ALMA uncovers a hidden black hole Read time: 3 minutes Black holes are the most mysterious objects in the Universe. Their gravity is dangerously strong. They suck in everything that comes too close. Moreover, they don’t emit any light. As a result, they’re completely invisible. Still, there are ways to detect black holes. That’s because they exert a tell-tale influence on their immediate surroundings. For instance, gas that falls towards a black hole is heated up. Eventually, it becomes high enough to emit energetic X-rays. Also, stars or gas clouds close to a black hole are seen to whirl around at very high speeds. Using ALMA, Japanese astronomers have now uncovered a black hole close to the center of our Milky Way galaxy. The astronomers studied a strange, dark cloud of molecular gas, known as HCN-0.009-0.044. The gas cloud looks remarkably compact. The ALMA observations revealed that the cloud whirls around rapidly. A compact gas cloud with a fast rotation – this suggests that it harbors a small, massive object. From the observations, the astronomers deduce that this mysterious object must be some 30,000 times more massive than the Sun, but smaller than our solar system. However, it doesn’t emit any light. Therefore, the astronomers think it must be a black hole. In the very center of the Milky Way sits a much more massive black hole. Known as Sagittarius A*, it is four million times the mass of our Sun. The newly discovered black hole is only some 20 light-years away from this dark monster. In the future, it will probably fall into Sagittarius A*, adding slightly to its mass. Astronomers believe that small black holes merge and grow into ever larger ones. The discovery of a ‘medium-mass black hole’ weighing in at 30,000 Suns supports this view. What? HCN-0.009-0.044 is a so-called high-velocity compact cloud (HVCC). Several of these clouds are known in the central region of the Milky Way. However, HCN-0.009-0.044 is even more compact than the others: it measures only a few light-years across. The cloud is some 25,000 light-years away from the Earth, in the constellation Sagittarius. In fact, it sits close to the very center of our Milky Way galaxy. The new ALMA observations suggest that it contains a medium-mass black hole, weighing in at some 30,000 times the mass of the Sun. Who? The ALMA observations of HCN-0.009-0.044 were carried out by a group of astronomers led by Shunya Takekawa of the National Astronomical Observatory of Japan. Shunya worked together with his Japanese colleagues Tomoharu Oka, Yuhei Iwata, Shiho Tsujimoto, and Mariko Nomura. They have published their results on January 20, 2019 in the Astrophysical Journal – a professional astronomy magazine. ALMA URL