Radio Recombination Lines from the Nuclear Regions of Starburst Galaxies
Radio Recombination Lines from the Nuclear Regions of Starburst Galaxies
复制标题
星暴星系核区的射电重组线
DOI：
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发表时间：
1996
期刊：
影响因子：
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通讯作者：
F. Viallefond
中科院分区：
文献类型：
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作者：
Jun;K. Anantharamaiah;W. Goss;F. Viallefond
使用角分辨率为3英寸的甚大阵列（VLA），我们检测到了来自星系Arp 220、M83和NGC 2146的氢复合线H92α。线发射来自核区域，线与连续谱的比率为1%或更小。为了拟合观察到的 H92α 线和核区的连续体数据，我们考虑了两种类型的模型。首先，我们利用具有 H ii 区域集合的模型。该模型需要大量紧凑的 H ii 区域。当电子温度在 5 × 103-1 × 104 K 范围内以及电子密度范围内时，该模型可以同时解释线强度和连续谱。在大多数情况下，由于 H ii 区域内出现自由连续体，H92α 线主要由内部受激发射主导。在 Arp 220 的低密度情况下 (ne = 50 cm−3)，由于背景非热源，大约一半的线发射来自外部受激发射。从这些模型预测，M83 的电离光子的典型速率为 ∼5 × 1052 s−1，NGC 2146 为 ∼4 × 10−53 s−1，Arp 220 为 5 × 1054 s−1。我们推断 Arp 220 需要 105 颗 O5 星，这比 NGC 2146 的数量级大了 2 个数量级。大于 M83。或者，几个 Te ⩾ 5 × 103 和 ne 在 50 − 1 × 104 cm−3 范围内的均匀板模型似乎既适合 H92α 线，又适合 Arp 220 和 M83 的连续谱数据。在低密度模型中，背景非热辐射的受激发射似乎在低频下占主导地位，而较高频率的谱线主要来自自发发射。均匀板模型比 H ii 区域模型需要更高的电离光子速率。没有任何具有合理 Te 的平板模型可以拟合在 M83 和 NGC 2146 中观测到的数据。将之前发布的数据与这些新观测结果相结合，已经观测到了 13 个星系与 VLA 的射电复合线 (RRL) 样本。 13 个星系中的 9 个已在 H92α 线中被发现。虽然 H92α 线的光度似乎与 Bra 线的光度相关，但我们发现几乎所有 RRL 星系的 H92α 线的光度都比预期的 LTE 值显着过量。 H92α 线通量的过量表明非 LTE 效应对于这些星爆核中的 H92α 线很重要。还发现 H92α 与 HCN/HCO+ 分子线之间存在很强的相关性，表明 RRL 发射体可能与致密分子核心在空间上相关。推断出的高电子密度也表明 RRL 与这些星系中的致密分子介质之间存在密切关系。
Using the Very Large Array (VLA) with an angular resolution of 3″, we have detected the hydrogen recombination line H92α from the galaxies Arp 220, M83, and NGC 2146. The line emission arises from the nuclear regions with a line-to-continuum ratio of 1% or less. In order to fit both the observed H92α line and continuum data in the nuclear regions, we have considered two types of models. First, we utilize a model with a collection of H ii regions. A large number of compact H ii regions are required in this model. With electron temperatures in the range 5 × 103-1 × 104 K and a range of electron densities, this model can account for both the line intensity and the continuum spectrum. In most cases, the H92α line is dominated by internal stimulated emission due to free-free continuum arising within the H ii regions. In a low-density case (ne = 50 cm−3) for Arp 220, about half the line emission comes from external stimulated emission due to the background nonthermal source. Typical rates of ionizing photons predicted from these models are ∼5 × 1052 s−1 for M83, ∼4 × 10−53 s−1 for NGC 2146, and 5 × 1054 s−1 for Arp 220. We infer that 105 O5 stars are required in Arp 220, which is an order of magnitude greater than in NGC 2146 and 2 orders of magnitude greater than in M83. Alternatively, several uniform slab models with Te ⩾ 5 × 103 and ne in the range of 50 − 1 × 104 cm−3 appear to fit both the H92α line and continuum data of Arp 220 and M83. In the low-density models, stimulated emission by the background nonthermal radiation appears to be dominant at low frequencies, and the lines at higher frequencies arise primarily from spontaneous emission. The uniform slab model requires a higher ionizing photon rate than the H ii region model. No slab models with reasonable Te can fit the data observed in M83 and NGC 2146. Combining previous published data with these new observations, a sample of 13 galaxies has been observed for radio recombination lines (RRLs) with the VLA. Nine out of the 13 galaxies have been detected in the H92α line. While the H92α line luminosity appears to be correlated with the Bra line luminosity, we find that nearly all the RRL galaxies show a significant excess in H92α line compared to the expected LTE value. The excess in the H92α line flux suggests that non-LTE effects are important for the H92α line in these starburst nuclei. A strong correlation between H92α and the molecular lines of HCN/HCO+ is also found, indicating that the RRL emitters may be spatially associated with the dense molecular cores. The inferred high electron density also suggests an intimate relation between the RRLs and the dense molecular medium in these galaxies.