Dark matter and black holes are some of the most mysterious things in the Universe, so a connection between the two is absolutely thrilling. In a new study, astronomers report a strange link between the amount of dark matter in a galaxy and the size of its supermassive black hole. That’s an amazing new black hole fact!

Artist rendition of a supermassive black hole. Image via Red Orbit

Most galaxies have a black hole at their centers – millions or even billions times heavier than our Sun. The origin of supermassive black holes remains an open question, with several competing hypothesis being put forth. The bigger the galaxy is, the bigger and heavier the black hole is… but why does this even happen? Why are the two connected? A new study of football-shaped collections of stars called elliptical galaxies suggests that the key here may be dark matter – the mysterious “thing” which makes up for most of the mass in the Universe. So, an amazing new

“There seems to be a mysterious link between the amount of dark matter a galaxy holds and the size of its central black hole, even though the two operate on vastly different scales,” says lead author Akos Bogdan of the Harvard-Smithsonian Center for Astrophysics (CfA).

Dark matter is really strange – we don’t know what it is and we can’t see it. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the Universe. In other words, we can’t see it, but we can see its effects… and we pretty much have no idea what it is. It’s one of the biggest mysteries of astrophysics, especially when you consider that dark matter outweighs normal matter 6 times.

To investigate the surprising link between dark matter and black holes, Bogdan and his team studied more than 3,000 elliptical galaxies. They used star motions as a tracer to weigh the galaxies’ central black holes and conducted X-Ray measurements to weigh the dark matter halo. The more dark matter a galaxy has, the more hot gas it can hold onto and the bigger halo it has. They found that the more dark matter a galaxy has, the bigger its black hole tends to be – even more so than the logical connection between a black hole and the galaxy’s stars.

It’s not clear exactly why this happens, but the likely explanation is connected to the way a galaxy grows. There are several types of galaxies, the most common ones being elliptical galaxies (smooth, featureless light distributions and appear as ellipses in images), spiral galaxies (consist of a flattened disk, with stars forming a spiral structure) and lenticular galaxies (consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure).

Elliptical galaxies form when two smaller galaxies merge, with their stars, planets and black holes mingling together. Because the dark matter is heavier than everything else, it molds the formation and development of the new galaxy, even from the outskirts to the central black hole.

“In effect, the act of merging creates a gravitational blueprint that the galaxy, the stars and the black hole will follow in order to build themselves,” explains Bogdan.

The teacup galaxy. Red/yellow shows the radio emission “bubbles”, blue shows the gas and the bright yellow in the centre shows where the jets are driving into the gas. Image via CNET.

This study, which was conducted on the Teacup Glaaxy, might also provide information about black hole formation and development in galaxies – something on which we have very little information.

“This particular piece of work on the Teacup Galaxy has provided new insight into how the black holes [drive energy] in ordinary galaxies,” Harrison said. “They appear to be capable of driving jets of charged particles that collide into the gas. You could imagine the ‘jet’ as like a water cannon being driven into a crowd of people – the water cannon collides with the crowd and causes it to break up and disperse rapidly. In this analogy, the crowd represents the gas in the galaxy that is trying to form stars, but is destroyed by the jet.”

The fact that our galaxy also has a supermassive black hole at its center further gives astronomers study material. In fact, there may be significant similarities between the Milky Way and the Teacup Galaxy (aside for their beverage names).

“There is a supermassive black hole at the center of our Milky Way,” Harrison added. “There is now good evidence that this black hole has driven large amounts of energy into the galaxy in the past, through the so-called ‘Fermi Bubbles,’ as well as other evidence. It is likely that billions of years ago the Milky Way was forming stars much more rapidly and the black hole may have played a role in shutting this down. However, this is not well understood. It is worth pointing out that the galaxies where we believe supermassive black holes have had the most influence are ‘dead’ with little-to-no stars forming. In contrast, the Milky Way is still forming stars (around one per year).”

The paper describing this work has been accepted for publication in the Astrophysical Journal.


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