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

Abstract

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.

A new parameter space study of cosmological microlensing

“…The contrary behaviour was anticipated: maps with low numbers of microlenses should be more prone to lens position systematics (e.g. random clustering in the lens plane) while for higher numbers (∼ 10 9 ) the purely smooth matter limit should be reached (Garsden et al 2012). The highest number of microlenses in our maps does not exceed 2.5×10 5 (for κ = 1.05 and γ = 0.0), therefore, further investigation is warranted to determine the cause for atypical maps ap-pearing in this intermediate region.…”

Gravitational probes of dark matter physics

“…A number of ideas have bubbled up in recent years to detect such small halos, but require additional validation. A number of authors have suggested different ways to use time-domain observations, in all of the strong, weak, and microlensing regimes, to detect halos in this mass regime [646,647,648,649,650,651]. These ideas need to be validated in an observational context, but they point to a very important fact: the current and next generation of optical wide-field surveys (Table 1) are time-domain surveys by design.…”

Resolving small-scale dark matter structures using multisource indirect detection

“…Current gravitational lensing probes are only sensitive to relatively massive halos (> 10 6 M ⊙ ) [32][33][34]. Nanolensing [35,36] or proper motion detection [37,38] might be able to probe smaller scales. The presence of microhalos, however, changes the clustering property of DM structures, which is encoded in the clumping factor, hδ 2 ðzÞi [25][26][27]39], defined as the mean of the matter overdensity squared.…”