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arXiv 1605.04040astro-ph.GA
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作为活动星系周围多相星际介质探测器的吸收变率

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

Jean-Pierre Macquart, Steven Tingay

摘要:

我们研究了一个模型的变量自由和中性氢吸收推断对一些紧凑的射电星系的核心,在其中的空间波动介质漂移在前面的源。我们与吸收引起的强度波动的统计基础的不透明度波动。我们调查模型中的吸收介质包括离散云或幂律谱的不透明度波动。我们研究的变化特性的介质组成的高斯形的云,其中的中性和电离的物质是共同定位的,其中的云包括球形的恒定密度的中性核心包围电离鞘。互功率谱反映了中性物质和电离物质之间的空间关系,并区分了两种模型,高斯模型中的功率呈无特征的幂律下降,而电离鞘层模型中的功率则在正负值之间振荡。我们展示了HI和自由-自由功率谱的比较如何揭示介质的电离和中性分数的信息。背景源作为底层不透明度功率谱的低通滤波器,其将时间波动限制在频率$ω< \dotθ_v / θ_{\rm src}$,其中$\dotθ_v$是源前面的物质的角漂移速度,并且其在小于源大小$θ_{\rm src}$的尺度上淬灭不透明度结构的可观测性。对于漂移速度的$\sim 10^3\,$公里的$^{-1}$和源亮度温度$\sim 10^{12}\,$ K,这种限制的时间不透明度波动的时间尺度为几个月到几十年。

英文摘要:

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 $ω< \dotθ_v / θ_{\rm src}$, where $\dotθ_v$ 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 $θ_{\rm src}$. 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.

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