worry over how these satellites could impact scientific research. Earlier this year, a study of Hubble Space Telescope observations showed how some images were being ruined by streaks of light coming from satellites -- and while only a small percentage of Hubble images were affected, the authors raised concerns that with the projected number of satellites set to explode in the next decade, the problem could become serious.

Now, astronomers at the Space Telescope Science Institute (STScI), which runs Hubble, have come up with a tool to deal with satellite streaks in Hubble images. "We developed a new tool to identify satellite trails that is an improvement over the previous satellite software because it is much more sensitive. So we think it will be better for identifying and removing satellite trails in Hubble images," said Dave Stark of STScI in a statement.