A team of researchers at the University of Waterloo and the Perimeter Institute led by postdoc Andrej Obuljen, have just announced the detection of galaxy assembly bias. The study is the first to reach a level of detection at what physicists call the 5-sigma level, generally accepted as conclusive proof. The preprint of the paper is available at: https://arxiv.org/abs/2004.07240 .
Galaxies in the Universe, each consisting of billions of stars, act as a set of lighthouses outlining the distribution of matter in the Universe. The exact way in which galaxies trace that matter is of great importance to cosmologists, who are interested in the distribution of matter but can only see the galaxies. The link between galaxies and mass is termed “the bias” of the distribution, and the simpler it is, the easier it is to understand the Universe using only observations of galaxies.
The new study shows that the link between galaxies and mass, the bias, is actually quite complicated. Galaxies are lined up along filaments and across sheets in the Universe, and their shape and orientation depends on the environment - particularly the gravitational tidal field felt by the galaxy. By carefully selecting samples of galaxies using properties of each galaxy - the study used the width of the distribution of line-of-sight velocities of the stars in each galaxy - they found galaxy samples that showed different bias along and across the line of sight to the galaxies, termed “assembly bias”.
This assembly bias means that cosmologists now have to think harder about how to interpret observations and use the distribution of galaxies to understand structure formation within the Universe.
The amount that galaxies cluster in the Universe as a function of scale, a quantity called the power spectrum. Measurements are shown for two galaxy samples selected based on their orientation to us. The upper measurements (filled circles) show the average power spectrum in all directions, which match between the two galaxy samples. However, the lower set of measurements (open circles) show the anisotropic component of the power spectrum, which is different between the two galaxy samples - a clear signature of assembly bias. The research team used these measurements to conclude that these two galaxy samples trace matter in our Universe in different ways. The team found 5-sigma evidence for this unusual type of bias. 5-sigma means that if we were able to look at 3.5 million Universes and there was no assembly bias in any, then there would only be one Universe that looked like ours.