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

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 $\omega <{\dot{\theta }}_v/{\theta }_{\mathrm{src}}$, 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 ${\theta }_{\mathrm{src}}$. 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.

16 pages, MNRAS accepted

References in corpus (5)

• Science with ASKAP - the Australian Square Kilometre Array Pathfinder
• Science With The Australian Square Kilometre Array Pathfinder
• Discovery of HI gas in a young radio galaxy at $z=0.44$ using the Australian Square Kilometre Array Pathfinder
• X-ray properties of the Youngest Radio Sources and their Environments
• The Spectral Variability of the GHz-Peaked Spectrum Radio Source PKS 1718-649 and a Comparison of Absorption Models

Cited by in corpus (1)

• The First Large Absorption Survey in HI (FLASH): I. Science Goals and Survey Design