Primordial black holes in the main lensing galaxy of FBQ 0951+2635

Although dark matter in galaxies may consist of elementary particles different from those that make up ordinary matter and that would be smoothly distributed (still undetected), the so-called primordial black holes (PBHs) formed soon after the initial Big Bang are also candidates to account for a certain fraction of mass in galaxies. In this paper, we focus on the main lensing galaxy ( z = 0.260) of the doubly imaged, gravitationally lensed quasar FBQ 0951+2635 ( z = 1.246) for probing possible PBH populations. Assuming that the mass of the galaxy is due to smoothly distributed matter (SDM), stars, and PBHs, the 16-yr microlensing variability observations were compared in detail with simulated microlensing signals generated by 90 different physical scenarios. Among other details, the simulated signals were sampled as the observed one, and the observed variability in its entirety and over the long term were used separately for comparison. While none of the scenarios considered can reproduce the overall observed signal, the observed long-term variability favours a small mass fraction in PBHs with a mass of the order of the mean stellar mass. Furthermore, it is possible to obtain strong constraints on the galaxy mass fraction in Jupiter-mass PBHs, provided that a reverberation-based measurement of the source size is available and relatively small. To constrain the mass fraction in ∼10 M ⊙ PBHs, light curves five times longer are probably required.