Stray black hole in Milky Way could be a cosmic bullet

Think "Milky Way" and "black hole," and the supermassive pit of gravity at the center of our galaxy is bound to come to mind. But there are a collection of other rogue black holes wandering through our cosmic spiral neighborhood, and researchers from Japan believe they've just spotted one zipping through the gaseous remnants of a supernova explosion.

Working for the National Astronomical Observatory of Japan (NOAJ), a research team was using the ASTE Telescope in Chile and the 45-m Radio Telescope at Nobeyama Radio Observatory in Nagano, Japan to examine supernova remnant W44, which is found about 10,000 light years from Earth. They were studying the way in which energy transferred in the system from the exploded star to the cloud of surrounding gas. But they saw something unexpected – a tightly packed molecular cloud that was hurtling through space at 100km/s (about 62 mi/s) and traveling in the reverse direction of the Milky Way's rotation.

They nicknamed the cloud the "Bullet" and set to figuring out just what it was.

"Most of the Bullet has an expanding motion with a speed of 50 km/s, but the tip of the Bullet has a speed of 120 km/s," said Yamada. "Its kinetic energy is a few tens of times larger than that injected by the W44 supernova. It seems impossible to generate such an energetic cloud under ordinary environments."

So, searching for an extraordinary solution, the team posited the existence of a black hole that could have been influencing the molecular cloud in one of two ways.

The first scenario posits that as the supernova expanded outwards, it enveloped a nearby black hole that sucked its material towards it, as black holes do. The result of having all of that molecular material condensed so tightly was an explosion that hurled the gas outwards at the impressive speeds seen, which "exceeds the speed of sound in interstellar space by more than two orders of magnitude" according to a report on the research from NOAJ.

The second theory is that the black hole itself was moving, passed through the molecular cloud and pulled a bunch of the gas along with it, much like a bullet through an apple.

If the first case is true, the researchers estimate that the black hole is about 3.5 the mass of our sun. If the second is true, they estimate that it would measure 36 times the solar mass. By way of comparison, the supermassive black hole at the center of our galaxy measures 4.6 million times the mass of our sun.

According to the NOAJ, there should be approximately 100 million to 1 billion black holes in our galaxy, although only 60 have been spotted so far. The next step for the research team is to try to find out which theory fits the observed scenario better, possibly arming astronomers with a new tool for spotting the spheres of intense gravity elsewhere in the Milky Way.

Their work has been published in the Astrophysical Journal Letters.