From quasars to gravitational lenses

The second part of the thesis aimed to make predictions about the scientific contribution of the telescope. Because the local gravity is used to shape the mirror, the latter cannot be tilted so the telescope points permanently towards the Zenith. However, thanks to the Earth rotation, the telescope will have access to a strip of sky that it will image every night, making it possible to study all the objects in the strip.

For five years, the ILMT will take images of the same strip of sky every night and acquire a great quantity of data on the variations in the luminous fluxes of all the objects in this patch of sky. By comparing the images of the same fields acquired night after night, the ILMT is an ideal instrument for detecting photometrically variable objects such as quasars, those active nuclei of galaxies, some of which are several billion light-years away and whose light were emitted when the universe was only half as old as it is now or even earlier than that. 

The first theoretical stage was to determine how many quasars the researchers could expect to observe. Based on other studies, the average distribution of quasars in space was known. From this, all that was needed was to simulate the population of quasars in the strip of sky accessible to the ILMT and, knowing the faintest luminous flux that could be detected by the ILMT, keeping only sources that were sufficiently bright to be detected. In this way, François Finet estimated that more than 9,000 quasars should be detected by the telescope. This was a prospective study, based on knowledge and theories currently shared about the composition of our universe. “But what really interested me, and this was the second stage of this part, was to estimate how many gravitational lenses we could expect to observe among the quasars detected. By calculating the probability of having a deflector between the source and the observer, sufficiently close to the line of sight to form a gravitational lens, we expect that there will be between 28 and 29 multiply imaged sources, of which 22 or 23 should be detected, because of the finite angular resolution of the instrument, i.e. its capacity to distinguish two sources that are angularly very close”. In other words, we therefore expect that among the 9,000 quasars detected, 29 would have a deflector sufficiently close to their line of sight, to lead to the formation of multiple images of these sources.

From gravitational lenses to the universe expansion rate

Why was the young researcher so excited to see 23 gravitational lenses for a total of 9,000 quasars? Because they can teach us a lot about the expansion of the universe and the amount of matter it contains. “Observations of gravitational lenses can be used in different ways. We can use them individually and this can teach us two things. Firstly, the deflection of the light depends on the total mass of the deflector which includes the visible matter and the dark matter. If we know the distance of the source as well as that of the deflector, we can then determine the distribution of the total mass of the deflector. Secondly, if we know the distribution of the mass of the deflector, we will be able to measure the local expansion rate of the universe, the Hubble constant, by measuring the temporal delays”, explains the astrophysicist.