A Tadpole-Shaped Cloud of Gas is Whirling Around a Black Hole

In the 1930s, astrophysicists theorized that at the end of their life cycle, particularly massive stars would collapse, leaving behind remnants of infinite mass and density. As a proposed resolution to Einstein’s field equations (for his Theory of General Relativity), these objects came to be known as “black holes” because nothing (even light) could escape them. By the 1960s, astronomers began to infer the existence of these objects based on the observable effects they have on neighboring objects and their surrounding environment.

Despite improvements in instruments and interferometry (which led to the first images of M87 and Sagittarius A*), the study of black holes still relies on indirect methods. In a recent study, a team of Japanese researchers identified an unusual cloud of gas that appears to have been elongated by a massive, compact object that it orbits. Since there are no massive stars in its vicinity, they theorize that the cloud (nicknamed the “Tadpole” because of its shape) orbits a black hole roughly 27,000 light-years away in the constellation Sagittarius.

The research team was led by Miyuki Kaneko, a School of Fundamental Science and Technology (SFST) at Keio University. He was joined by astrophysicists and engineers from the SFST, the Technology Institute of Science and Technology (Keio University), the National Astronomical Observatory of Japan (NAOJ), Kanagawa University, and the Center for AStronomy at Ibaraki University. The paper that describes their findings was recently published in The Astrophysical Journal.

Annotated map of the Milky Way Galaxy with the constellations that cross the galactic plane in each direction and the known most prominent components. Credit: Pablo Carlos Budassi

The team used data from the James Clerk Maxwell Telescope at the East Asian Observatory and the NAOJ’s Nobeyama 45-meter Radio Telescope to observe the Tadpole molecular gas cloud. They noted that the cloud is unique due to its characteristic head-tail structure, position, and velocity. Based on its kinematics and changes in line intensity along its orbit, the team determined that the best fit was a black hole. They were also able to constrain its mass, which they estimated to be 1 million times the mass of our Sun.

This would make it an intermediate-mass black hole (IMBH), placing it between a stellar mass and a supermassive black hole (SMBH). The presence of a black hole roughly one-fourth as massive as Sgr A* and located in the Galactic Bulge not far from where Sgr A* resides (25,640 light-years away) raises many interesting questions. In the near future, the team plans to use the Atacama Large Millimeter/submillimeter Array (ALMA) to search for more evidence of a black hole at the gravitational center of the Tadpole’s orbit.

These examinations could lead to some major discoveries. For example, could this IMBH be destined to merge with Sgr A* someday? Such an event would cause the black hole at the center of the Milky Way to become up to 20% more massive. It would also trigger a huge release of gravitational waves (GWs) that observatories would NOT fail to notice!

Further Reading: NAOJ, The Astrophysical Journal

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