Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Indication of Another Intermediate-mass Black Hole in the Galactic Center

Published January 2019

Main takeaway:

By analyzing the internal motions of a cloud of molecular gas using the Atacama Large Millimeter/submillimeter Array (ALMA), a team of scientists led by Shunya Takekawa (Nobeyama Radio Observatory, NAOJ, Japan) has found evidence for a 30,000 solar-mass black hole drifting near our galaxy’s center.

Why it’s interesting:

Why this discovery may lead to others:

A weight of 30,000 times that of the Sun would categorize this object as an intermediate-mass black hole (IMBH) — a black hole of 100 to 100,000 solar masses, a category that we’ve never directly observed. Though we’ve long suspected that supermassive black holes —black holes of millions to billions of solar masses that often lurk at the centers of galaxies, like our own Sgr A* at the center of the Milky Way — may grow via the mergers of IMBH seeds, we’ve lacked evidence for this theory. The possible IMBH discovered by Takekawa and collaborators lies within 20 light-years of Sgr A*, suggesting that such a merger may be in our galaxy’s future.

The discovery of this black hole is interesting for another reason: it was found despite the fact that it’s extremely faint. Theory predicts that there are perhaps a hundred million black holes in the Milky Way. Since the vast majority of these objects are dark, however, we’ve found very few of them — only a few dozen exceptions that are bright due to active accretion. Takekawa and collaborators found the faint intermediate-mass black hole not by looking for its emission, but by examining the motions of a gas cloud around it: the gas streams of the cloud were being flung in unexpected directions due to the gravitational forces of an unseen source. By using this same technique in the future, we may be able to discover additional otherwise-invisible black holes.

Citation

Shunya Takekawa et al 2019 ApJL 871 L1. doi:10.3847/2041-8213/aafb07