A survey of submillimeter C and CO lines in nearby galaxies
A survey of submillimeter C and CO lines in nearby galaxies
复制标题
对附近星系中亚毫米 C 和 CO 线的调查
DOI：
10.1051/0004-6361:20053872
复制
发表时间：
2006
影响因子：
6.5
通讯作者：
A. Contursi
中科院分区：
文献类型：
--
作者：
E. Bayet;M. Gerin;T. Phillips;A. Contursi
目标。虽然在遥远的星系中寻找分子气体是基于对亚毫米CO旋转线的探测，但目前对附近星系的CO测量仅限于毫米CO旋转线。亚毫米CO谱线是在温暖致密的分子气体中形成的，因此对物理条件很敏感，而CO(J=1→0)谱线是分子气体总质量的示踪物。为了能够比较近星系和远星系中分子气体的性质，我们利用加州理工大学的亚毫米天文台(CSO)观测了近星系样品中的C和CO亚毫米线(包括^(12)CO(6-5)和^(12)CO(7-6)线)。
方法：研究方法。通过将亚毫米CSO数据与文献中的观测结果相结合，我们获得了CO冷却曲线(也称为CO光谱能量分布)的完整图像。我们利用大速度梯度模型分析了观测到的CO冷却曲线，预测了所研究星系从J=1→0到J=15→14的CO谱线强度，并导出了热致密分子气体的物理性质：动力学温度(T_K)、气体密度(n(H2))、CO柱密度除以谱线宽度N(^(12)CO)/Δv。
结果。我们展示了CO SED如何随星系恒星形成活动而变化。对于活性核，峰位于^(12)CO(6-5)或^(12)CO(7-6)旋转线附近，而对于正常核，大部分能量由^(12)CO(4-3)和^(12)CO(5-4)线携带。无论原子核的光谱类型是什么，观测到的C冷却速率都比观测到的CO冷却速率低(低≥4倍)。近星暴星系(如NGC253)的CO冷却曲线与遥远星系的CO冷却曲线形状非常相似。因此，CO冷却曲线对于诊断遥远天体的恒星形成活动是有用的。
Aims. While the search for molecular gas in distant galaxies is based on the detection of submillimeter CO rotational lines, the current CO surveys of nearby galaxies are restricted to the millimeter CO lines. The submillimeter CO lines are formed in warm and dense molecular gas and are therefore sensitive to the physical conditions whereas the CO (J = 1 → 0) line is a tracer of the total molecular gas mass. In order to be able to compare the properties of molecular gas in nearby and distant galaxies, we have observed C and CO submillimeter lines (including the ^(12)CO(6-5) and ^(12)CO(7-6) lines) in a sample of nearby galaxies using the Caltech Submillimeter Observatory (CSO).
Methods. We have obtained a complete view of the CO cooling curve (also called CO spectral energy distribution) by combining the submillimeter CSO data with previous observations found in the literature. We made use of Large Velocity Gradient (LVG) models to analyse the observed CO cooling curve, predict CO line intensities from J = 1 → 0 to J = 15 → 14 in the studied galaxies, and derive the physical properties of the warm and dense molecular gas : the kinetic temperature (T_K); the gas density (n(H2)); the CO column density divided by the line width N(^(12)CO)/Δv. The predictions for the line intensities and for the total CO cooling power, obtained from LVG modelling have been compared with predictions from Photo Dissociation Regions (PDR) models.
Results. We show how the CO SED varies according to the galaxy star forming activity. For active nuclei, the peak is located near the ^(12)CO(6-5) or ^(12)CO(7-6) rotational lines, while, for normal nuclei, most of the energy is carried by the ^(12)CO(4-3) and ^(12)CO(5-4) lines. Whatever the spectral type of the nucleus, the observed C cooling rate is lower than the observed CO cooling rate (by a factor of ≥ 4). The CO cooling curve of nearby starburst galaxies (e.g. NGC 253) has a quite similar shape to the CO cooling curve of distant galaxies. Therefore, the CO cooling curves are useful diagnostics for the star forming activity in distant objects.