The cosmic history of the spin of dark matter haloes within the large scale structure

What's the goal of astronomy and astrophysics? Pretty Hubble pictures? Whacky statements about multi-verses and black hole entropy? Nope, not really. The goal is to understand the Universe in terms of the laws of physics.

Like "real" science, there is a strong link between theory and observation and experimentation, each feeding off each other, and as I noted a little while ago, the development of new instrumentation on telescopes often opens a new window on the Universe. Instruments like SAMI will let us do something new, namely measure the spins and chemistry of a large number of objects.

But now the key question - If you were going to measure the spins of a whole load of galaxies, what would you expect to see? Would they be randomly orientated? Would patches of galaxies spin together in unison? Would nearby galaxies spin in opposite directions? And how would you even begin to answer this question?

There are paper-and-pen approaches to answering this question, basically known as tidal torquing theory, which basically says that as the mass that forms galaxies starts to collapse, nearby masses tug unevenly on each other, causing them to start to spin.

The problem is that we know that galaxies don't simply form from essentially isolated collapsing masses. It is a lot messier, with galaxies crashing together, and small systems being cannibalized by larger galaxies, and what we end up with at the end of the day is a cosmic web!

We see clusters (bright yellow), dark voids and filaments connecting it all. Just how are galaxy spins aligned in this mess?

Well, you can just measure it (and the just in there does some serious disservice to how tricky this really is to do). You can find the collapsed masses, and look what they are doing and where they live, and measure what their spins are.

Minor interlude: Here is a fantastic little intro from Andrew Pontzen on what we actually do when we mean galaxy here.

PhD student, Holly Trowland, Joss Bland-Hawthorn and myself have just submitted a paper to The Astrophysical Journal doing such spin measurements on cosmological simulations, finding that there are a whole range of correlations of the spins with environment, with mass and over cosmic history. In fact, it shows that if you simply take the paper-and-pen tidal torque theory, it just fails when you get into the mess of the cosmic web.


One issue is that our simulations have been looking at only dark matter, the dominant mass in the Universe. But galaxies are made of atoms and we need to know how these spin within the spinning dark matter halos. This is trickier than it sounds, but Holly is working on it, and we should have some results later in the year. But for now, well done Holly!


The cosmic history of the spin of dark matter haloes within the large scale structure


Holly E. Trowland, Geraint F. Lewis, Joss Bland-Hawthorn

(Submitted on 30 Jan 2012)

We use N-body simulations to investigate the evolution of the orientation and magnitude of dark matter halo angular momentum within the large scale structure since z=3. We look at the evolution of the alignment of halo spins with filaments and with each other, as well as the spin parameter, which is a measure of the magnitude of angular momentum. It was found that the angular momentum vectors of dark matter haloes at high redshift have a weak tendency to be orthogonal to filaments and high mass haloes have a stronger orthogonal alignment than low mass haloes. Since z=1, the spins of low mass haloes have become weakly aligned parallel to filaments, whereas high mass haloes keep their orthogonal alignment. This recent parallel alignment of low mass haloes casts doubt on tidal torque theory as the sole mechanism for the build up of angular momentum. We find a significant alignment of neighboring dark matter haloes only at very small separations, r<0.3Mpc/h, which is driven by substructure. A correlation of the spin parameter with halo mass is confirmed at high redshift.