CHRIS LINTOTT
SKY AT NIGHT PRESENTER
Chris explains why the tidal streams left by galaxy mergers are pointing to the existence of dark matter.
“If gravity is behaving oddly, there is no reason for it not be odd here too”
The fact that most of the Universe’s matter is in the form of mysterious dark matter – something we don’t understand – is embarrassing and fascinating in equal measure. Instinctively, resorting to filling space with a new kind of particle seems desperate, and many of the people I meet when I give talks would prefer an alternative.
For several decades now, that alternative has tended to take the form of theories which muck about with gravity. Make gravity behave differently on the largest of scales, or in the emptiest parts of space, and you might just be able to get rid of the need for dark matter.
What’s needed are scenarios that can test these rival ideas, and in a recent paper a team led by Federico Lelli of Case Western University in Cleveland have put them head to head. The challenge is to explain observations of an unusual type of galaxy, formed not in the loneliness of deep space but as the result of a violent merger between two larger systems.
These are the tidal dwarfs, ‘recycled’ objects that form from the debris of major galaxy mergers. Think of the long streams of stars, gas and dust that stretch away from the centre of famous merging systems like the Antennae Galaxies. If these tidal tails contain enough material, then they can collapse, forming stable, if small, dwarf galaxies with a newly independent existence.
So what? The material that ends up in those tidal tails comes from the discs of the colliding galaxies, and according to the standard theory discs are dominated not by dark matter but by normal, baryonic matter. The dark matter forms a spherical halo within which the disc sits.
If dark matter rules the day, then these galaxies should be a rare exception where what we see – in the form of ‘normal’ matter – is all we get. If gravity is behaving oddly, there’s no reason for it not be odd here too and these galaxies should have something strange going on. So who wins?
The result is a near-knockout in favour of dark matter. Six galaxies were included in the study, and each of them was found to have formed a rotating disc; in other words, they are well on their way to being proper galaxies. The behaviour of these discs, and especially the speed with which they are rotating, was found to be consistent with models in which no dark matter was needed, and hence which provided no opportunity to fiddle with gravity.
Indeed, when the rotational speed of the disc was plotted against the observed mass of normal matter – forming an iconic plot called the Tully-Fisher relation – these tidal dwarfs stood apart from other small systems. Further confirmation that these things are odd, and odd in precisely the way you’d expect if they were uniquely dark matter free.
Until we get a good understanding of what dark matter actually is, doubts will remain. Experiments like this one, though, convince me that the most likely explanation is that the Universe is filled with the stuff – whether astronomers like it or not!