Galaxy NGC3799 lies around 16 million light years from Earth. Any event observed today within that galaxy took place 16 million years ago. One such event was observed in February 2023 when a surge in brightness in the core was followed by a rapid dimming. The observations that followed revealed that the event was a star being torn apart by a supermassive black hole at the heart of the galaxy. This is not the first time such an event has been observed but it is the first to be within our galactic backyard suggesting it may be more common that first thought.
Normal stellar mass black holes form when massive stars reach the end of their lives. The star ceases fusion in its core, the star collapses leading to a rebound visible as supernova explosions. The remains, if the star was massive enough, is a black hole. These black holes tend to be between 5 and 50 times the mass of the Sun yet at the core of most galaxies seem to be black holes that can be up to several billion times the mass of the Sun.
Our own Milky Way hosts one such supermassive galaxy with its gravitational pull that is so immense that even light cannot escape. The presence of these colossal objects has an influence on the dynamics of the galaxy and can reshape the orbit of stars and gas clouds around them. The origin and evolution of supermassive black holes has been the subject of much debate over recent years.
Researchers at the University of Hawaii Institute of Astronomy (IfA) have recently published a paper detailing the nearest observation of a supermassive black hole shredding a star. The team co-led by Jason Hinkle (a graduate student from the IfA) used the All-Sky Automated Survey for Supernovae (ASAS-SN) to observe a sharp increase in brightness followed by a fading from the heart of NGC3799.
Following on from the discovery, subsequent observations were conducted using the Asteroid Terrestrial Last Alert System (ATLAS) on Maunaloa, the Keck Observatory and a few other ground and space based telescopes. These events occur when a star wanders too close to a supermassive black hole. The intense gravitational pull from the black hole varies greatly with distance so the unsuspecting star is torn apart. Eventually the star is consumed by the black hole.
The sun sets on Mauna Kea as the twin Kecks prepare for observing. Credit: Laurie Hatch/ W. M. Keck ObservatoryThe change in brightness was was the result of a flare released when the star was consumed. The event has been called ASASSN-23bd and was visible on all-sky cameras. It was unique in its proximity to Earth but unique for other reasons too; more energy released than previous Tidal Disruption Events (TDEs), closest discovered using visible light and a faster light curve profile than other events.
It’s not unusual to see stars being ripped apart by supermassive blackholes but the team have observed one closer than ever before. Willem Hoogendam, an IfA graduate student who co-led the study reported “This discovery holds the potential to significantly enhance our comprehension of the growth of supermassive black holes and their accretion of surrounding material.”
