Probing planetary mass dark matter in galaxies: gravitational nanolensing of multiply imaged quasars


  • Research undertaken as part of the Commonwealth Cosmology Initiative (CCI:, an international collaboration supported by the Australian Research Council.

E-mail: (HG); (NFB); (GFL)


Gravitational microlensing of planetary-mass objects (or ‘nanolensing’, as it has been termed) can be used to probe the distribution of mass in a galaxy that is acting as a gravitational lens. Microlensing and nanolensing light curve fluctuations are indicative of the mass of the compact objects within the lens, but the size of the source is important, as large sources will smooth out a light curve. Numerical studies have been made in the past that investigate a range of source sizes and masses in the lens. We extend that work in two ways – by generating high-quality maps with over a billion small objects down to a mass of 2.5 × 10−5 M, and by investigating the temporal properties and observability of the nanolensing events. The system studied is a mock quasar system similar to MG 0414+0534. We find that if a variability of 0.1 mag in amplitude can be observed, a source size of ∼0.1 Einstein radius (ER) would be needed to see the effect of 2.5 × 10−5 M masses, and larger, in the microlensing light curve. Our investigation into the temporal properties of nanolensing events finds that there are two scales of nanolensing that can be observed – one due to the crossing of nanolensing caustic bands, and the other due to the crossing of nanolensing caustics themselves. The latter are very small, having crossing times of a few days and requiring sources of size ∼0.0001 ER to resolve. For sources of the size of an accretion disc, the nanolensing caustics are slightly smoothed out, but can be observed on time-scales of a few days. The crossing of caustic bands can be observed on time-scales of about three months.