Astronomers see a massive black hole awaken in real time

"Imagine you've been observing a distant galaxy for years, and it always seemed calm and inactive," says Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study accepted for publication in Astronomy & Astrophysics. "Suddenly, its [core] starts showing dramatic changes in brightness, unlike any typical events we've seen before." This is what happened to SDSS1335+0728, which is now classified as having an 'active galactic nucleus' (AGN) -- a bright compact region powered by a massive black hole -- after it brightened dramatically in December 2019 [1].

Some phenomena, like supernova explosions or tidal disruption events -- when a star gets too close to a black hole and is torn apart -- can make galaxies suddenly light up. But these brightness variations typically last only a few dozen or, at most, a few hundreds of days. SDSS1335+0728 is still growing brighter today, more than four years after it was first seen to 'switch on'. Moreover, the variations detected in the galaxy, which is located 300 million light-years away in the constellation Virgo, are unlike any seen before, pointing astronomers towards a different explanation.

The team tried to understand these brightness variations using a combination of archival data and new observations from several facilities, including the X-shooter instrument on ESO's VLT in Chile's Atacama Desert [2]. Comparing the data taken before and after December 2019, they found that SDSS1335+0728 is now radiating much more light at ultraviolet, optical, and infrared wavelengths. The galaxy also started emitting X-rays in February 2024. "This behaviour is unprecedented," says Sánchez Sáez, who is also affiliated with the Millennium Institute of Astrophysics (MAS) in Chile.

"The most tangible option to explain this phenomenon is that we are seeing how the [core] of the galaxy is beginning to show (...) activity," says co-author Lorena Hernández García, from MAS and the University of Valparaíso in Chile. "If so, this would be the first time that we see the activation of a massive black hole in real time."

Massive black holes -- with masses over one hundred thousand times that of our Sun -- exist at the centre of most galaxies, including the Milky Way. "These giant monsters usually are sleeping and not directly visible," explains co-author Claudio Ricci, from the Diego Portales University, also in Chile. "In the case of SDSS1335+0728, we were able to observe the awakening of the massive black hole, [which] suddenly started to feast on gas available in its surroundings, becoming very bright."

"[This] process (...) has never been observed before," Hernández García says. Previous studies reported inactive galaxies becoming active after several years, but this is the first time the process itself -- the awakening of the black hole -- has been observed in real time. Ricci, who is also affiliated with the Kavli Institute for Astronomy and Astrophysics at Peking University, China, adds: "This is something that could happen also to our own Sgr A*, the massive black hole (...) located at the centre of our galaxy," but it is unclear how likely this is to happen.

Follow-up observations are still needed to rule out alternative explanations. Another possibility is that we are seeing an unusually slow tidal disruption event, or even a new phenomenon. If it is in fact a tidal disruption event, this would be the longest and faintest such event ever observed. "Regardless of the nature of the variations, [this galaxy] provides valuable information on how black holes grow and evolve," Sánchez Sáez says. "We expect that instruments like [MUSE on the VLT or those on the upcoming Extremely Large Telescope (ELT)] will be key in understanding [why the galaxy is brightening]."

Notes

[1] The SDSS1335+0728 galaxy's unusual brightness variations were detected by the Zwicky Transient Facility (ZTF) telescope in the US. Following that, the Chilean-led Automatic Learning for the Rapid Classification of Events (ALeRCE) broker classified SDSS1335+0728 as an active galactic nucleus.

[2] The team collected archival data from NASA's Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX), the Two Micron All Sky Survey (2MASS), the Sloan Digital Sky Survey (SDSS), and the eROSITA instrument on IKI and DLR's Spektr-RG space observatory. Besides ESO's VLT, the follow-up observations were conducted with the Southern Astrophysical Research Telescope (SOAR), the W. M. Keck Observatory, and NASA's Neil Gehrels Swift Observatory and Chandra X-ray Observatory.