arXiv 2404.055962024-04-08astro-ph.GA

高红移处作为分子气体示踪剂的 CO 与 [C I] 基态跃迁的比较研究

A Comparative Study of the Ground State Transitions of CO and [C I] as Molecular Gas Tracers at High Redshift

Marta Frias Castillo, Matus Rybak, Jacqueline A. Hodge, Paul Van der Werk, Ian Smail, Joshua Butterworth, Jasper Jansen, Theodoros Topkaras, Chian-Chou Chen, Sc… 展开作者

Marta Frias Castillo, Matus Rybak, Jacqueline A. Hodge, Paul Van der Werk, Ian Smail, Joshua Butterworth, Jasper Jansen, Theodoros Topkaras, Chian-Chou Chen, Scott C. Chapman, Axel Weiss, Hiddo Algera, Jack E. Birkin, Elisabete da Cunha, Jianhang Chen, Helmut Dannerbauer, E. F. Jiménez-Andrade, Soh Ikarashi, Cheng-Lin Liao, Eric J. Murphy, A. M. Swinbank, Fabian Walter, Gabriela Calistro Rivera, R. J. Ivison, Claudia del P. Lagos

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本文比较 z=2–5 多尘恒星形成星系中的 CO(1–0) 与 [C I](1–0)，验证其比值恒定并交叉校准气体质量转换因子，同时量化高温 CMB 对示踪剂及质量估计的影响。

摘要：

CO(1–0) 和 [C I](1–0) 发射线是近邻星系中成熟的冷分子气体质量示踪剂。在星际介质（ISM）可能更致密的高红移处，对这两种基态跃迁的直接比较仍然有限。本文研究了 20 个位于 z=2–5、未经引力透镜放大的多尘恒星形成星系样本中的 CO(1–0) 与 [C I](1–0) 发射。CO(1–0)/[C I](1–0) 比值至少在 z=5 以前保持恒定，这支持将 [CI](1–0) 用作气体质量示踪剂。对现有数据进行 PDR 建模表明，H2 密度中位数为 log(n [cm^-3])=4.7±0.2，紫外辐射场为 log(G_UV [G0])=3.2±0.2。我们利用 CO(1–0)、[C I](1–0) 和 3 mm 尘埃连续谱测量对各自的气体质量转换因子进行交叉校准，发现这些因子既不依赖红移，也不依赖红外光度。若采用可变的 CO 转换因子，则意味着 [C I] 和尘埃转换因子不同于通常假定的标准值，但与我们源所预期的太阳/超太阳金属丰度一致。辐射转移建模显示，高红移处更暖的 CMB 会显著影响 [C I] 以及 CO 发射；对于预期中最冷的动力学气体温度，这可使推导出的分子气体质量改变至多 70%。尽管如此，我们表明该效应对两种示踪剂比值的影响幅度处于已知的 L'_CO–L'_[CI] 关系弥散范围内。进一步确定各谱线强度的绝对下降，将需要采样充分的谱线能量分布（SLEDs），以便更详细地模拟气体激发条件。

英文摘要：

The CO(1--0) and [\ion{C}{1}](1--0) emission lines are well-established tracers of cold molecular gas mass in local galaxies. At high redshift, where the interstellar medium (ISM) is likely to be denser, there have been limited direct comparisons of both ground state transitions. Here we present a study of CO(1--0) and [\ion{C}{1}](1--0) emission in a sample of 20 unlensed dusty, star-forming galaxies at $z=2-5$. The CO(1--0)/[\ion{C}{1}](1--0) ratio is constant up to at least $z=5$, supporting the use of [CI](1-0) as a gas mass tracer. PDR modelling of the available data indicates a median H$_2$ density of log$(n~[$cm$^{-3}])=4.7\pm0.2$, and UV radiation field log$(G_{\mathrm{UV}} [G$_0$])=3.2\pm0.2$. We use the CO(1--0), [\ion{C}{1}](1--0) and 3mm dust continuum measurements to cross--calibrate the respective gas mass conversion factors, finding no dependence of these factors on either redshift or infrared luminosity. Assuming a variable CO conversion factor then implies [\ion{C}{1}] and dust conversion factors that differ from canonically assumed values but are consistent with the solar/super-solar metallicities expected for our sources. Radiative transfer modelling shows that the warmer CMB at high redshift can significantly affect the [\ion{C}{1}] as well as CO emission, which can change the derived molecular gas masses by up to 70\% for the coldest kinetic gas temperatures expected. Nevertheless, we show that the magnitude of the effect on the ratio of the tracers is within the known scatter of the $L'_\mathrm{CO}-L'_\mathrm{[CI]}$ relation. Further determining the absolute decrease of individual line intensities will require well-sampled spectral line energy distributions (SLEDs) to model the gas excitation conditions in more detail.

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