Black holes have long been the staple of science fiction, being monstrous beasts with a gravitational pull that prevents even light from escaping.
As well as being useful plot devices, offering mechanisms to travel to alternate universes, while also harbouring destructive creatures, scientists have realised that black holes play a crucial role in the evolution of the universe.
Over the last few decades, astronomers have found that supermassive black holes, with masses of millions to billions of times that of our sun, lurk at the heart of almost all galaxies, including our Milky Way – and the more massive the galaxy, the larger the black hole.
Astronomers this week announced the discovery of the most massive black holes that we know of, some 320 million light years from Earth, with masses around ten billion times greater than the mass of our sun. Their location, in the hearts of relatively nearby giant galaxies, show that the growth of such black holes was implicitly intwined with the growth of the galaxy in which they reside, suggesting a complex interplay over cosmic time.
Fixing a hole
Before we go any further, and the fundamental concept of black holes is called into question, it’s important to understand what the words “black hole” actually mean. Einstein published his new description of gravity, general relativity, in 1915, thinking the mathematics involved were too complex to be solved.
But one year later, the German astronomer and physicist Karl Schwarzschild, who was serving on the Eastern Front in World War One, was able to calculate the gravitational field of a spherical ball of mass using Einstein’s equations.
His mathematics showed the gravitational pull far from the mass matches the predictions of Newton, but as you get closer, gravity influences space and time, and things get a little strange. If the mass is squeezed within a special radius, known as the event horizon, then the mass collapses to a point, and nothing we currently know in science can stop it.
The mass is locked in a singularity (which, despite its almost mythical status in popular science, is nothing but a sign that general relativity has broken down), hidden within the event horizon, from which mass and light cannot escape.
Even for a black hole with a mass a billion times that of the our sun, the event horizon is small, only a few times the size of our solar system. While this might sound large, with light taking roughly a year to cross the distance, it is tiny compared to the total extent of the galaxy, with light taking more than 100,000 years to cross from one side to the other.
Tiny but mighty
This size is too small for an astronomer to see, and with nothing being emitted from the event horizon, it may seem as if they are undetectable objects. What astronomers can see, however, is the influence of stars orbiting in the vicinity of a supermassive black hole. From observing their immense speeds (thousands of kilometres a second) they can calculate the mass in the hole.
So when astronomers say they have seen a black hole, what they mean is that they have “measured an immense amount of mass, from the velocities of stars, and this mass is concentrated in a small volume, and the only objects we know of in modern science would have such bizarre properties are the black holes of Einstein’s theory of relativity”. I think you would agree that is not much of a sound-bite.
Super size me
But to the discovery in hand. Published this week in Nature, graduate student Nicholas J. McConnell and colleagues report the discovery of two new supermassive black holes, in galaxies only a few hundred million light years away.
At this distance, we cannot see individual stars in the galaxies, but using a novel technique, known as integral field spectroscopy, they are able to measure the light from various patches of stars near the galactic centres.
Each of the stars in the patch will have been Doppler-shifted, due to their extreme speeds, and by looking at the absorption patterns in the spectra, they can calculate this, and the central masses. The result, each galaxy harbours a black hole with a mass of roughly ten billion solar masses.
Should we be surprised? The answer is “not really”. As I hinted at the start, every big galaxy we look at appears to harbour supermassive black holes, and the larger the galaxy, the more massive the hole. This M-sigma, or Magorrian, relationship has been known for more than a decade, and as observations have improved, the more distant (and more massive) galaxies we have been able to observe.
Time and again, the mass of the supermassive black hole is strongly related to the mass of the galaxies in which they reside (although these two new ones are a little heavier than expected).
And that leaves us with a problem. Supermassive black holes are small, while galaxies are large. How did the process that drives one influence the other? The answer is that we don’t really know, but we have a number of good ideas, the key one being feedback.
Galaxies are formed when gas collapses, first on galactic scales, feeding gas to the forming black hole, and then through a series of gas fragmentations into billions of stars. However, when you feed gas to a black hole, it does not swallow it in one swoop, but swishes it around like an experienced wine-taster.
This gas, on the brink of being consumed, gets very hot and drives out a lot of the remaining gas in the galaxy, effectively stopping the formation of more stars.
This is a nice picture, but galaxy formation and evolution is a rather violent affair, with a series of massive mergers between infant galaxies, and a continual feeding of gas and dark matter over cosmic time. Can we maintain this strong correlation between galaxies and their black hole masses in such a picture?
We are still trying to work that out, but clearly, the monsters at the hearts of galaxies have a lot of influence on the growth of their hosts.