Nearly a decade ago, NASA's Chandra X-ray Observatory saw what appeared to be a black hole snacking on gas at the middle of the nearby Sculptor galaxy. Now, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), which sees higher-energy X-ray light, has found the black hole asleep.
The Sculptor galaxy, also known as NGC 253, is the nearest starburst galaxy actively producing stars. It hosts a central black hole with a mass five million times that of the sun, and 25 percent higher than the black hole at the center of the Milky Way.
Both Chandra and NuStar were trained on NGC 253 in late 2012 and researchers found that the X-ray emission seems to have stopped.
"Our results imply that the black hole went dormant in the past 10 years," said Bret Lehmer of the Johns Hopkins University, Baltimore, and NASA's Goddard Space Flight Center.
The find, to be published in Astrophysical Journal, has astronomers perplexed because star formation and black hole activity usually go together.
"Black holes feed off surrounding accretion disks of material. When they run out of this fuel, they go dormant," said co-author Ann Hornschemeier of Goddard. "NGC 253 is somewhat unusual because the giant black hole is asleep in the midst of tremendous star-forming activity all around it."
Nearly all galaxies are suspected to harbor supermassive black holes at their centers, the most massive of which are thought to grow at the same rate that stars form.
In the Sculptor galaxy, astronomers do not know if star formation is winding down or ramping up. It is possible that the black hole was not actually active 10 years ago, and Chandra observed a different source of X-rays.
"Periodic observations with both Chandra and NuSTAR should tell us unambiguously if the black hole wakes up again. If this happens in the next few years, we hope to be watching."