Astronomers are witnessing a spectacle never before seen in the cosmos: the awakening of a supermassive black hole at the center of a distant galaxy.
In late 2019, a team of astronomers spotted an unusual galaxy called SDSS1335+0728, 300 million light-years away in the constellation Virgo. A sudden bump in the galaxy’s brightness was automatically detected by the Zwicky Transiting Object Telescope at the Palomar Observatory in California.
With its extreme wide-field view, the camera scans the entire northern sky every two days, capturing data on celestial objects such as near-Earth asteroids as well as distant and bright supernovae.
An interdisciplinary team of astronomers and engineers followed Zwicky’s observation using information from space and ground-based telescopes to see how the galaxy’s brightness changed over time.
To their surprise, the researchers realized they were witnessing a unique moment as a cosmic monster awoke. Their study findings have been accepted for publication in the journal Astronomy & Astrophysics.
“Imagine you’ve observed a distant galaxy for years, and it always seemed quiet and inactive,” lead study author Paula Sánchez Sáez, an astronomer at the European Southern Observatory in Germany, said in a statement. “Suddenly, its (core) starts to show dramatic changes in brightness, unlike any typical event we’ve seen before.”
The team classified the galaxy as having an active galactic nucleus, or a bright, compact region powered by a supermassive black hole.
A number of celestial scenarios can cause a galaxy to suddenly brighten, such as supernova explosions or when stars come too close to black holes and break apart during a phenomenon called tidal disruption events.
But such events last only tens or hundreds of days – and SDSS1335+0728 has continued to grow in brightness for more than four years after researchers first saw it flash in brightness like the flick of a cosmic light switch .
And the brightness variations in the galaxy are unlike anything astronomers have seen before, which only confused them further.
An unprecedented cosmic event
To find the answers, 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 taken by the European Southern Observatory’s Very Large Telescope, or VLT, in Chile, the Southern Astrophysical Research Telescope in Chile, the WM Keck Observatory in Hawaii, and the space-based Neil Gehrels Swift and Chandra of NASA. X-ray observatories.
Together, the data sets presented a broad portrait of the galaxy both before and after the December 2019 observation, revealing that the galaxy shifted to emit much more ultraviolet, visible and infrared light in recent years, and X-ray starting in February – which is unprecedented behavior, said Sánchez Sáez.
Given that the galaxy is 300 million light-years away, the events the astronomers are seeing happened in the past – but the light from these events has only just reached Earth after traveling through space for millions of years. A light year is the distance light travels in one year, which is 5.88 trillion miles (9.46 trillion kilometers).
“The most plausible option to explain this phenomenon is that we are seeing how the (core) of the galaxy is starting to show (…) activity,” said study co-author Lorena Hernández García, an astronomer at the Millennium Astrophysics Institute and The University of Valparaíso, both in Chile, said in a statement. “If so, this would be the first time we’ve seen a massive black hole activate in real time.”
The sleeping celestial giants
Supermassive black holes are classified as having a mass greater than 100,000 times that of our sun. They can be found at the center of most galaxies, including the Milky Way.
“These giant monsters are usually sleeping and not directly visible,” study co-author Claudio Ricci, associate professor at Diego Portales University in Chile, said in a statement. “In the case of SDSS1335+0728, we were able to observe the wake of the massive black hole, (which) suddenly began to enjoy the available gas in its surroundings, becoming very bright.”
Previous research has noted dormant galaxies that appeared to become active after a few years, which is usually caused by black hole activity, but the process of waking up a black hole has never been observed before, until now, he said. Hernández García.
The same scenario could happen with Sagittarius A*, the supermassive black hole at the center of the Milky Way, but astronomers aren’t sure how likely it is, Ricci said.
Astronomers cannot rule out that their observation could be an extremely slow tidal disruption event, or a new unknown celestial phenomenon.
“Regardless of the nature of the variations, (this galaxy) provides valuable information about how black holes grow and evolve,” Sánchez Sáez said. “We expect that instruments like (MUSE on the VLT or those on the upcoming Extremely Large Telescope) will be key to understanding (why the galaxy is shining).”
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