JPL News: In Colliding Galaxies, a Pipsqueak Shines Bright

In the nearby Whirlpool galaxy and its companion galaxy, M51b, two supermassive black holes heat up and devour surrounding material. These two monsters should be the most luminous X-ray sources in sight, but a new study using observations from NuSTAR (Nuclear Spectroscopic Telescope Array)—a Caltech-led mission managed by the Jet Propulsion Laboratory, which is managed by Caltech for NASA—shows that a much smaller object is competing with the two behemoths.

The most stunning features of the Whirlpool galaxy, officially known as M51a, are the two long, star-filled "arms" curling around the galactic center like ribbons. The much smaller M51b clings like a barnacle to the edge of the Whirlpool. Collectively known as M51, the two galaxies are merging. 

At the center of each galaxy is a supermassive black hole millions of times more massive than the sun. The galactic merger should push huge amounts of gas and dust into those black holes and into orbit around them. In turn, the intense gravity of the black holes should cause that orbiting material to heat up and radiate, forming bright disks around each that can outshine all the stars in their galaxies. 

But neither black hole is radiating as brightly in the X-ray range as scientists would expect during a merger. 

"I'm still surprised by this finding," said study lead author Murray Brightman, a postdoctoral scholar in physics at Caltech. "Galactic mergers are supposed to generate black hole growth, and the evidence of that would be strong emission of high-energy X-rays. But we're not seeing that here."

Read the full story from JPL News.