First Galaxies and Faint Dwarfs

Back in Oz after a week in the US at the University of California at Santa Barbara (UCSB). I was at a conference called First Galaxies and Faint Dwarfs at the Kavli Institute of Theoretical Physics.

The week started with us being told that this was going to be the best conference we would ever attend, and I must agree that it was. Instead of the usual barrage of 15min talks, we had 40mins of review talks, with discussion sessions, and it was great.

What was the point of the meeting? Well, that was summarized by the opening slide by Leon Koopmans;

Essentially, the goal was to bring together two communities, those that study the high redshift universe and the first forming galaxies, and those that look at the tiny dwarf galaxies in our local universe. Why? Because understanding galaxy formation and the links between the nearby and faraway will reveal the inner workings of the universe, especially the nature of dark matter.

I'm not going to go through the conference in detail (as I have to get to work), so here's few slides. All the talks are public, and there are podcasts so you can hear the discussions, and I recommend that you take a looksie.

Notice what there is not. There is not wailing and gnashing of teeth with screams of "oh what is dark matter, we don't know what it is, oh this is a disaster, ohhhh, woe is me". No. That is not where we are in the subject. The point is that we know dark matter (or, with a much lower prior, modified gravity) is out there, and we want to know what the consequences are.

So, what are the big questions. Well, they may seem quite technical, but  it is where the front lines of cosmological science currently are.

• Where are the dark matter subhalos? Basically, our picture of the large scale structure of the Universe, one driven by dark matter and dark energy, works really well in predicting where galaxies should be. But on small scales, it predicts that a galaxy like the Milky Way should be surrounded by a host (thousands of..) dwarf galaxies. But we only see a few 10s of these little things out there. So where are they? If you believe Carlos Frenk, the problem is solved - the first burst of star formation in the universe blew the gas out of the halos, and so they could not form stars. The basic result of this would be that we surrounded by dark halos, loads of them. But how can you see if their there? Well, we can use gravitational lensing, and we're working on it. Just as an example, here is a simulated gravitational lens, followed by an actual lens, the Horseshoe. By studying the details, we'll be able to measure the the amount of subclumps of mass out there. Cool eh?

• Are dark matter halos cored or cusped?  This is quite technical, but our cosmological models make some firm predictions on the distribution of mass in galaxies, predicting that the mass density should increase rapidly towards the centre. This peaky mass distribution is called cusped and so if we can measure it, we can directly test whether our models are correct. If the mass does not rise so quickly (a cored distribution) then this would be a bit of a problem. Except, things are not so simple, as the existence of baryons (to you and me, basically means gas and atoms and stars and stuff) can apparently change cusps into cores. But gas physics is hard, especially including them in the complex computer simulations of structure formation in the Universe. If halos are cored, rather than cusped, then it might means that dark matter is not cold, which brings me to the next question.
•  Is dark matter hot, warm or cold? Is dark matter cold (i.e. composed of things moving slowly) or hot (things moving relativistically), or somewhere in between (i.e. warm)? This is related to the above two questions, as warm dark matter doesn't form the little halos we expect around the Milky Way, and is likely to influence the core/cusp question. Currently, we don't know.
• Are dwarf galaxies we see today like the dwarf galaxies that formed the Milky Way? Again, this is a tricky one. The Milky Way was built of little galaxies that fell in. But are the ones falling in now like the ones that fell in long ago? The answer is "we don't really know", but there are some clues, namely coming from the mass-metallicity relationship, which means that smaller galaxies have less metals (which, to an astronomer, is all elements heavier than helium. But stellar chemistry is quite complex, and the observations challenging, but there is some really good ground being made in this area.

There are more, of course, but these are the key questions. At the meeting we had the usually argy-bargy about theorists and observers not believing or trusting each other, but it's all part of the fun.

On a closing note, I've now visited three Kavli Institutes around the world, the one in Santa Barbara, one in Bejing and another in Cambridge. These are wonderful research environments, with great programs bringing people together. But similar to the Perimeter Institute in Canada, and the Keck Telescope, these are funded by private endowments, rather than being solely funded by government.  Any Australian philanthropists out there, it would be nice to have a similar institute here :)