对两个z=4.05亚毫米镓GN20和GN20的中性碳的研究
A Search for Neutral Carbon towards two z=4.05 Submillimetre Galaxies, GN20 and GN20.2
摘要：
用IRAM Plateau-de-Bure干涉仪（PdBI）对中性碳（Cl（2->1），nu_rest = 809 GHz）和~（12）CO（J=7->6）的上部精细结构谱线进行了研究。（nu_rest= 806 GHz）向亚毫米星系（SMG）GN 20（SMM J123711.9+622212，z = 4.055）和GN20.2（SMM J123708.8+622202，z = 4.051）。在GN 20中以8 σ显著性检测到远红外（FIR）连续谱，其中通量密度为S_1.8mm = 1.9+-0.2mJ γ，而在GN20.2中未检测到连续谱。在CI和CO（7-6）谱线中，这两个源在统计上都未被探测到;我们推导出CI和CO的谱线光度极限为L '<~ 2x10^10 K km s^-1 pc ^2。假设碳激发温度T_ex = 30 K（星系测量的尘埃温度），我们推断CI质量极限为M_CI < 5.4x10^6 Msun（GN 20）和M_CI < 6.8x10^6 Msun（GN20.2）。推导出的CI丰度极限为GN 20 <1.8x10^-5，GN20.2 <3.8x10^-5，这意味着这些系统具有银河系水平的中性碳富集度（X[CI]/X[H2]）或更低，类似于高红移碳探测系统（5x 10 ^-5），但比本地星暴星系的中性碳富集度低约50倍。利用GOODS-N高分辨率MERLIN+VLA射电图中GN 20和GN20.2的观测值进一步限定活动区的大小和位置。我们的结论是，年轻的快速发展系统，如GN 20和GN20.2的物理气体性质可能显着不同的星暴/ULIRG环境中的本地宇宙，但类似于z~2 SMG。除非能找到引力放大的例子，否则像GN 20这样的星系的观测将需要阿塔卡马大毫米阵列（ALMA）的灵敏度增加一个数量级，以限制它们的CI和高J CO含量，尽管它们是z~4处最亮的系统。
英文摘要：
Using the IRAM Plateau-de-Bure Interferometer (PdBI) we have searched for the upper fine structure line of neutral carbon (CI(2->1), nu_rest = 809 GHz) and 12CO(J=7->6) (nu_rest=806GHz) towards the submillimetre galaxies (SMGs) GN20 (SMM J123711.9+622212, z = 4.055) and GN20.2 (SMM J123708.8+622202, z = 4.051). The far-infrared (FIR) continuum is detected at 8sigma significance in GN20, with a flux density of S_1.8mm = 1.9+-0.2mJy, while no continuum is detected in GN20.2. Both sources are statistically undetected in both CI and CO(7-6) lines; we derive line luminosity limits for both CI and CO of L'<~2x10^10K km s^-1 pc^2. Assuming carbon excitation temperatures of T_ex = 30K (the galaxies' measured dust temperatures), we infer CI mass limits of M_CI < 5.4x10^6 Msun (GN20) and M_CI < 6.8x10^6 Msun (GN20.2). The derived CI abundance limits are <1.8x10^-5 for GN20 and <3.8x10^-5 for GN20.2 implying that the systems have Milky Way level neutral carbon enrichment (X[CI]/X[H2]) or lower, similar to high-redshift carbon-detected systems (at 5x10^-5) but about 50 times less than the neutral carbon enrichment of local starburst galaxies. Observations of GN20 and GN20.2 in high-resolution MERLIN+VLA radio maps of GOODS-N are used to further constrain the sizes and locations of active regions. We conclude that the physical gas properties of young rapidly evolving systems like GN20 and GN20.2 are likely significantly different than starburst/ULIRG environments in the local Universe yet similar to z~2 SMGs. Unless gravitationally amplified examples can be found, observations of galaxies like GN20 will require the order of magnitude increase in sensitivity of the Atacama Large Millimetre Array (ALMA) to constrain their CI and high-J CO content, despite the fact that they are the brightest systems at z~4.
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