Substructure Detection Reanalyzed: Dark Perturber shown to be a Line-of-Sight Halo

TL;DR

This study reanalyzed a sub-galactic structure in gravitational lensing, revealing it as a line-of-sight dark matter halo rather than a satellite, significantly impacting dark matter research and future survey interpretations.

Contribution

It demonstrates that a previously identified substructure is actually a line-of-sight halo, using redshift fitting with multi-plane lensing, which is a novel application in this context.

Findings

The perturber's redshift is estimated at z=1.42.

The structure's mass is an order of magnitude greater than previously thought.

First evidence of a dark perturber being a line-of-sight halo via lensing.

Abstract

Observations of structure at sub-galactic scales are crucial for probing the properties of dark matter, which is the dominant source of gravity in the universe. It will become increasingly important for future surveys to distinguish between line-of-sight halos and subhalos to avoid wrong inferences on the nature of dark matter. We reanalyze a sub-galactic structure (in lens JVAS B1938+666) that has been previously found using the gravitational imaging technique in galaxy-galaxy lensing systems. This structure has been assumed to be a satellite in the halo of the main lens galaxy. We fit the redshift of the perturber of the system as a free parameter, using the multi-plane thin-lens approximation, and find that the redshift of the perturber is $z_\mathrm{int} = 1.42\substack{+0.10 \\ -0.15}$ (with a main lens redshift of $z=0.881$). Our analysis indicates that this structure is more massive than the previous result by an order of magnitude. This constitutes the first dark perturber shown to be a line-of-sight halo with a gravitational lensing method.