密集群中红外“绿谷”中增强的暖H2发射
Enhanced Warm H2 Emission in the Compact Group Mid-Infrared "Green Valley"
摘要：
我们目前的结果来自斯皮策，中红外光谱研究的样本74星系位于23希克森紧凑组，选择在一个动态活跃的阶段HI耗尽。我们在14个星系（约20%）中发现了增强的温暖H2发射（即高于恒星形成区的紫外线激发）的证据，其中8个星系的L（H2 S（0）-S（3））/L（7.7微米PAH）的极端值超过0.07。这种发射以前曾在致密星系群HCG 92（Stephan's Quintet）中看到过，并被证明与机械能的耗散有关，这种耗散与一个星系群成员以高速与星系群内介质中的潮汐碎片碰撞时产生的大规模激波有关。类似地，激波激发或湍流加热可能是致密星系群中H2辐射增强的原因，因为其他加热源（恒星形成或活动星系核的紫外线或X射线激发）不足以解释观测到的辐射。星系群主要落在中红外色-色空间区域，这一区域被先前的研究确定为与HCG星系演化中的快速转变有关。此外，大多数H2增强星系位于蓝色云和红色序列之间的光学“绿谷”中，并且主要是早型盘系统。我们认为，H2增强系统可能代表了一个特定的阶段，在致密的环境中的星系的演化，并提供了新的洞察机制，将星系的光学红色序列。
英文摘要：
We present results from a Spitzer, mid-infrared spectroscopy study of a sample of 74 galaxies located in 23 Hickson Compact Groups, chosen to be at a dynamically-active stage of HI depletion. We find evidence for enhanced warm H2 emission (i.e. above that associated with UV excitation in star-forming regions) in 14 galaxies (~20%), with 8 galaxies having extreme values of L(H2 S(0)-S(3))/L(7.7micron PAH), in excess of 0.07. Such emission has been seen previously in the compact group HCG 92 (Stephan's Quintet), and was shown to be associated with the dissipation of mechanical energy associated with a large-scale shock caused when one group member collided, at high velocity, with tidal debris in the intragroup medium. Similarly, shock excitation or turbulent heating is likely responsible for the enhanced H2 emission in the compact group galaxies, since other sources of heating (UV or X-ray excitation from star formation or AGN) are insufficient to account for the observed emission. The group galaxies fall predominantly in a region of mid-infrared color-color space identified by previous studies as being connected to rapid transformations in HCG galaxy evolution. Furthermore, the majority of H2-enhanced galaxies lie in the optical "green valley" between the blue cloud and red-sequence, and are primarily early-type disk systems. We suggest that H2-enhanced systems may represent a specific phase in the evolution of galaxies in dense environments and provide new insight into mechanisms which transform galaxies onto the optical red sequence.
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