Largest-ever study of galaxy motions
In general, galaxies ‘go with the flow’: they move further apart as the Universe expands. But that’s not the whole story. Galaxies are also drawn together by gravitational attraction. And this gives them an additional component of motion.
By measuring the speed and direction of galaxies’ individual movements, researchers can map the gravitational forces that are tugging on the galaxies, and so determine how matter, seen and unseen, is distributed. Researchers recently used this technique to define the ‘Laniakea supercluster’: the relatively nearby galaxies, including our Milky Way, that are being drawn together by gravitational attraction.
Now Christopher Springob (ICRAR/UWA) and colleagues have publicly released data on the movements of almost 9,000 galaxies—double the number measured by the largest previous homogeneous study of this type. And those galaxies are spread over a region that is 1.5 billion light-years across and nearly ten times the volume of the Laniakea supercluster. The size of this survey will allow researchers to test for the first time if our local region is representative of the Universe as a whole, and whether our standard cosmological model correctly predicts galaxy movements.
Astronomers have measured the positions in space of hundreds of thousands of galaxies. But we have far fewer measurements of galaxy movements because they are more difficult to compute. To derive them, you have to compare the distance predicted by a galaxy’s redshift with the distance predicted by its internal properties. Not all galaxies lend themselves to this second calculation.
The procedure also depends on accurately measuring the width of the galaxy’s spectral lines. That requires a high-resolution spectrograph, such as the one purpose-built by the Australian Astronomical Observatory for this survey.
This new dataset of galaxy motions is derived from the 6dF Galaxy Survey (6dFGS), which was carried out with the 1.2-m UK Schmidt Telescope at Siding Spring Observatory in northwest NSW. The survey recorded redshifts for more than 110 000 galaxies over 80% of the Southern sky, out to about two thousand million light-years from Earth (z = 0.15).
The ‘smoothed’ dataset of galaxy motions. Red indicates regions where galaxies are moving away from us and blue, regions where galaxies are moving towards us. Labels refer to superclusters of galaxies. The box is 1.5 billion light-years on each side. Interactive 3D plots of the data.
Christopher M. Springob, Christina Magoulas, Matthew Colless, Jeremy Mould, Pirin Erdoğdu, D. Heath Jones, John R. Lucey, Lachlan Campbell, Christopher J. Fluke. “The 6dF Galaxy Survey: Peculiar Velocity Field and Cosmography”. http://arxiv.org/abs/1409.6161