The Universe appears to be inexplicably lopsided
Dwarf galaxies cluster between larger ones in the Local Group and elsewhere, but we don’t know why
Our Local Group of galaxies is lopsided, and this has been worrying cosmologists. The group is made up of two large systems (Andromeda and our own Milky Way) and a host of smaller satellites, and it’s the latter that are the problem. Rather than being distributed randomly, it turns out they are much more likely to be in the region between the two biggest galaxies than anywhere else, and it isn’t immediately obvious why that should be the case.
The effect isn’t small. About 80 per cent of the satellite galaxies that orbit around Andromeda are on the side that faces us, and similar effects have been seen around other pairs of large galaxies observed by the Sloan Digital Sky Survey. This result inspired a team led by Marcel Pawlowski in Irvine, California, to investigate whether the same thing happens in large-scale simulations of the Universe. If differences were found, it would suggest that something is wrong with our picture of how cosmology has steered the evolution of the Universe over the last 13.8 billion years. The currently favoured ‘Lambda-CDM’ cosmology already has problems accounting for lots of small-scale features of the Universe, such as the number of satellite galaxies seen, so a new test would be very welcome.
The experiment is an easy one to imagine. All you have to do is take one simulation of the Universe, and count the small galaxies that appear around pairs of massive systems within it. The authors used a suite of different simulations, torn between picking one that includes the smallest galaxies and choosing one that covers a large enough volume of the Universe to make for a good test.
We know much more about these simulated Universe than the real one. It is, for example, possible in the simulation to identify with cast-iron confidence which galaxies really are satellites of any given pair, while observers looking at the real thing might be confused by background galaxies that just
“About 80 per cent of the satellite galaxies that orbit around Andromeda are on the side that faces us”
happen to appear nearby. This problem is made worse because we only have estimated redshifts and hence distances for most of the smaller galaxies, but with some careful statistics the authors can make a sensible comparison.
When they do, it seems that in the simulations, just as in reality, the presence of a pair of big galaxies is associated with a lopsided distribution of smaller systems. The effect is seen in all but one simulation, and though that one is the most complex, including much more physics, it is also the smallest and so the absence of an effect is probably just coincidence. Cosmology passes the test, but we’re still left with a mystery.
Satellites are most likely to be found between two large galaxies, but the next most likely simulated position is on the far side of each of the pair of galaxies. It’s not at all clear why this is the case, or whether that effect holds in the real world. Plenty for both observers and theorists to get to grips with, therefore – even if cosmology passes the test this time.
Milky Way The Andromeda Galaxy (M31) The Milky Way and M31 are dominant galaxies in the Local Group; M33, the third largest, may be a satellite of M31 The Triangulum Galaxy (M33)
CHRIS LINTOTT is an astrophysicist and co-presenter of The Sky
at Night on BBC TV. He is also the director of the Zooniverse project.