Meet Stefi A. Baum and Christopher O’Dea. Baum is the dean of the faculty of science as well as a professor in the physics and astronomy department, while O’Dea is a professor in the same department, in the field of observational astronomy. They are also in the centre of one of the strangest recent discoveries about black holes.
For the first time, astronomers have discovered a black hole that appears to be experiencing rain. The supermassive black hole-300 million times the size of the sun, to be exact-seems to be accompanied by rain clouds. These rain clouds are lumped-together cold gas, about the mass of a million suns, and appear to be feeding the black hole, and contributing to its constant growth in the process.
What makes this discovery so interesting is that it changes so much about what we know about black holes. While we know that star formation is fuelled by cold gas clouds, this is the first time experts have found a black hole doing the same thing.
Led by a former graduate student of theirs, Grant Tremblay, Baum and O’Dea were part of a group of researchers that made the discovery. The Manitoban had a chance to sit down with both Baum and O’Dea to discuss the discovery, as well as further research they’re working on.
How it works
The fuelling of a black hole, formally called accretion, was originally thought to be a simple, logical process. The conventional view of black hole accretion is that they grow and feed on hot gas in the form of a ring, which slowly and constantly accretes into the black hole.
What has happened here, however, is that a huge cloud releasing cold lumps of molecular gas is fuelling its activity, and it may be that the cold gas is the primary fuel source.
“There may be some black holes that do feed on hot gas, but there’s a lot of cold gas in galaxies and we know that galaxy activity – that is, when they become active and release energy – is when there has been some kind of interaction or merger,” said O’Dea.
“And that’s going to provide cold gas to the black hole. So, now it seems plausible that it’s really cold gas that’s supplying the fuel for black hole activity.”
The study, which was published by Nature, provides several new insights into the all-consuming nature of black holes. The clouds that host this cosmic rain are only about 150 light years away from the black hole. Because they are so close, as the black hole slowly grows and absorbs these cold gas clouds, eventually the clouds will fall in. This research marks the first time concrete evidence has been found that black holes can be fuelled by clouds like these.
A long time coming
The first known sighting of cold gas clouds falling into a black hole was observed in 1994, in a galaxy located in the Abell 2597 Cluster. Baum and O’Dea continued to gather data from the same galaxy with the use of the Hubble telescope.
With the later launch of the Herschel space observatory in May 2009, they had enough data to make a thesis project, and had their graduate student Grant Tremblay work on the particular galaxy that housed the black hole.
When the Atacama Large Millimeter/submillimeter Array (ALMA) came online in March 2013, it allowed them to take a better look at the object in question and observe the black hole in action.
“Basically, as you can see, anytime there was a new technology or new observatory that came online with some new capability, we would turn it on this object because we knew it was a really interesting object,” Baum said.
Expect the pair to jump on the next available technology that can allow them to take another good look at this fascinating phenomenon.
Further work to be done
Thanks to ALMA, they were able to use the evidence from 1994 and see it more clearly . However, O’Dea noted there is still a bit more work to be done. Only the brightest clouds have been observed, and being able to go deeper to take a look at the fainter gas would give them a better idea of how the clouds are moving and how the black hole’s fuelling process works.
“The thing with ALMA is it’s hard to get time on it, and it takes quite a while to reduce the data. So, you know, every year, you can get maybe another more detailed observation or another object or whatever so it builds up slowly over time,” said Baum.
But, as Baum noted, it is possible to gather a group of galaxies that are showing the same type of activity as the one in question, and they do exist. By doing that, additional information can be gathered while waiting to try and get new observations.
Clearly, you need to make the most of your time with ALMA, and they have done just that.
Along with this discovery, Baum and O’Dea have many other projects they’re working on. The New General Catalogue (NGC) is a collection of deep-sky objects that contains a different galaxy called the NGC 1068 that they have been observing for some time.
“We’ve used ALMA on a different type of object. It’s a spiral galaxy and it has a black hole that’s being fuelled,” O’Dea said.
Given how close this object is, they were able to see the finer details. What they found was a ring of molecular gas around the black hole, forming a donut shape.
“This is the first time that we’ve been ever been able to see things, this molecular donut, around these active galaxies. For 30 years we thought it had to be there, but this is the first time we’re actually observing it, ” said O’Dea.