The polarisation of the light reveals an alignment

The orientation of the axis of the accretion disc can be determined thanks to the observation and analysis of the polarisation of light. Usually, light is not polarised. It is a transversal wave which vibrates and propagates in all directions in a homogeneous way from its source and in all vibration planes. Light is polarised when the oscillation of the wave preferentially occurs in one direction. “In normal circumstances”, explains Damien Hutsemékers, “the reflection of light on a puddle of water, on a wind-screen or a mirror constitute phenomena that produce polarisation. In the case of quasars, a similar mechanism is at work: a source of light which is assumed not to be polarised, is reflected by particles such as electrons or dust. This reflection causes polarisation”. Vincent Pelgrims continues, “The angle of polarisation indicates the direction in which the electric field of the photons that reach us is the strongest. We have one a direction and therefore an axis which is related to that of the accretion disc”.  

For the 93 quasars observed, 19 candidates emitted a sufficient quantity of polarised light to enable them to be studied. “With Vincent and Dominique”, continues Damien Hutsemékers, “we have looked at how the angles of polarisation organize themselves on the structure. At first, we observed that they were aligned despite the fact that they are separated by billions of light years. We then went further and we noticed that they also tended to be aligned with the axis of the filamentary structure in which they were found. This was surprising, even though we somehow expected it. This was what we had been trying to observe”.

The predictions of the standard model therefore seemed to have been exceeded. “The information about the polarisation backed up the idea that we were witnessing something unique and whose constituent parts were experiencing a phenomenon of alignment”, summarize the researchers. “This may prove that there is an ingredient missing in our current models”. In any case, the nature of alignments on such scales is not easy to understand. “The nucleus of a quasar is made up of a supermassive black hole which can reach a mass equivalent to several billion times the mass of the sun. It has not been established that such objects behave like less massive galaxies. It is a very interesting first channel of investigation. We can legitimately suspect, by extrapolating from what we know about galaxies that the same mechanism can apply to quasars. The theory must be tested firstly by verifying that what explains alignments on smaller scales can include such vast sprawling structures, and secondly, must be confirmed by new observations”.

Towards other groups of quasars

This web of quasars is a challenge to astrophysicists today. According to theory, the expanding Universe did not have time to form such a huge structure. “It is intriguing that we have found such an ordered structure that is one gigaparsec in size. This could be due to a statistical fluctuation. But if we find others, we will have to reconsider the model and integrate new factors. We also need to check that we are not in the presence of a poorly-defined structure. We must characterize these alignments to verify whether they have a real meaning. All this work on quasars is very recent and there is a lot of work still to be done. We are observers more than theorists but we will further study the question. In the shorter term, we have requested more observing time on the VLT. We will look at other clusters of quasars to see if we notice the same behaviour and reinforce our observations”, the researchers point out.