Hunting for dark halo substructure using submilliarcsecond-scale observations of macrolensed radio jets

TL;DR

This paper investigates the potential of VLBI observations of macrolensed quasar jets to detect small-scale dark matter substructures, such as primordial black holes and ultracompact minihalos, within specific mass ranges.

Contribution

It demonstrates the feasibility of using high-resolution VLBI to identify or constrain compact dark matter substructures in lensing systems, focusing on alternative dark matter models.

Findings

VLBI can detect primordial black holes >1% of dark matter in lensing galaxies.

Detection of ultracompact minihalos is possible if they constitute >10% of dark matter.

Standard cold dark matter subhalos are not detectable with this method.

Abstract

Dark halo substructure may reveal itself through secondary, small-scale gravitational lensing effects on light sources that are macrolensed by a foreground galaxy. Here, we explore the prospects of using Very Long Baseline Interferometry (VLBI) observations of multiply-imaged quasar jets to search for submilliarcsecond-scale image distortions produced by various forms of dark substructures in the 1e3-1e8 Msolar mass range. We present lensing simulations relevant for the angular resolutions attainable with the existing European VLBI Network (EVN), the global VLBI array, and an upcoming observing mode in which the Atacama Large Millimeter Array (ALMA) is connected to the global VLBI array. While observations of this type would not be sensitive to standard cold dark matter subhalos, they can be used to detect more compact forms of halo substructure predicted in alternative structure formation scenarios. By mapping ~5 strongly lensed systems, it should be possible to detect or robustly rule out primordial black holes in the 1e3-1e6 Msolar mass range if they constitute >1% percent of the dark matter in these lenses. Ultracompact minihalos are harder to detect using this technique, but 1e6-1e8 Msolar ultracompact minihalos could in principle be detected if they constitute >10% of the dark matter.