Finding a Stray

Black holes, by their nature, are difficult to see, which makes detecting them a challenge. Especially the quiet ones. (It’s always the quiet ones.) Astronomers expect that millions of them should be inside our Milky Way galaxy, but to date, we’ve only identified 60 or so.

Some are easier to spot than others. If a black hole has a companion star, gas streaming onto the black hole piles up around it and forms a disk. The disk heats up due to friction from material accelerated by the black hole’s enormous gravitational pull, and infalling material emits intense radiation which we can observe. But a stray black hole—floating all alone in space—creates no observable emission.

Recently, Japanese researchers observing molecular clouds around the supernova remnant W44 (located 10,000 light years away) found quite a surprise. When they measured how much energy was transferred from the supernova explosion to the surrounding molecular gas, they happened to find signs of a hidden black hole at the edge of W44.

The team found a compact molecular cloud with enigmatic motion they named the “Bullet.” It has a speed of more than 100 kilometers per second (well over 200,000 miles per hour), or something like 100 times the speed of sound in interstellar space (yeah, in space, no one can hear you scream, but physicists can still define a “speed of sound” in the diffuse interstellar medium). In addition, the Bullet moves backward against the rotation of the Milky Way. Again, quite a surprise!

To investigate the origin of the Bullet, the team performed intensive observations of the gas cloud with ASTE Telescope in Chile and the 45-meter radio telescope at Nobeyama Radio Observatory. The data indicate that the Bullet seems to jump out from the edge of the W44 supernova remnant with immense kinetic energy. “Most of the Bullet has an expanding motion with a speed of 50 kilometers per second, but the tip of the Bullet has a speed of 120 kilometers per second,” says Masaya Yamada, lead author of the study, published in the Astrophysical Journal Letters.

Some of that energy comes from the ancient supernova that created the nebula—but not all of it! “Its kinetic energy is a few tens of times larger than that injected by the W44 supernova,” says Yamada. “It seems impossible to generate such an energetic cloud under ordinary environments.”

The team proposes two scenarios for the formation of the Bullet. In both cases, a dark and compact gravity source, quite possibly a black hole, plays an important role.

One scenario is the “explosion model” in which an expanding gas shell of the supernova remnant passes by a static black hole (just sitting there minding its own business). The black hole pulls the gas very close to it, giving rise to an explosion, which accelerates the gas toward us after the gas shell has passed the black hole. In this case, the astronomers estimate that the mass of the black hole would be 3.5 times our sun’s mass or larger.

The other scenario is the “irruption model” in which a high speed black hole storms through dense gas, and the gas is dragged along by the strong gravity of the black hole to form a stream. In this case, researchers estimate the mass of the black hole would be 36 times the Sun’s mass—or larger! Until they can collect more data, it is difficult for the team to distinguish which scenario is more likely.

But the researchers are happy with their results. “We found a new way of discovering stray black holes,” says co-author Tomoharu Oka.

The team expects to disentangle the two possible scenarios and find more solid evidence for a black hole in the Bullet with higher resolution observations using a radio interferometer, such as the Atacama Large Millimeter/submillimeter Array (ALMA). Stray black holes can’t hide so well when they leave streamers of supersonic gas in their wake!

Combined X-ray and infrared image of W44 from Herschel SPIRE/PACS/ESA and ESA/XMM-Newton