A faint, ghostly galaxy 10 million light-years away has delivered one of black hole astronomy’s holy grails.
At the bottom is a black hole that appears to belong to an elusive middleweight class of intermediate-mass black holes, a discovery that could help us understand how some of the more massive black holes form.
The galaxy in question is a dwarf galaxy called Mirach’s Ghost (or, less poetically, NGC 404), and is suspected of harboring one of these ‘missing link’ intermediate mass black holes. Now, a new technique seems to have validated this suspicion, with scientists discovering a black hole inside the Mirach Phantom with a mass 550,000 times greater than that of the Sun.
While the boundaries between intermediate mass black holes (IMBH) and supermassive black holes (SMBH) are currently not very well defined, IMBHs are generally considered larger than a typical collapsed star (up to 100 solar masses) but not supermassive (between a million and a billion times more mass than a typical stellar black hole).
So the seemingly medium mass of the new discovery makes it an important object in understanding how supermassive black holes form and grow.
Supermassive black holes are a huge puzzle. We have a good handle on how the smallest stellar-mass black holes form: they are the dead and collapsed cores of massive stars, and they can have up to a few dozen solar masses.
But there is an upper limit to this formation model imposed by the mass of the precursor star. If the star begins with a mass between 130 and 250 solar masses, it ends its life in what is called a pair instability supernova that makes the star explode.
You may have noticed that there is a large gap between stellar-mass black holes and supermassive black holes. That’s where intermediate-mass black holes should fall, but they have been incredibly difficult to find.
This poses a problem, because if black holes start from small stellar mass, as a model of evolution proposes, and become powerful beasts accumulating much and much matter for a long time, then intermediate mass black holes would logically be the passage between the two.
The other possibility is that supermassive black holes were born that way, directly collapsing from a huge group of matter already found in the nuclei of galaxies. And supermassive black holes have been found in the early Universe, too soon after the Big Bang to have had time to grow from stellar-mass black holes.
However, if this were the case, there would be a lower limit on the mass of supermassive black holes.
One way to learn more would be to discover intermediate mass black holes. They wouldn’t necessarily invalidate the direct collapse model, but they would be a big brand in favor of the accretion model.
In fact, we’ve had some pretty convincing indirect observations that suggest these mean weights exist, but astronomers believe that even stronger evidence can be found in the nuclei of small galaxies, also known as dwarf galaxies.
Dwarf galaxies tend to preserve clues to their history of black hole evolution far better than larger, more battle scarred relatives. Understanding the characteristics of its intermediate-mass black holes would be a great victory in understanding how they grow.
Cue Mira’s Ghost, so called because it is very difficult to see, obscured by a much closer and very bright star. A decade ago, astronomers found evidence that a black hole of at least a few tens of thousands of solar masses was at its center, but because the galaxy is difficult to see, it was very difficult to learn more.
Two things have happened since then. The Atacama Large Millimeter / submillimeter Array (ALMA) in Chile, a state-of-the-art telescope with incredible resolution, became operational in 2011. Then, in 2014, astrophysicists validated a technique to derive mass from a black hole based on the gas movements around it.
This is what a team of astronomers led by Tim Davis of Cardiff University did. They used ALMA to observe the Mirach Phantom in high resolution, mapping the movement of the gas around its nucleus at a very high resolution of 1.5 light years in diameter.
They then used simulation software to predict gas distribution and kinematics, comparing these results with observations to obtain the best fit.
This is how they calculated the mass of the black hole. Since the definition of ‘intermediate mass’ is not well defined, its classification in that category could be debatable. But, much more interesting, it provides support for both models of evolution of supermassive black holes.
“The SMBH at Mirach’s Ghost appears to have a mass within the range predicted by the ‘direct collapse’ models,” Davis said.
“We know that it is currently active and is swallowing gas, so some of the more extreme ‘direct collapse’ models that only produce very massive SMBHs may not be true. This alone is not enough to definitely distinguish the difference between the image of ‘seed’ and ‘direct collapse’ – we need to understand the statistics for that – but this is a massive step in the right direction. “
There have been other similar low-mass supermassive black holes. A galaxy called NGC 4395 has a black hole calculated at 360,000 solar masses, and the black hole at the heart of a galaxy called POX 52 was measured at 160,000 solar masses.
It is only by finding many more of these objects that astronomers will be able to start piecing together the puzzle.
“Our study shows that with this new technique we can really begin to explore the properties and origins of these mysterious objects,” said Davis.
“If there is a minimum mass for a supermassive black hole, we haven’t found it yet.”
The research has been published in the Monthly notices from the Royal Astronomical Society.
.
