NuSTAR Observes Dormant Black Hole In Nearby Sculptor Galaxy
June 12, 2013

NuSTAR Observes Dormant Black Hole In Nearby Sculptor Galaxy

April Flowers for - Your Universe Online

NASA´s Chandra X-ray Observatory caught signs of what appeared to be a black hole eating gas at the middle of the nearby Sculptor Galaxy nearly a decade ago. A new observation by NASA´s Nuclear Spectroscopic Telescope Array (NuSTAR) has found the black hole asleep.

Details of NuSTAR´s findings were published in the Astrophysical Journal.

"Our results imply that the black hole went dormant in the past 10 years," said Bret Lehmer of the Johns Hopkins University, and NASA's Goddard Space Flight Center. "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."

The black hole at the center of the Sculptor Galaxy is approximately 5 million times the mass of our sun. The Sculptor galaxy, known as NGC 253, is a starburst galaxy actively giving birth to new stars approximately 13 million light-years away from our own Milky Way, making it one of our closest cosmic neighbors.

Compared to the Sculptor galaxy, the Milky Way is much quieter. In the Milky Way, far fewer new stars are being made, and the black hole at its center is also asleep.

"Black holes feed off surrounding accretion disks of material. When they run out of this fuel, they go dormant," said 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."

Astronomers are using the findings to learn how galaxies grow over time. They suspect that nearly every galaxy harbors supermassive black holes at their hearts. In the largest of these, the scientists believe the black holes grow at the same rate that new stars form, until blasting radiation from the black holes ultimately shuts down star formation. They are unsure, however if the Sculptor Galaxy is ramping up or winding down.

"Black hole growth and star formation often go hand-in-hand in distant galaxies," said Daniel Stern, a co-author and NuSTAR project scientist at NASA's Jet Propulsion Laboratory (JPL). "It's a bit surprising as to what's going on here, but we've got two powerful complementary X-ray telescopes on the case."

Chandra made its first observations of what appeared to be Sculptor´s feeding supermassive black hole in 2003. Material spiraling into the center of a black hole heats up to tens of millions of degrees. This causes it to glow in the X-ray spectrum that is visible to Chandra and NuSTAR.

In September and November of 2012, both telescopes observed the same region simultaneously. NuSTAR, which launched in June of 2012, was the first-ever to detect focused, high-energy X-ray light from the region, which allowed the researchers to say conclusively that the black hole is not accreting material.

There are two theories to explain the difference. In the first, the black hole has gone to sleep. In the second theory, scientists propose that the black hole wasn´t awake 10 years ago and that Chandra actually observed a different source of X-rays. The team hopes that future research with both telescopes will solve the puzzle.

"The combination of coordinated Chandra and NuSTAR observations is extremely powerful for answering questions like this," said Lou Kaluzienski, NuSTAR Program Scientist at NASA Headquarters in Washington. "Now, we can get all sides of the story."

NuSTAR´s observations also revealed a smaller, flaring object that the researchers identified as an "ultraluminous X-ray source," (ULX), which is a black hole feeding off material from a partner star. ULXs shine more brightly than typical stellar-mass black holes generated from dying stars, but are fainter and more randomly distributed than the supermassive black holes at the centers of massive galaxies. The origins, size and physics of ULXs are still mysteries.

"These stellar-mass black holes are bumping along near the center of this galaxy," said Hornschemeier. "They tend to be more numerous in areas where there is more star-formation activity."

The NuSTAR and Chandra teams plan to check back on the system over time, so that if Sculptor´s sleeping giant wakes up amidst all the commotion they will be able to monitor the situation.