The rotation axes of several quasars, those active nuclei of galaxies hosting a supermassive black hole at their core, are aligned with each other but also with the axis of the cosmic web filament to which they belong. This group may constitute a complex structure formed at about one third of the current age of the Universe, that extends over a distance of three billion light years. This discovery was made thanks to the analysis of the polarisation of the light emitted by those quasars. The observations were carried out in Chile by astrophysicists from the University of Liege. Though the alignment phenomenon was expected by the researchers, the detection of such a large structure remains very surprising since it is not predicted by the standard model of the Universe.

In March 2014, researchers from the Department of Astrophysics, Geophysics and Oceanography of the University of Liege traveled to the ESO observatory in Chile to observe the cosmos through one of the VLTs (Very Large Telescopes), which are among the largest telescopes in the world. They knew precisely where to look. They concentrated their attention towards a part of the sky containing a high number of quasars. These are the active nuclei of galaxies which contain supermassive black holes around which the significant accretion of matter generates so much energy that they are brighter than all the stars of their host galaxy put together. They are so bright that, despite being several billion light years away, they are visible from the Earth. “Of course”, explains Damien Hutsemékers, senior research associate with the FNRS in the Department of Astrophysics, Geophysics and Oceanography (AGO) of the University of Liege, and the first author of the publication(1), “what we see are mere point-like sources like when observing the stars Their name was derived from that property ; quasar stands for quasi-stellar radio source. In the sixties, spectroscopy allowed to establish that these sources of light were not located within our galaxy but much further away”.

Since then, the quest to catalog and better understand these objects at the edge of the Universe has continued to grow. In the last few years, large sky surveys have discovered lots of these objects. From just a few thousand objects ten years ago, the list of known quasars today exceeds one million, a number which makes these objects a robust and interesting statistical sample for different reasons. “Their distance and brightness are good assets”, explains Lorraine Braibant, a doctoral student at the AGO and co-author of the publication. “We use them as beacons for sketching the distant and therefore very young Universe. For example, the group we have just studied has enabled us to go back ten billion years in time, at a time when the Universe was about one third of its current age. We can also study quasars for themselves”. When studied for themselves they are also objects of great curiosity. Not only does their core consist of a supermassive black hole, a very mysterious cosmic object, but they are also among the most massive objects of the Universe and seem able to organize themselves into structures as illustrated by this study. Studying quasars sheds some light on a poorly understood aspect of the standard cosmological model. We understand quite well the behaviour of what surrounds us but, on a bigger scale, that is to say on scales of billions of light years in terms of time and space, our knowledge of the Universe remains theoretical and full of extrapolations that need to be validated through observation.

Damien Hutsemékers and his team have focussed their interest on nearly one hundred quasars. They could have delegated the responsibility for data gathering to the astronomers at of the observatory in Chile. “But we preferred to go there ourselves”, explains Lorraine Braibant. “There was a risk that the weather conditions might not be good when we arrived. But in our case we did not require optimal atmospheric conditions. Finally, we were very lucky with the weather. We wanted to point the telescope at 70 quasars which we succeeded in doing quite quickly. We had some remaining observation time to use.  We were therefore able to observe another sub-group of quasars and increase the number of quasars observed to 93”.   

(1) Damien Hutsemékers, Lorraine Braibant, Vincent Pelgrims, Dominique Sluse, Alignment of quasar polarizations with large-scale structures, Astronomy & Astrophysics, 19 novembre 2014.