The Dense Gas Fraction in Galactic Center Clouds
The Dense Gas Fraction in Galactic Center Clouds
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DOI：
10.3847/1538-4357/aae581
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发表时间：
2018-09
期刊：
影响因子：
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通讯作者：
E. Mills;A. Ginsburg;K. Immer;J. Barnes;L. Wiesenfeld;A. Faure;M. Morris;M. Requena-Torres
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文献类型：
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作者：
E. Mills;A. Ginsburg;K. Immer;J. Barnes;L. Wiesenfeld;A. Faure;M. Morris;M. Requena-Torres
我们分析了银河系中心R=300pC的气体密度，重点分析了三个云：GCM0.02-0.07(50公里的S−1云)，GCM0.13-0.08(20公里的S−1云)和GCM0.25+0.01(“砖”)。密度是通过观察分子HC3N的J=(3-2)、(4-3)、(5-4)、(10-9)、(18-17)、(19-18)、(21-20)和(24-23)跃迁来确定的。我们发现了HC3N至少有两种激发机制的证据，并将低激发组分的密度限制在10~4 cm−3以下，高激发组分的密度限制在10~5到106 cm−3之间。这比在银河系中心最活跃的恒星形成云SGR B2中发现的107 cm−3的密度要小得多。这与银河系中心恒星形成的密度阈值高于银河盘的典型密度的要求是一致的。我们还可以限制每个组分的柱密度，以确定这些云的“稠密”气体(n>；105 cm−3)的质量分数。我们发现，这三种云的∼都是15%。将我们的模型结果应用于整个中心R=300pC的(10-9)和(3-2)线的比率，我们发现致密(n>；104 cm−3)气体的比例在半径为∼140pC的范围内增加，这与最近对该区域气体动力学的模型的预测一致。我们的观测表明，HC3N是探测银河系中心云密度结构的一个很好的分子。
We present an analysis of gas densities in the central R = 300 pc of the Milky Way, focusing on three clouds: GCM –0.02–0.07 (the 50 km s−1 cloud), GCM –0.13–0.08 (the 20 km s−1 cloud), and GCM 0.25+0.01 (the “Brick”). Densities are determined using observations of the J = (3–2), (4–3), (5–4), (10–9), (18–17), (19–18), (21–20), and (24–23) transitions of the molecule HC3N. We find evidence of at least two excitation regimes for HC3N and constrain the low-excitation component to have a density less than 104 cm−3 and the high-excitation component to have a density between 105 and 106 cm−3. This is much less than densities of 107 cm−3 that are found in Sgr B2, the most actively star-forming cloud in the Galactic center. This is consistent with the requirement of a higher-density threshold for star formation in the Galactic center than is typical in the Galactic disk. We are also able to constrain the column density of each component in order to determine the mass fraction of “dense” (n > 105 cm−3) gas for these clouds. We find that this is ∼15% for all three clouds. Applying the results of our models to ratios of the (10–9) and (3–2) line across the entire central R = 300 pc, we find that the fraction of dense (n > 104 cm−3) gas increases inward of a radius of ∼140 pc, consistent with the predictions of recent models for the gas dynamics in this region. Our observations show that HC3N is an excellent molecule for probing the density structure of clouds in the Galactic center.