Where did the dark matter go?

Galaxias are supposed to be made mostly of stars, gas, dust and  … dark matter. Dark matter is the most mysterious substance in the Universe. We know HOW MUCH there is, we know WHERE it is, but we don’t know WHAT is it.  Dark matter has eluded detection by multiple experiments here on earth yet we continue gathering evidence of its existence on cosmological scales. Dark matter is responsible for the accelerated rotation of galaxies in the outer regions of these galaxies. It is also reponsible for the distribution of matter in the Universe on very large scales and also it is largely responsible for the so called gravitational lensing effect.  In galaxies, most of the mass is in the form of dark matter as evidenced by the rotational curves of galaxies that tell us ho much mass is inside a given distance from the centre of the galaxy. In some heavy galaxies, the amount of dark matter is so large that the space warps around these galaxies producing the optical illusion that another galaxy that is far behind these galaxies is seen in two, sometimes in three or more locations. This is the gravitational lensing effect predicted by the  general theory of relativity , thanks to which we can study the distribution of dark matter in these galaxies.


The most spectacular examples of the gravitational lensing effect can be found in galaxy clusters where the concentration of dark matter is greatest. Among the observations of the gravitational lensing effect, the recent Hubble Frontier Program is producing the best data around colliding galaxy clusters offering a unique opportunity to study dark matter (in a manner that tries to emulate particle accelerators that smash particles against each other).  In a recent work we use the gravitational lensing effect around a special type of galaxies, known as BCG (or Brightest Cluster Galaxy) and find something unexpected. Our results show that these particular galaxies have no dark matter at all (or a very small amount). Our study relies on two gravitationally lensed galaxies (marked with 7.2, 7.3, 19.1 and 19.2 in the figure above) that can be explained only if the two BCG galaxies in these cluster have most of their mass (if not all) in the form of stars and no dark matter (or very little). If these galaxies were normal galaxies, that is, having a significant amoount of dark matter , the two images shown above would be curved  towards the BCG galaxies, something that can be ruled by the observations. If confirmed, our findings would require new developments on galaxy formation  to explain this type of galaxies. We discuss several scenarios that could result on this type of galaxies. One of the most promising ones and that has not been explored sufficiently in the past is that these galaxies form as a consequence of cooling and not as the result of merging of several smaller galaxies. Future observations could confirm or reject this hypothesis but that will be a different story …

Link to paper: http://arxiv.org/abs/1609.04822