The biggest object in our galaxy is remarkably difficult to see. The core of our galaxy houses a supermassive black hole that weighs in at over a million times our Sun's mass. And when it's actively feeding on matter, it should be very bright. Yet for years, all we knew was that there was some sort of radio source there.

Evidence of a black hole at the center of our galaxy came indirectly by tracking the orbit of a nearby star. This demonstrated that there had to be something extraordinarily heavy in a very small region of space, strengthening the case that the object was an immense black hole.

Now, researchers are making a similar case for what may be the second-biggest black hole in the Milky Way. The object appears to be buried in a gas cloud that's keeping it obscured. But the gas itself is moving fast enough that calculations suggest that a 100,000 solar-mass black hole is holding it together. That would make the object an intermediate-mass black hole. While intermediate-mass black holes play a key role in many cosmological models, we have yet to confirm any actually exist.

Fast gas

Even by astronomical standards, the gas cloud in question has a pretty dull name: CO–0.40–0.22. And it's a standard looking molecular cloud—a body of gas cool enough for its atoms to get together and form molecules—unless you happen to look more closely, as a team of Japanese researchers did back in 2015.

Individual molecules within the cloud have specific signatures, places on the electromagnetic spectrum where they absorb or emit photons. If the gas is moving, however, these signatures get red- or blue-shifted by the Doppler effect, depending on whether they've moving away from us or toward us, respectively. If a cloud of gas is rotating or turbulent, parts of it will be moving toward us while other parts will be moving away. Here, you get red and blue shifts on both sides of the wavelength at the molecule's signature, resulting in its broadening.

CO–0.40–0.22 had a very large broadening, indicating its contents were moving rapidly. This placed the object in what the researchers refer to as a "peculiar" category called "high-velocity compact clouds." It's peculiar because CO–0.40–0.22 doesn't seem to have enough mass for its gravity to hold the cloud together against the rapid motion of its contents.

So what could? The authors ran a variety of models and came up with one possible solution: a very large black hole.

Thinking black

In a new paper, the same team is back and looking more carefully at the idea of a black hole lurking (lurking is their term) in the cloud of gas. If it were, in fact, a black hole, it would provide the first confirmation of the existence of intermediate-mass black holes.

Black holes are formed by the death of massive stars. Supernovae create black holes up to several times the mass of the Sun, and there's a prospect of slightly larger black holes being formed by a non-explosive mechanism. The black hole mergers observed using LIGO seem to involve objects up to 30 times the mass of the Sun. At the other end of the spectrum are supermassive black holes, millions of times the mass of the Sun, which sit at the center of galaxies.

Supermassive black holes appear in the Universe too early for there to have been enough time for any small black holes to have fed on enough material to become supermassive. So cosmologists have posited the existence of intermediate-mass black holes. Formed on the heavy side to begin with through the merger of stars, these grow to tens of thousands of solar masses before merging to create supermassive black holes.

But if this idea is right, then there are going to be some un-merged intermediate-mass black holes hanging around. And, although we have a number of candidates, we haven't confirmed the existence of any of them yet.

The authors' modeling suggested that the black hole in CO–0.40–0.22 was probably 100,000 solar masses, which would place it squarely in the intermediate-mass camp. That would also make it the second largest black hole known in our galaxy, after the supermassive one right at the center.

To get a better view of what's going on inside the cloud, the researchers turned to the ALMA telescope array, which is sensitive to wavelengths that pass through gas and dust. ALMA identified a point source of radiation that, within its ability to resolve objects, appears to be inside the gas cloud. The amount of light coming out of it at these wavelengths were about 1/500 of that emitted by the supermassive black hole at our galaxy's center.

The temperature of the gas cloud, measured through separate observations, suggests it's too cool for the gas itself to be the source of this radiation. And a check of X-rays using the XMM-Newton satellite shows that there's a point source in the region that also appears to be emitting about 1/500 of the radiation coming out of the Milky Way's supermassive black hole.

We're not quite at confirmation yet, but the evidence is leaning that way. So, the authors consider an obvious question: how did the black hole get there? Some models suggest that intermediate-mass black holes can form in dense star clusters. Yet, in these models, the size of the black hole tends to be about 0.1 percent the size of the cluster. And, given the size of the one in CO–0.40–0.22, that would mean something bigger than a cluster: a dwarf galaxy.

Galaxies like the Milky Way are thought to have been formed from the merger of multiple dwarf galaxies, and the remains of some continue to orbit our galaxy. So it's possible that this black hole is simply left over from the process that build the Milky Way. And, since it's in the vicinity of the galactic core, there's a chance that, one day, it too will merge with the supermassive black hole at the center.

Nature Astronomy, 2017. DOI: 10.1038/s41550-017-0224-z (About DOIs).