Massive clouds dim the powerful X-rays coming from the centers of some galaxies.
At the hearts of most big galaxies, including our own Milky Way, there lurks a supermassive black hole weighing millions to billions of times the sun's mass. As gas falls toward a monster black hole, it gathers into a disk and becomes compressed and heated, ultimately emitting X-rays. In 2014, using data from NASA's Rossi X-ray Timing Explorer satellite, scientists uncovered a dozen instances where X-ray signals from active galaxies dimmed as a result of a cloud of gas moving across our line of sight. Measurements show these clouds vary in shape and size but average 4 billion miles across, or large enough to span the solar system from the sun to beyond Pluto’s orbit. The findings will help explain how gas funnels into the hot accretion disk that feeds a supermassive black hole. Watch the video to see the cloudy structure of an active galaxy's core.