2024-07-11 08:15:06
In late 2019, the previously unremarkable galaxy SDSS1335+0728 began to shine brighter than ever. To understand why, an international team used data from several space and ground-based observatories, including the European Southern Observatory’s (ESO) Very Large Telescope (VLT), to study how brightness changed the galaxy. The researchers concluded that they are witnessing unprecedented changes in a galaxy, which are likely due to the sudden awakening of the supermassive black hole at its core.
“Imagine that you have been looking at a distant galaxy for many years and it always looks calm and inactive,” says Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study. Suddenly, its core begins to show large changes in brightness, in a completely different way than previously observed.” This is what happened to SDSS1335+0728 when, in December 2019, it increased its brightness in a disturbing way. That is why it is now classified as an “active galactic nucleus” (AGN, a dense, bright region fed by a supermassive black hole).
Certain phenomena, such as supernova explosions or tidal disruption events (when a star gets too close to a black hole and breaks apart), can cause galaxies to burst into flames. But these brightness variations usually last a few dozen or, at most, a few hundred days. Today, more than four years after it was first seen “flaring up,” SDSS1335+0728 continues to increase in brightness. Furthermore, the variations detected in the galaxy, located 300 million light-years away in the constellation Virgo, unlike anything observed before, point to a different explanation.
The team sought to understand these brightness variations using a combination of archival data and new observations from several facilities, including the X-shooter instrument, installed on ESO’s VLT in the Atacama Desert. By comparing data taken before and after December 2019, the researchers found that SDSS1335+0728 has significantly more light radiating in ultraviolet, optical and infrared wavelengths. The galaxy also started emitting X-rays in February 2024. “This behavior is unprecedented,” says Sánchez Sáez, who is also affiliated with the Millennium Astrophysics Institute (MAS), in Chile.
“The most tangible option to explain this phenomenon is that we are seeing how the nucleus of the galaxy is starting to show activity,” affirms co-author Lorena Hernández García, from MAS and the University of Valparaiso, in Chile. “If so, this would be the first time we’ve seen the occupation of a supermassive black hole in real time.”
Artist’s impression of the galaxy SDSS1335+0728 increasing in brightness due to the increased activity of the central black hole’s gravitational pull. (Illustration: ESO / M. Kornmesser. CC LE 4.0)
Most galaxies, including the Milky Way, have a supermassive black hole at their center (with a mass over a hundred thousand times that of our Sun). “These giant monsters are usually asleep and not directly visible,” explains co-author Claudio Ricci from the University of Diego Portales, also in Chile. “In the case of SDSS1335+0728, we were able to observe the wake of the supermassive black hole, which suddenly began feasting on the available gas around it, becoming very bright.”
This process has never been observed,” confirms Hernández García. Previous studies have shown that inactive galaxies become active after several years, but this is the first time that the process itself, the awakening of the black hole, has been observed in Ricci, which also affiliated with the Kavli Institute for Astronomy and Astrophysics at Peking University in China, says: “This is something that could also happen to our own A*Score, the supermassive black hole located at the center of our galaxy”, but it is not clear how likely is this to happen.
Follow-up observations are still needed to rule out other explanations. Another possibility is that we are witnessing an unusually slow tidal disruption event, or even a new phenomenon. If this is indeed a tidal disturbance event, this would be the longest and weakest event ever observed. “Regardless of the nature of the variations, this galaxy provides valuable information about how black holes grow and evolve,” says Sánchez Sáez. “We hope that instruments like MUSE on the VLT or those on the upcoming Very Large Telescope (ELT) will help determine why the galaxy is glowing.”
The study of the behavior of the core of SDSS1335+0728 has recently been published in the academic journal Astronomy & Astrophysics. (Source: ESO. CC LE 4.0)
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