Large scale galaxy motions match expectations for dark matter
24 November 2015
For nearly a century, astronomers have known that the universe is expanding – most galaxies are moving away from each other. When we measure the motion of a distant galaxy, the overall expansion of the universe is, in most cases, by far the dominant contributor to that galaxy’s movement. However, astronomers have long been fascinated by a secondary contributor to galaxy motions: the gravitational attraction of nearby matter. By studying the motions of galaxies, we can measure the distribution of all matter in the nearby universe, including dark matter.
One important statistic that can be used to understand the large scale motions of galaxies is the "bulk flow". It is the average motion of all galaxies within a large region of the universe. The faster the bulk flow, the stronger the gravitational attraction of nearby matter on large scales. Two studies of galaxy motions presented this month by researchers in the CAASTRO Dark Universe research theme show that this bulk flow is consistent with our expectations.
Dr Morag Scrimgeour, a former CAASTRO PhD student at ICRAR-UWA who was awarded the Charlene Heisler Prize of the Astronomical Society of Australia (ASA) for her thesis, and and the 6dF Galaxy Survey team measured the bulk flow of galaxies in the 6dF Galaxy Survey. They found that the bulk flow of galaxies in the southern sky out to a depth of 300 million light-years is 243 +/- 58 km/s. This is within the range of theoretical predictions for the bulk flow taken from the standard model for the universe, albeit on the high end of that range. The analysis concluded that the galaxies covering this large volume are collectively moving in a direction that aims roughly towards the Shapley Supercluster, an extremely massive supercluster of galaxies about 600 million light-years away.
This study shows that the bulk flow is consistent with theoretical expectations drawn from the "standard model" of the universe, and now another study from the CAASTRO Dark Universe research theme suggests that the bulk flow is also consistent with expectations relating to the specific geometry of the galaxies that we see in the nearby universe. Dr Christopher Springob (ICRAR-UWA) and collaborators have taken the bulk flow as measured for the 2MASS Tully-Fisher Survey and compared it to what we would expect it to be if the dark matter is distributed across the nearby universe in the same pattern that we observe for the galaxies themselves. Again, the model and our measurements are largely in agreement. Assuming that dark matter is more heavily concentrated wherever galaxies are more heavily concentrated (such as the Shapley Supercluster) gives a prediction for the bulk flow that is consistent with what we actually observe.