The Dangers of Swimming While Consuming Ice Cream

July has been a hectic month in Europe for some CANDELS astronomers. In the last post you heard from Fernando, who did a great job of organising a conference on "Declining Star Formation" at the UK National Astronomy Meeting in St Andrews, before jetting off to Finland for the European Week of Astronomy to discuss how to "Build Elliptical Galaxies" and the "Co-evolution of Galaxies and Black Holes". 

Taking a much needed break from the conference season

After a brief pause for some sea kayaking in the north-west highlands of Scotland, it's back to the summer conference season for me: this week I'm in Leiden in the Netherlands, where we're still talking about supermassive black holes [see the conference website here]. 

And it seems there's still a lot to talk about. To summarise the first two days in a nutshell, astronomers don't agree about how the supermassive black holes (SMBHs) that live in the centres of massive galaxies got to be as "supermassive" (i.e. heavy) as they are. The story starts with observations made over 10 years ago, which showed that the mass (i.e. weight) of a SMBH is correlated with the mass of the galaxy in which it lives. [For more info, read about the M-sigma relation here]. So, the more massive the galaxy, the more massive the supermassive black hole in its centre.

The problem is that a correlation between two observables does not necessarily mean they are causally connected. Take a more down to Earth example: it can be shown that as ice cream sales increase, the rate of deaths by drowning increases. If you take this statement at face value and don't stop to think about it, you would naturally conclude that ice cream consumption causes drowning. But of course there is a "hidden variable" that needs to be taken into account: the weather. This is a very appropriate example for the unusually warm temperatures we are enjoying here in the Netherlands this week! Obviously hot weather causes more people to eat ice cream and more people to take a swim, unfortunately sometimes fatally, but probably not while eating their ice cream.

So the problem we are trying to solve is: is there a "causal connection" between galaxy mass and supermassive black hole mass? Or is there a "hidden variable" that causes them both to grow together? Let's look at the two main contenders:

Hidden variable: gravity

 

The Mice merger: a collision of massive galaxies with plenty of
gas in the local Universe. The tails of the Mice are caused
by strong gravitational forces acting on the gas and the stars in the
galaxies as they passed one another for the first time about
two hundred million years ago. The two galaxies will collide in around
700 million years time as gravity finally manages to pull them together.
If both galaxies have supermassive black holes in their centres,
they will merge at the same time. Image Credit:
http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/

There are several important forces in physics that attract or repel objects towards or away from each another (I'm sure you can all recall these from your school physics lessons!). Luckily for astronomers interested in how galaxies form and evolve, the only dominant force that acts over large enough distances to be important to galaxies is the force of gravity (ok, this is not quite true, ask a cosmologist about dark energy, but it's not so relevant here). In the early Universe gas was distributed almost, but not completely, homogeneously. Over time, the regions with the highest densities (i.e. amount of mass in a given volume) had the largest gravity force and therefore attracted more gas, making them denser. Stars formed from this gas, and we think that the first black holes must have formed from these first stars. Time keeps progressing, and the regions with the highest densities always have the largest gravity force and always attract more gas, more stars and more black holes. So this looks trivial right? The biggest black holes form in the biggest galaxies, because of gravity.

The problem is, if you get your hands on a large supercomputer and try to recreate this process, you find that you very rapidly build galaxies that have orders of magnitude more stars than those that we observe in the real Universe. And the SMBHs in the centres of the simulated galaxies have a rather bad habit of swallowing everything they can get their hands on -- because of the force of gravity. So no nice tight correlation between the mass of the galaxies and the mass of the SMBH in the centre. Well, observational astronomers always tend to think the theoretical astronomers don't know what they are doing, and that might still be the case (yes, I'm mostly an observational astronomer). But when they *all* tell us it's impossible, we have to at least question whether we've missed something in our observations.

Causal connection: energy

 

In "simulation land" the problem is easily solved -- the SMBH uses some spare energy to regulate both its growth and the growth of its host galaxy. Effectively, when galaxies collide under the force of gravity, the black hole gets so much to eat that it gets indigestion, throws a tantrum and expels all the gas from around itself and from the whole galaxy. This stops the galaxy from forming more stars, starves the SMBH, and with a little time and a few mergers you can get a nice self-regulation set up so that galaxies build up mass at the same rate as the SMBHs in their centres - Hey Presto, tight correlation between black hole mass and galaxy mass. Unfortunately, as you can tell from my description,  the theorists have very little clue how this might actually work in the real world (and if you meet one who tells you they know, go and ask another one - there's almost as many ideas as there are theoretical astronomers when you dig down to the details). 

Now the observers have a problem: how can we find new observations that help the theorists to explain what's happening? We know that stuff falling into a black hole releases a lot of energy -- we see some of this energy being released in "Active Galactic Nuclei" (AGN - see the blog post by Carolin here about how we are  looking for links between AGN and merging galaxies). But lots of energy doesn't help on its own: this energy needs to affect the gas in the galaxies, and we just don't see any evidence of this happening in the majority of galaxies in the Universe, as would be needed to satisfy the theorists.

And that's where I'm going to stop - we've still got a lot of work to do to fully understand how SMBHs are formed and why their mass correlates so strongly with their host galaxy. Evolutionary biologists spend a lot of time wondering about the difference between correlation, co-evolution and causation. Both biologists and astronomers work with enormous datasets, in which many different correlations can easily be found. Astronomers have a unique advantage, with surveys like CANDELS we can look back in time nearly to when the first galaxies were forming, we use this (and other clever techniques) to see the co-evolution of galaxies and SMBHs directly. But understanding the cause of the correlations and co-evolution is a more difficult challenge. We'll certainly make some good progress over the next few days here in Leiden, and we'll be going home with new ideas to try out. And of course new collaborations started this week will bring together complementary skills to tackle the problem from new angles in the future. 

Dinner with two galaxy formation simulators, Peter Johansson and Kelly Holly-Bockelmann