The Awakening of a Massive Black Hole

Observations from several telescopes, including the European Southern Observatory's Very Large Telescope (VLT) in the Atacama Desert in Chile, show a galaxy undergoing dramatic changes over the past several years. These dramatic changes in brightness began in December 2019, a phenomenon first noticed by the Zwicky Transient Facility (ZTF), which operates from the 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory.

Various phenomena can cause a temporary increase in brightness in a galaxy, including supernova explosions or tidal disruptions (when a star is torn apart as it passes close to a supermassive black hole). But the behavior of this galaxy, called SDSS1335+0728, is novel, at least compared to previous observations. Rather than brightening temporarily, for dozens or a few hundred days, the galaxy is still growing brighter today, more than four years after the change was first observed.

The most likely explanation for this behavior is the turning on of a black hole at the center of SDSS1335+0728, though other explanations have not yet been ruled out. When supermassive black holes like this turn on, they are referred to as active galactic nuclei.

"We have found several million active galactic nuclei to date, and with the new generation of time-domain sky surveys like that at ZTF, we have found about 700 that are changing significantly in brightness," Matthew Graham, research professor of astronomy at Caltech and project scientist for ZTF, explains. "But up to now we have not observed any galactic nuclei that are in the actual process of turning on, of transitioning from a quiescent state to an active state with material falling into the supermassive black hole at the heart of the galaxy."

New observations from ESO's VLT and other locations show SDSS1335+0728 is radiating more ultraviolet, optical, and infrared light, and most recently, X-rays.

"As far as we can tell," Graham says, "there is nothing particularly unusual about this galaxy. We've just caught it at a somewhat unique moment. We expect most galaxies go through a phase like this since most galaxies have a supermassive black hole at their center. Further study of this galaxy will help us to better understand this process and also help us find other examples."

This research was published in Astronomy & Astrophysics under the title "SDSS1335+0728: The awakening of a ∼106M⊙ black hole."

Read the full story from ESO online.