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arXiv 0904.4239astro-ph.CO
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Stephan五重星系碰撞中H2的形成和激发

H2 formation and excitation in the Stephan's Quintet galaxy-wide collision

P. Guillard, F. Boulanger, G. Pineau des Forets, P. N. Appleton

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19 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics Minor editing and typos
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摘要：

上下文斯皮策太空望远镜在斯蒂芬五重星系（SQ）中探测到了一个强大的（L（H2）~10^41 erg s-1）中红外H2辐射，该辐射朝向星系范围的碰撞。这一发现之后，又发现了更遥远的河外氢光源，几乎没有恒星形成的光谱特征。这些观测将分子气体置于一个新的背景下，人们必须描述它作为星系演化的高能阶段的冷却剂的作用。目标。SQ激波后介质被观察到是多相的，H2气体与热的（~ 5 10^6 K）X射线发射等离子体共存。H_2谱线的表面亮度超过X射线，0-0 S（1）H_2谱线宽度约为900 km·s ~（-1），与碰撞速度在同一数量级。这些观测结果提出了三个问题，我们打算回答：（i）为什么H2存在于激波后气体？(ii)我们如何解释H2激发？(iii)为什么H2是主要的冷却剂？方法.我们考虑两个多相含尘气体流的碰撞。我们的模型量化的气体冷却，尘埃的破坏，H2的形成和激发在激波后介质。结果(i)激波速度、激波后温度和气体冷却时间尺度取决于激波前气体密度。碰撞速度是充满云间气体的低密度体中的激波速度。这产生了~ 5 10^6 K的无尘X射线发射等离子体。激波在云中的速度较小。我们表明，气体加热到低于10^6 K的温度冷却，保持其尘埃含量，并成为氢在SQ碰撞年龄（~ 5 - 10^6年）。(ii)由于大量的动能的H2气体是占主导地位的能源水库，我们认为，H2的排放是由动能湍流能量的耗散。（节录）

英文摘要：

Context. The Spitzer Space Telescope has detected a powerful (L(H2)~10^41 erg s-1) mid-infrared H2 emission towards the galaxy-wide collision in the Stephan's Quintet (SQ) galaxy group. This discovery was followed by the detection of more distant H2-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations set molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H2 gas coexisting with a hot (~ 5 10^6 K), X-ray emitting plasma. The surface brightness of H2 lines exceeds that of the X-rays and the 0-0 S(1) H2 linewidth is ~ 900 km s-1, of the same order of the collision velocity. These observations raise three questions we propose to answer: (i) Why H2 is present in the postshock gas ? (ii) How can we account for the H2 excitation ? (iii) Why H2 is a dominant coolant ? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H2 formation and excitation in the postshock medium. Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume filling intercloud gas. This produces a ~ 5 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is smaller in clouds. We show that gas heated to temperatures less than 10^6 K cools, keeps its dust content and becomes H2 within the SQ collision age (~ 5 10^6 years). (ii) Since the bulk kinetic energy of the H2 gas is the dominant energy reservoir, we consider that the H2 emission is powered by the dissipation of kinetic turbulent energy. (Abridged)

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