Astronomers watched a giant black hole awaken in real time

M. Kornmesser/ESO

An artist’s illustration shows a massive black hole raging at the center of a distant galaxy. The black hole relies on surrounding gas to pull a growing disk of matter toward it, causing the galaxy to glow.

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Astronomers are witnessing an unprecedented sight in the universe: the awakening of a supermassive black hole at the center of a distant galaxy.

At the end of 2019, a team of Astronomers noticed a normal galaxy called SDSS1335+0728 300 million light-years away in the constellation Virgo. The sudden increase in the galaxy’s brightness was automatically detected by the telescopes of the Zwicky Transient Facility at the Palomar Observatory in California.

With its extremely wide field of view, the camera scans the entire northern sky every two days, collecting data on celestial objects such as near-Earth asteroids as well as distant, bright supernovae.

An interdisciplinary team of astronomers and engineers followed up on Zwicky’s observation using information from space and ground-based telescopes to see how the galaxy’s brightness changed over time.

The researchers were surprised to discover that they were witnessing a unique moment when a cosmic monster awoke. The findings of their study have been accepted for publication in the journal Astronomy & Astrophysics.

“Imagine that you’ve been observing a distant galaxy for years, and it’s always seemed quiet and inactive,” Paula Sanchez Saez, an astronomer at the European Southern Observatory in Germany and the study’s lead author, said in a statement. “Suddenly, its (core) starts to show dramatic changes in brightness, something we’ve never seen before.”

The team classified the galaxy as an active galactic nucleus, or a bright, dense region powered by a supermassive black hole.

A number of astronomical scenarios can cause galaxies to suddenly brighten, such as supernova explosions or when stars get too close to a black hole and are torn apart during a phenomenon called a tidal disruption event.

But such events only last for dozens or hundreds of days — and SDSS1335+0728 has continued to increase in brightness since researchers first spotted it, like the flick of a cosmic light switch.

And the changes in brightness in the galaxy didn’t match anything astronomers had seen before, leaving them even more puzzled.

To find the answer, the team consulted archival data from NASA’s Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer, the Two Micron All Sky Survey, the Sloan Digital Sky Survey and other observatories.

The researchers compared the data with follow-up observations made by the European Southern Observatory’s Very Large Telescope (VLT) in Chile, the Astrophysical Research Telescope of the Southern Hemisphere in Chile, the W.M. Keck Observatory in Hawaii, and NASA’s space-based Neil Gehrels Swift and Chandra X-ray observatories.

Together, the datasets offer a comprehensive portrait of the galaxy before and after the December 2019 observation, revealing that the galaxy began emitting much more ultraviolet, visible and infrared light in recent years, and X-rays starting in February — which is unprecedented behavior, Sánchez Sáez said.

Given that the galaxy is 300 million light-years away, the events astronomers are seeing happened in the past — but the light from these events is reaching Earth now, after traveling through space for millions of years. A light-year is the longest beam of light on Earth. distance covered by light in one yearwhich is 5.88 trillion miles (9.46 trillion kilometers).

“The most convincing option to explain this phenomenon is that we are seeing how (…) the galaxy’s (core) is starting to show activity,” Lorena Hernández García, an astronomer at the Millennium Institute of Astrophysics and the University of Valparaíso in Chile, said in a statement. “If this is the case, it would be the first time we would observe the activation of a massive black hole in real time.”

Supermassive black holes are classified as having a mass more than 100,000 times that of our Sun. They can be found at the center of most galaxies, including the Milky Way.

“These giants are usually asleep and not directly visible,” Claudio Ricci, an associate professor at Diego Portales University in Chile and a co-author of the study, said in a statement. “In the case of SDSS1335+0728, we were able to observe the awakening of the supermassive black hole, (which) suddenly started feeding on the gas that was around it, and became very bright.”

Previous research has pointed to inactive galaxies becoming active after many years, usually due to black hole activity, but the process of black hole awakening had never been directly observed until now, Hernández García said.

Ricci said the same scenario could also happen with Sagittarius A*, the supermassive black hole at the center of the galaxy, but astronomers aren’t sure how likely that is.

Astronomers cannot rule out the possibility that their observation could be an unusually slow tidal disruption event or an unknown new astronomical phenomenon.

“Whatever the nature of the variations, (this galaxy) provides valuable information about how black holes grow and evolve,” Sanchez Saez said. “We hope that (instruments on MUSE or the upcoming Extremely Large Telescope on the VLT) will be key to understanding (why the galaxy is shining).”


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