Strong Lensing by Subhalos in the Dwarf-Galaxy Mass Range II: Detection Probabilities

TL;DR

This paper evaluates the likelihood of detecting dark matter subhalos through strong gravitational lensing of quasars, finding low probabilities for point sources but potential for extended sources at radio wavelengths with sufficient resolution.

Contribution

It provides a detailed estimation of lensing probabilities by subhalos, challenging previous assumptions and highlighting conditions under which detection may be feasible.

Findings

Optical depth for lensing by subhalos is very low for point sources.

Extended sources like radio quasars offer higher chances of detecting substructure lensing.

Radio observations can set upper limits on subhalo abundance even without multiple images.

Abstract

The dark halo substructures predicted by current cold dark matter simulations may in principle be detectable through strong-lensing image splitting of quasars on small angular scales (0.01 arcseconds or below). Here, we estimate the overall probabilities for lensing by substructures in a host halo closely aligned to the line of sight to a background quasar. Under the assumption that the quasar can be approximated as a point source, the optical depth for strong gravitational lensing by subhalos typically turns out to be very small (tau < 0.01), contrary to previous claims. We therefore conclude that it is currently not feasible to use this strategy to put the simulation predictions for the dark matter subhalo population to the test. However, if one assumes the source to be spatially extended, as is the case for a quasar observed at radio wavelengths, there is a reasonable probability for witnessing substructure lensing effects even at rather large projected distances from the host galaxy, provided that the angular resolution is sufficient. While multiply-imaged, radio-loud quasars would be the best targets for unambiguously detecting dark matter subhalos, even singly-imaged radio quasars might be useful for setting upper limits on the abundance and central surface mass density of subhalos.