Absorption variability as a probe of the multiphase interstellar media surrounding active galaxies

TL;DR

This paper models how variable absorption caused by fluctuating interstellar media around active galaxies affects radio observations, revealing insights into the medium's structure, ionization, and neutral content through spectral analysis.

Contribution

It introduces a novel framework linking absorption variability to the statistical properties of the interstellar medium, distinguishing cloud and sheath models via power spectrum analysis.

Findings

Power spectra differ between cloud and sheath models.

Comparison of HI and free-free spectra reveals ionization states.

Temporal fluctuations are limited to months or decades by source size and drift speed.

Abstract

We examine a model for the variable free-free and neutral hydrogen absorption inferred towards the cores of some compact radio galaxies in which a spatially fluctuating medium drifts in front of the source. We relate the absorption-induced intensity fluctuations to the statistics of the underlying opacity fluctuations. We investigate models in which the absorbing medium consists of either discrete clouds or a power-law spectrum of opacity fluctuations. We examine the variability characteristics of a medium comprised of Gaussian-shaped clouds in which the neutral and ionized matter are co-located, and in which the clouds comprise spherical constant-density neutral cores enveloped by ionized sheaths. The cross-power spectrum indicates the spatial relationship between neutral and ionized matter, and distinguishes the two models, with power in the Gaussian model declining as a featureless power-law, but that in the ionized sheath model oscillating between positive and negative values. We show how comparison of the HI and free-free power spectra reveals information on the ionization and neutral fractions of the medium. The background source acts as a low-pass filter of the underlying opacity power spectrum, which limits temporal fluctuations to frequencies $\omega < \dot{\theta}_v / \theta_{\rm src}$, where $\dot{\theta}_v$ is the angular drift speed of the matter in front of the source, and it quenches the observability of opacity structures on scales smaller than the source size $\theta_{\rm src}$. For drift speeds of $\sim 10^3\,$km s$^{-1}$ and source brightness temperatures $\sim 10^{12}\,$K, this limitation confines temporal opacity fluctuations to timescales of order several months to decades.