An unusually dense galaxy could be the first clear evidence for the existence of an unconventional form of 鈥渟ticky鈥� dark matter, altering our understanding of this mysterious cosmic substance.
In the standard picture of cosmology, so-called cold dark matter only interacts with the rest of the universe through gravity, which causes it to bunch together in invisible, puffy clouds around galaxies. We can map these clouds indirectly by measuring the gravitational pull they exert, which bends light from other galaxies on its way to Earth.
But astronomers have found that some of these clouds, called dark halos, appear to bunch together in ways that shouldn鈥檛 be possible if dark matter particles can only interact through gravity. One solution is to assume that dark matter is 鈥渟ticky鈥� and able to exert a force on itself beyond that of gravity, allowing it to form much denser halos.
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Astronomers have previously found some evidence for this sticky, or self-interacting, dark matter, but it has been difficult to pin down its location within a galaxy to match exactly with predictions.
Now, at the University of Portsmouth, UK, and his colleagues have identified a dark halo in a galaxy called SDSS J0946+1006 that could change that picture, as it appears to be an unusually dense cloud exactly at the galaxy鈥檚 centre 鈥� just as predicted. 鈥淭he dark matter density profile is consistent with what some of these self-interacting dark matter models predict,鈥� says Collett.
The researchers pinned down the halo by applying a statistical model to measurements previously taken by the Hubble Space Telescope of SDSS J0946+1006, which acts as a gravitational lens. Such lenses occur when a massive galaxy鈥檚 gravity bends the light that we observe coming from other galaxies behind it. In this case the particular arrangement of the lens聽allowed the team to calculate how dark matter is distributed in it.
They found that the dark halo is almost certainly near the galaxy鈥檚 centre, as well as calculating the gravitational distribution of matter and dark matter, which matches self-interacting dark matter models. 鈥淭here aren鈥檛 really many ways of actually detecting dark matter because it doesn鈥檛 emit light, so lensing gives you this wonderful, clean way of learning about mass,鈥� says Collett.
鈥淭he system is fascinating. If the density estimates are accurate, it鈥檚 hard to explain with standard cold dark matter,鈥� says at the Carnegie Science Center in Pennsylvania. But while self-interacting dark matter is a plausible explanation, it is too early to say definitively, he says.
鈥淚t鈥檚 super exciting,鈥� says at Durham University, UK. 鈥淭here are now several of these clumps of dark matter, all of which seem to be more densely packed than expected. It might be a selection bias, that there is a range but the most compact ones are easiest to see, but it might also be the first hints of dark matter doing something more than just gravity.鈥�
Journal Reference: Monthly Notices of the Royal Astronomical Society
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