Black hole menu

The Postdoc Perspective was a blog for the Physics and Astronomy Department at McMaster University in Canada that I kept while I was a postdoctoral researcher. Many of the topics were talks presented at the McMaster Origins Institute seminar series.  

Credit: NASA/JPL-Caltech

Credit: NASA/JPL-Caltech

It is a depressing fact that over 95% of the Universe can't be bothered to interact with you. By looking at the speed with which our galaxy is rotating, we can infer that the amount of mass present must greatly exceed what we can see in stars and gas. This 'dark matter halo' is the cocoon in which our brightly lit spiral galaxy lives.

One of the puzzling features of these galactic cocoons is their wide range of sizes. It is surprising because the size of a galaxy is proportional to its dark matter halo, yet there are no galaxies found in very small or very large halos. It's a little like looking at the cities in Ontario, and finding everyone lives in Hamilton or London, but there is not a soul to be found in Toronto or Ancaster. Additionally, the large galaxies that do exist tend to have predominantly old stars, with very little cold gas from which new stars could form. So, in our Ontario analogy, Ottawa would be populated only by people over the age of 65.

The absence of small galaxies in small halos is explainable by the violent deaths of stars. As a star such as our sun reaches the end of its life, it will throw a large fraction of its substance into the surrounding area in an explosion called a supernova. In a small galaxy, this can blow so much gas out of the halo cocoon that it destroys the galaxy, leaving behind a star-less dark matter halo.

The absence of very large galaxies in the biggest halos however, is more of a mystery. The amount of mass in these galaxies would be so large that any gas that is ejected away from the disc by supernovae will be dragged back down by the gravitational pull of the remaining matter. In the last Origins talk of the semester, Professor Tim Heckmen from John Hopkins University in Baltimore proposed that the answer lies with super-massive black holes.

The most sinister objects in the Universe, a black hole is where so much mass has been squeezed into such a tiny volume that the speed needed to escape its gravitational pull is greater than the speed of light (and that's the fastest speed there is!). For the Earth to become a black hole, it would have to be compressed down to the size of a grape. Super-massive black holes containing the mass of billions of suns, reside at the centre of galaxies. How they have formed is hotly debated but what is known is that the larger the galaxy, the larger its super-massive black hole. Of course, something that destroys everything that enters it does not have the best PR, so going to these objects for answers feels like asking a kraken to attack a single ship; the probability of having any vessels left at the end of its foraging seems rather low.

Professor Heckman's theory is that gas close to the black hole is pulled towards it like water swirling down a drain. As it approaches the edge of the hole, the energy the gas is loosing (by dropping down the black hole-drain) is converted into heat at a rate that is much more efficient than nuclear fusion. The resulting radiation is the most intense source in the Universe. If enough gas falls in, the black hole can go through a 'feeding frenzy' and produce jets that evacuate huge holes in the galaxy. These jets fill the halo with hot gas, removing all the cold star-forming gas from the disc. If the jets are strong enough, the galaxy could be destroyed completely. If it isn't, then the resulting cavity around the black hole removes its food supply and the jets turn off. Yet, as the ejected gas cools, it falls back down to the galaxy, serving up another black hole dinner.

Why though, would this mechanism not occur in our own Milky Way, destroying us and smaller galaxies along with the bigger ones, kraken style? The answer, Professor Heckman explains, is that the super-massive black hole needs feeding with a lot of gas to produce the powerful jets. One possible source of this food is from supernovae as they blow away their outer layers. This gas might initially go into the halo, but as it returns to the galaxy, it could be drawn into the black hole which would then start to feed and produce jets. Larger galaxies will have more stars and therefore more supernovae, increasing the food supply to a point where jets can be formed. This means that the lifecycle and star production of a galaxy is intimately linked to its super-massive black hole. To understand one, Professor Heckman said, you need to understand the other.

At the end of the talk, there was one important question on the audience's mind:

Which came first; the black hole or the galaxy?