It has been known for about a decade that the Galaxy 2MASX J21240027+340911 has an active core. At its core, a supermassive black hole was feeding on interstellar material, gas or dust, that came too close. Recently, astronomers discovered a repeating signal from this object, hinting at more complexity: it’s not one but two supermassive black holes at the core of this galaxy – and they’re sharing a meal.
The supermassive black holes have a combined mass 40 million times that of the Sun and are about one light-day apart, about 26 billion kilometers (16 billion miles). They are destined to collide in about 70,000 years, and they orbit each other closer and closer every 130 days. It is this orbital motion that gave rise to the repeated event observed.
“It is a very strange event, called AT 2021hdr, that keeps repeating every few months,” lead author Lorena Hernández-García, an astrophysicist at the Millennium Institute of Astrophysics and the University of Valparaíso in Chile, said in a statement. “We think a gas cloud has engulfed the black holes. As they orbit each other, the black holes interact with the cloud, disrupting and consuming the gas. This produces an oscillating pattern in the light of the system.”
Artist’s impression of the pair of monstrous black holes swirling in a gas cloud
NASA/Aurore Simonnet (Sonoma State University)
The eruption was first seen by the Caltech-led ZTF (Zwicky Transient Facility) at the Palomar Observatory. The facility continued to observe the event every 60 to 90 days and was then monitored by NASA’s Swift telescope.
“Although this outburst was initially thought to be a supernova, outbursts in 2022 made us think of other explanations,” said co-author Alejandra Muñoz-Arancibia, also at the Millennium Institute of Astrophysics and the Center for Mathematical Modeling at the University of Chile. . “Each subsequent event has helped us refine our model of what is happening in the system.”
The team considered several options. It could be a common behavior in an active core. Alternatively, it could have been a star that got too close to the supermassive black hole, torn apart and slowly swallowed it. But a pair of supermassive black holes shrouded in a cloud of gas, feasting as they orbit each other, are the most compelling.
The team plans to continue monitoring this event to better model what is going on and to study the host galaxy undergoing a merger. The galaxy is located 1 billion light years away from Earth.
The article was published in the journal Astronomy & Astrophysics.