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arXiv 1306.1842astro-ph.GA
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银河系中心附近温暖电离气体的密度：低射频观测

Density of warm ionized gas near the Galactic Center: Low radio frequency observations

Subhashis Roy

备注：
19 pages, 2 figures, accepted for publication in ApJ
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摘要：

我们用GMRT观测了0.154和0.255GHz的银河系中心区。总共探测到62个致密的可能的河外源。它们的散射尺寸随着与GC的角距离的增加而线性下降，直到约1度。在银河系359.5度< l < 0.5度和-0.5度<b <0.5度（超强散射区）范围内，源的表观散射尺寸比NE 2001银河系电子分布模型预测的小一个数量级以上。高自由光学深度观察到对大多数的扩展非热源在0.6度的GC。光学深度的显著变化表明吸收介质在10'的角尺度下是片状的，并且电子密度为约10个/cc，这与NE 2001模型相匹配。该模型预测的河外（EG）源，以解决了从1.4 GHz的干涉测量。然而，该地区预计的10个可能的EG源中有8个存在于1.4 GHz目录中。密集分子云与周围介质的电离界面最有可能是强散射和低射频吸收的原因。然而，密集的GC云跟踪CS $J=1-0$发射被发现有一个狭窄的分布~0.2度整个银河系平面。在$b$中，通过所谓的超强散射区看到的大多数EG源的角分布是随机的，并且通常10个源中的约7个将不会被看到通过致密的分子云，这解释了为什么它们中的大多数在1.4 GHz处没有散射展宽。

英文摘要：

We have observed the Galactic Center (GC) region at 0.154 and 0.255 GHz with the GMRT. A total of 62 compact likely extragalactic sources are detected. Their scattering sizes go down linearly with increasing angular distance from the GC up to about 1 deg. The apparent scattering sizes of sources are more than an order of magnitude down than predicted earlier by the NE2001 model of Galactic electron distribution within 359.5 deg < l < 0.5 deg and -0.5 deg <b <0.5 deg (Hyperstrong scattering region) of the Galaxy. High free-free optical depths are observed towards most of the extended nonthermal sources within 0.6 deg from the GC. Significant variation of optical depth indicate the absorbing medium is patchy at an angular scale of 10' and electron density is ~10 per cc that matches with the NE2001 model. This model predicts the extragalactic (EG) sources to be resolved out from 1.4 GHz interferometric surveys. However, 8 likely EG sources out of 10 expected in the region are present in 1.4 GHz catalog. Ionized interfaces of dense molecular clouds to the ambient medium are most likely responsible for strong scattering and low radio frequency absorption. However, dense GC clouds traced by CS $J=1-0$ emission are found to have a narrow distribution of ~0.2 deg across the Galactic plane. Angular distribution of most of the EG sources seen through the so called Hyperstrong scattering region are random in $b$, and typically ~7 out of 10 sources will not be seen through to the dense molecular clouds, and it explains why most of them are not scatter broadened at 1.4 GHz.

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