希克森致密群57中强远红外冷却线、特殊的CO动力学和可能的恒星形成抑制
Strong Far-IR Cooling Lines, Peculiar CO Kinematics and Possible Star Formation Suppression in Hickson Compact Group 57
摘要：
我们从Hickson紧致群HCG 57的毫米天文学研究（CARMA）组合阵列的Herschel和CO（1-0）图中给出[C II]和[O I]观测结果，重点是星系HCG 57 a和HCG 57 d。HCG 57 a先前已显示含有与冲击和/或湍流加热一致的增加量的热分子氢。我们的观察结果表明，HCG 57 d与L $_{\rm FIR}$相比具有较强的[C II]发射和较弱的CO（1-0），而HCG 57 a的[C II]和CO（1-0）都很强。HCG 57 a位于[C II]/CO和[C II]/FIR比率的正态分布的上端，并且其远IR冷却支持落在可接受的PDR模型的边界附近的低密度暖扩散气体。然而，在[C II]和温暖的H $_2$发射辐射的功率具有相似的幅度，在其他冲击主导的系统和最近的模型预测。我们认为，冲击加热的[C II]是一个可行的替代光电加热的剧烈扰动扩散气体。激波的存在也与星系中特有的CO运动学相一致，表明存在高度非圆运动。这些运动学上受到干扰的CO区域也显示出恒星形成受到抑制的证据，在肯尼库特-施密特关系中比正常星系低10-30倍。我们认为，这两个星系的特殊性质是一致的HCG 57 d和57 a之间的高度耗散的偏心碰撞遇到，在两个系统中创建环状的形态。高耗散的气体对气体碰撞在密集的群体中可能更常见，因为可能会重复多次遭遇。激波引起的SF抑制的可能性可以解释为什么这些HCG星系的一个子集以前被发现落在中红外绿谷。
英文摘要：
We present [C II] and [O I] observations from Herschel and CO(1-0) maps from the Combined Array for† Research in Millimeter Astronomy (CARMA) of the Hickson Compact Group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG 57a has been previously shown to contain enhanced quantities of warm molecular hydrogen consistent with shock and/or turbulent heating. Our observations show that HCG 57d has strong [C II] emission compared to L$_{\rm FIR}$ and weak CO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a lies at the upper end of the normal distribution of [C II]/CO and [C II]/FIR ratios, and its far-IR cooling supports a low density warm diffuse gas that falls close to the boundary of acceptable PDR models. However, the power radiated in the [C II] and warm H$_2$ emission have similar magnitudes, as seen in other shock-dominated systems and predicted by recent models. We suggest that shock-heating of the [C II] is a viable alternative to photoelectric heating in violently disturbed diffuse gas. The existence of shocks is also consistent with peculiar CO kinematics in the galaxy, indicating highly non-circular motions are present. These kinematically disturbed CO regions also show evidence of suppressed star formation, falling a factor of 10-30 below normal galaxies on the Kennicutt-Schmidt relation. We suggest that the peculiar properties of both galaxies are consistent with a highly dissipative off-center collisional encounter between HCG 57d and 57a, creating ring-like morphologies in both systems. Highly dissipative gas-on-gas collisions may be more common in dense groups because of the likelihood of repeated multiple encounters. The possibility of shock-induced SF suppression may explain why a subset of these HCG galaxies have been found previously to fall in the mid-infrared green valley.
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