2024-07-12 06:00:12
In late 2019, the previously inconspicuous galaxy SDSS1335+0728 suddenly began to shine brighter than ever. To understand why, astronomers used data from several space and ground-based observatories, including the Very Large Telescope (VLT) of theEuropean Southern Observatoryin order to monitor variations in brightness of the galaxy. In a study published today, they conclude that they are witnessing changes never before observed in a galaxy, probably due to the sudden awakening of the black hole massif which is located in its heart.
“Imagine you’ve been observing a distant galaxy for years and it’s always seemed quiet and inactive,” says Paula Sánchez Sáez, an astronomer at ESO in Germany and lead author of the study, which has been accepted for publication in Astronomy & Astrophysics. “Suddenly, its nucleus starts to show dramatic changes in brightness, unlike any typical event we’ve seen before.” That’s what happened to SDSS1335+0728, which is now classified as an “active galactic nucleus” (AGN) – a bright, compact region driven by a massive black hole – following it brightened up dramatically in December 2019. (1).
Certain phenomena, such as supernova explosions or tidal disruptions — when a star gets too close to a black hole and is torn apart — can cause galaxies to suddenly brighten. But these brightness changes typically last only a few dozen or, at most, a few hundred days. SDSS1335+0728 continues to brighten today, more than four years following it was first seen “lighting up.”
Moreover, the variations detected in the galaxy, located 300 million light-years away in the constellation Virgo, are unlike any other observed so far, suggesting another explanation.
The team attempted to understand these brightness variations by combining archival data and new observations from several facilities, including the X-shooter instrument on ESO’s VLT in the Atacama Desert in Chile. (2). Comparing data taken before and following December 2019, they found that SDSS1335+0728 is now radiating much more light in ultraviolet, optical, and infrared wavelengths. The galaxy also began emitting X-rays in February 2024. “This behavior is unprecedented,” says Paula Sánchez Sáez, who is also affiliated with the Institute ofastrophysics of millennium (MAS) in Chile.
This animation shows the growth of the disk of material around the massive black hole at the center of the galaxy SDSS1335+0728. In late 2019, this galaxy suddenly began to shine brighter than ever and was classified as having an active galactic nucleus, powered by the central black hole feeding on surrounding matter.
Credit: ESO/M. Kornmesser
“The most tangible option to explain this phenomenon is that we see how the [noyau] “The galaxy is starting to show (…) activity,” explains Lorena Hernández GarcĂa, from MAS and the University of ValparaĂso in Chile, co-author of the study. “If this is the case, it would be the first time we have seen theactivation of a massive black hole in real time.”
Massive black holes – with masses more than a hundred thousand times greater than our Sun – exist at the centre of most galaxies, including the Milky Way. “These giant monsters are usually dormant and are not directly visible,” explains Claudio Ricci of 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 began to feed on the gas available in its environment, becoming very luminous.”
“This process (…) has never been observed before,” says Lorena Hernández GarcĂa. Previous studies have reported that inactive galaxies become active following several years, but this is the first time that the process itself – the awakening of the black hole – has been observed in real time. Claudio Ricci, who is also affiliated with the Kavli Institute for Astronomy and Astrophysics at Peking University in China, adds: “This is something that might also happen to our own Sgr A*, the massive black hole (…) located at the center of our galaxy,” but it is not clear how likely it is that this will happen.
Further observations are still needed to rule out other explanations. It is also possible that we are witnessing an unusually slow tidal disturbance, or even a new phenomenon. If this is indeed a tidal disturbance, it would be the longest and weakest event ever observed. “Regardless of the nature of the variations, [cette galaxie] provides valuable information on how black holes grow and evolve,” says Paola Sánchez Sáez. “We hope that instruments like [MUSE sur le VLT ou ceux du futur Extremely Large Telescope (ELT)] will be essential to understand [pourquoi la galaxie brille]”.
Notes:
(1) The unusual brightness variations of the galaxy SDSS1335+0728 were detected by the Zwicky Transient Facility (ZTF) telescope in the United States. Then, the group led by the Chilean Automatic Learning for Rapid Event Classification (ALeRCE) team 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. In addition to ESO’s VLT, complementary observations were made with the Southern Astrophysical Research Telescope (SOAR), the WM Keck Observatory and the Neil Gehrels Swift Observatory, and NASA’s Chandra X-ray Observatory.
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