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What's The Mass Of A Supermassive Black Hole?

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Caroline Reid

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640 What's The Mass Of A Supermassive Black Hole?
The central region of NGC 1097. Red indicates gas moving away from us and purple gas is coming toward us. ALMA (ESO/NAOJ/NRAO), K. Onishi (SOKENDAI), NASA/ESA Hubble Space Telescope

There is something monstrous sitting at the center of our galaxy. In fact, it is thought that these hungry beasts sit at the center of most galaxies. No, they're not ghosts or ghoulies or galactic space squid, but black holes. 

Black holes are the most massive objects in the universe. They are the final form of especially massive dying stars. At the end of such a star's life, it collapses under its own gravitational pull, dragging matter closer and closer together until it forms an unimaginably dense, tiny ball of matter. 


Scientists set out to measure exactly how enormous one of these black holes is: a supermassive black hole (SMBH) in the center of a galaxy named NGC 1097. Their results are published in The Astrophysical Journal. The black hole was measured to be a mind-boggling 140 million times the mass of the sun: and that's not even the biggest black hole that's out there!

Visualization of the mass of a supermassive black hole at the centre of the Phoenix Cluster. Its mass is 20 billion times that of the Sun. Clip from video below.

The research was based on data taken by a team led by Kyoko Onishi at the SOKENDAI (The Graduate University for Advanced Studies). The SMBH data was taken by Atacama Large Millimeter/submillimeter Array, or ALMA. It demonstrates the outstanding capacity that ALMA has for measuring the masses of SMBHs.

There are a few different ways to measure the mass of an SMBH, but most of the techniques are quite fussy. A technique that measures the mass of the SMBH at the center of an elliptical galaxy might not work for a spiral galaxy, for example. Taking NGC 1097's barred-spiral shape into consideration, the team derived the SMBH mass from the dynamics of the molecular gas in the central region of the galaxy surrounding the black hole. 


Looking at the data of the motion of molecular gas has an advantage over the data of ionized gas and stars. The motion of all of these things is affected by the SMBH's gravity, but also a host of other, distracting environmental sources. Molecular gas is affected the least by these distracting sources, so its motion is more associated with the gravitational pull of the SMBH. This makes it a god way to predict the SMBH's mass.

You can see the motion of the molecular gas in the super-colorful main image. The colors haven't been daubed on top of the image arbitrarily; purple-colored gas is moving toward Earth and red-colored gas is moving away from Earth. This information indicates that the galaxy is rotating, thanks to the central SMBH. The team could tell which direction and how fast the gas was moving by observing the light emissions coming from the gas. By comparing the spectrum of light emitted from gas in the galaxy to light emitted from the same gas on Earth, we can tell which way the space-gas is moving and how quickly, and thefore predict how big the central SMBH would need to be to have this effect on the gas. As it turns out, it takes 140 million solar masses to account for the motion!  

The team's next task is to measure the masses of more black holes so that they can start to solve the mystery of how galaxies and SMBHs formed and coevolved.

For a mind-blowing visualization of how much mass is squeezed into a black hole, check out the video below.




[Via SOKENDAI, The Astrophysical Journal]


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