Spin correlations, part II
Imagine throwing a handful of spinning ballerinas into an empty space (since they are in empty space now, there is no need to for them to be ice-skaters). We can associate a thumb with each one in a similar manner as we did with galaxies in the last post. This thumb points towards her head if the ballerina herself thinks that she is rotating anti-clockwise, and towards her feet if she things she is rotating clockwise. Alternatively, we can also wind our right-hand fingers around her body in the sense in which she is spinning and look at where the thumb is pointing – this will give consistent results.
If ballerinas are thrown in at random, there are no correlations in their spinning directions. We discussed the concept of correlation in the previous post: if there are no correlations, the two neighbouring galaxies are equally likely to have parallel, anti-parallel or perpendicular spin directions – in other words, by knowing a spin direction of one galaxy, you can’t deduce anything about the spin direction of its galactic neighbour. This is illustrated here by these five animated photos of a spinning girl:
The ballerinas-in-space (illustrated by the spinning girl in the photos above) are aligned in different directions, just as galaxies in the universe are aligned in different directions. You can talk about the direction in which the ballerinas are pointing by talking about the axis around which they are rotating.
If you imagine lining up the ballerinas into neat rows, you can see that you can line them up to spin along the same axis. But, even if you align them along the same axis, they might spin in different senses – some in the clockwise or “thumb pointing down” sense, and some in the anti-clockwise or “thumb pointing up” sense. See our spinning girl:
Conversely, we can induce these spin direction (“chiral”) correlations by making them spin not only along the same axis, but also in the same chosen sense (i.e. thumbs up or thumbs down). (As an aside: this does not necessarily require that they all points with their heads-up. All that matters is that the spin vectors or thumb direction point in the same direction. For galaxies this is not an issue, as they have no heads or legs and look the same from both ends). Here we have an example where all ballerina’s are spinning with a “thumb-up” spin:
The important message to take home is that axis correlations do not necessarily mean chiral correlations!
Now, replace ballerinas with galaxies. People have been looking at axis correlations for a long time, by studying the projected ellipticities, which can be calculated by a computer from the projected images. And they have indeed found that axes are correlated. This is good, as axes are supposed to be correlated.
With the Galaxy Zoo data, we can however study the other question: are the spin senses of neighboring galaxies also correlated? This was the main topic of our paper 6 (available online at the arXiv preprint server). The result was, that they are indeed correlated for close galaxies. The interesting news, however, is that these were not supposed to be correlated! So what is going on?
In the previous post I talked about the ice-skater pushing herself with her feet and then gaining angular speed by pulling her arms towards herself. Spiral galaxies pull their arms towards themselves by forming stars and cooling gas which contracts the ordinary matter’s component. But where does the gas get its initial kick? The most natural way to give gas this kick is to consider torquing (i.e applying force that spins it) of the local distribution of matter with the tidal fields. Tidal fields are essentially the variations of gravitational forces with space (which also cause Earth’s tides as well, hence the name). In the Universe, these tidal fields are given by the large-scale structure of the universe and vary only slowly in space (as opposed to Moon’s tidal fields that gives rise to the Earth’s tides). Local clumps of matter see these forces and respond by accelerating along the force lines produced by these gravitational fields (note that they are gravitational and so they pull everything that has mass). If the field varies from one point to another, different bits of local mass distribution will be pulled in different directions, and therefore, this pulling in different directions results in the emergence of an initial spin.
Now, one can make a reasonable assumption that for two neighbouring galaxies, the tidal field would be very similar, but that the initial distribution of matter, the so called “protohalo,” around each of these galaxies would be completely unrelated. It turns out, that under such assumption, the spin axis of these two galaxies should be, in average, correlated, i.e. being very roughly parallel, but also that the actual spin senses should not be. The former comes from the fact that there are axes in which a given tidal field cannot spin up the galaxy, regardless of how you arrange the initial distribution of matter. If both galaxies see the same tidal field, they would both have the same “forbidden” axes and hence somewhat correlated actual axes. However, the actual spin direction (i.e. up or down, left or right, etc. along a given axis) will be random.
This is because for every matter distribution which spins a galaxy around a certain axis, a mirror distribution will spin the galaxy in the opposite way around the same axis. Since they are both equally likely, you get correlated axes, but not correlated spin directions.
The major news of our new spin paper is that we seem to have an indications that the actual spins are somewhat correlated, when the galaxies under consideration are very close together. This indicates that something must be wrong with the picture above.
How would the local distribution of matter around galaxy A in the very early Universe know about the local distribution of matter around galaxy B? There are a couple of caveats that must be taken into account. First, we are detecting correlations only at very small separations. In other words, galaxies that are very close together (that is less than 500 kiloparsecs or about 1.5 million light- years – this is what “close” means in cosmology!) tend to have their spins aligned, but this is not true for galaxies that are separated at larger distances. This indicates that these galaxies probably belong to the same group of galaxies associated with some large amount of collapsed matter. In such scenarios, it is not unthinkable that different sub-clumps of matter “know” about each other. Second, we are looking at face-on spiral galaxies. We are not looking at randomly selected galaxies and we are not even looking at all spiral galaxies. But it is known that galaxies with different morphologies inhabit different parts of the universe. So, by picking face-on galaxies we are limiting ourselves to one peculiar family of astronomical objects.
This means that it is very tough to predict theoretically what correlations should one expect. Our best hopes lie in using computer models. By performing large computer simulations, we can actually simulate large chunks of the Universe. We can then find dark matter halos that are likely to host spiral galaxies in the simulated Universe and calculate their correlation properties, trying to follow the observation protocol (i.e. pick galaxies that are sufficiently face-on to be classified as such by a zooite) as closely as possible. This is definitely possible, but has not been done, yet. These computer models will be the next step in our adventure. But detecting the actual correlations is a major first step!