Cosmological nanolensing by dense gas clouds

TL;DR

This paper investigates how dense gas clouds in the universe can cause observable effects like lensing, extinction, and refraction on distant sources such as quasars, and proposes a new method to test their influence using angular variation patterns.

Contribution

It introduces a comprehensive classification of light curves caused by cosmological gas clouds and suggests a novel statistical test to identify nanolensing effects in quasar variability.

Findings

Gas clouds of ~10^{-4.5} M_sun can explain quasar variability amplitudes.

Extinction caused by these clouds is mostly grey, complicating detection.

A quadrupole angular pattern in variability could confirm nanolensing effects.

Abstract

We study the influence of a cosmological population of dense gas clouds on distant sources, with emphasis on quasar optical variability. In addition to gravitational lensing such clouds affect flux measurements via refraction in the neutral gas and via dust extinction, leading to a variety of possible light curves even in the low optical depth limit. We classify and illustrate the types of light curves that can arise. For sources as large as quasars we show that gravitational lensing and extinction are the dominant effects, with gas refraction playing only a minor role. We find that clouds with mass ~10^{-4.5+/-0.5} M_\odot can reproduce the observed distribution of quasar variation amplitudes, but only if such clouds make up a large fraction of the closure density. In that case there may also be substantial extinction of distant optical sources, which can in principle be constrained by data on "standard candles" such as type Ia supernovae. Unfortunately that extinction is essentially grey, even when the material opacity is strongly wavelength dependent, making it difficult to distinguish from the influence of the background geometry. We propose a novel statistical test of the origin of quasar variability, based on the angular structure of the variation timescale for a large number of quasars distributed all over the sky. If quasar variability is primarily due to nanolensing that angular structure is expected to include a quadrupole term of amplitude $\sim5\%$, which ought to be measurable with future data from the Gaia mission.