Astronomers Think They've Found The Second-Largest Black Hole In The Milky Way

An artist's impression of the new intermediate-mass black hole. Tomoharu Oka/Keio University

There are thought to be three types of black holes out there, each a different sized, star-swallowing, spacetime-warping colossus. A new study published in The Astrophysical Journal Letters claims to have discovered a fourth intermediary type hiding within our very own galaxy.

The first type of black holes, stellar black holes (SBHs), appear when extremely massive stars, having reached the end of their lives, are no longer able to perform nuclear fusion. The inwardly directed gravitational force of the star overcomes the externally directed generation of heat, and a catastrophic implosion occurs. The resulting SBH typically has a mass of around ten Suns (solar masses).

The second type, primordial black holes, are purely hypothetical at this stage. Formed by the compaction of matter present during the universe’s early expansion, they could be as small as a single atom, but with the mass of a large mountain.

Supermassive black holes (SMBHs) are typically found at the hearts of galaxies, and are easily the most gargantuan. Our Milky Way's Sagittarius A*, for example, is four million times more massive than the Sun. Their formation mechanism is still open to debate: Some scientists think that they grow by absorbing smaller black holes over time, whereas others believe they form from the collapse of enormous gas clouds early on during the galaxy formation.

The problem with this three-tiered categorization is that there isn’t much room for “intermediate-mass black holes” or IMBHs – those with masses somewhere between that of a SBH and a SMBH. In 2014, the discovery of one such black hole was claimed: A luminous object called X-1 in the constellation M82, with 400 solar masses, was spotted, and astronomers declared it an IMBH.

The SMBH, Sagittarius A*, at the core of the Milky Way, seen with respect to the new IMBH candidate, highlighted in magenta. The scale bar is in parsecs, where one parsec equals 3.26 light-years. Oka et al./The Astrophysical Journal Letters

Its mass was calculated by looking at the X-ray emissions jettisoning from it. Astronomers noticed that these emissions seem to beat like a drum, appearing at set 3-2 ratios – for example, 300 times per second and then 200 times per second. The frequency of these “beats” are inversely proportional to the mass of the black hole, so an emission rhythm of 150 times per second then 100 times per second would be coming from a more massive black hole than one with a 300-200 beat. X-1 had a rhythm of roughly 5-3.3 X-ray emissions per second, meaning that it had a mass of 428 Suns, making it an IMBH.

This more recent study wasn’t actually looking for IMBHs, but was instead peering at an enigmatic gas cloud called CO-0.40-0.22. Using Japan’s Nobeyama and Chile’s ASTE radio telescopes, the researchers noticed that the gas molecules in the cloud were all moving at a wide range of speeds, meaning something was accelerating them. X-ray observations, like those used to look at X-1, revealed nothing.

However, a simulation of the gas movement in the cloud suggested only one culprit: namely, an IMBH of around 100,000 solar masses is hiding in the cloud, using its powerful gravitational field to fling particles chaotically through space. If proven to be true by independent studies, this IMBH will be the first of its kind: it, and its gas cloud shield, are both within our own Milky Way. It will also be the second most massive black hole in our galaxy, after Sagittarius A*.

However, a controversy now exists. X-1 was detected using powerful X-ray emissions, and this new IMBH candidate was not. And since there is no reference IMBH for comparison, that these two studies therefore sharply disagree on which one, if any, is actually an IMBH.

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