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SUBMILLIMETER HCN AND HCO~+ EMISSION FROM GALAXIES

机译：来自银河系的舒米特HCN和HCO〜+排放

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摘要

We report the first detection of extragalactic HCN and HCO~+ J = 4 → 3 emission at λ = 850 μm. Because their critical densities (n ～10~7 cm~(-3)) and excitation energies (E_u ～ 45 K) are large, these lines trace dense, warm gas. We have detected HCN J = 4 → 3 toward NGC 253, NGC 4945, and IC 342 and report a sensitive upper limit toward M82. We have also detected HCO~+ J = 4 → 3 in both NGC 253 and M82. The HCN J = 4 → 3/J = 1 → 0 intensity ratio, which is larger for higher gas densities, is at least 12 times larger in NGC 253 than that in M82. Single-component gas excitation model calculations for NGC 253 show that, averaged over the central 300 pc, the molecular gas is warm (T approx > 50 K) and dense (n ～5 x 10~5 cm~(-3)). For M82, the lower HCN J = 4 → 3/J = 1 → 0 line ratio requires an average gas density at least 10 times smaller. In both M82 and NGC 253, the J = 4 → 3/J = 1 → 0 line ratios peak at the nucleus. Consequently, molecular gas is densest in the central 300 pc. The submillimeter HCN and HCO~+ lines reveal large differences in the dense gas properties, despite similar CO and far-infrared luminosities.
机译：我们报道了在λ= 850μm处首次检测到河外HCN和HCO〜+ J = 4→3发射。由于它们的临界密度（n〜10〜7 cm〜（-3））和激发能（E_u〜45 K）很大，因此这些线会跟踪稠密的温暖气体。我们已经检测到朝向NGC 253，NGC 4945和IC 342的HCN J = 4→3，并向M82报告了敏感的上限。我们还在NGC 253和M82中都检测到HCO〜+ J = 4→3。 HCN J = 4→3 / J = 1→0强度比（对于更高的气体密度而言更大）在NGC 253中比在M82中大至少12倍。 NGC 253的单组分气体激发模型计算表明，平均在中心300 pc处，分子气体是温暖的（T约> 50 K）和致密的（n〜5 x 10〜5 cm〜（-3））。对于M82，较低的HCN J = 4→3 / J = 1→0管路比要求平均气体密度小至少10倍。在M82和NGC 253中，J = 4→3 / J = 1→0线比在原子核处达到峰值。因此，在中心300 pc中，分子气体最密集。尽管具有相似的CO和远红外光度，但亚毫米HCN和HCO〜+线显示出致密气体属性的巨大差异。

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