arXiv 0802.39142008-02-26astro-ph

阻尼Lyman α系统的双峰性

Bimodality in Damped Lyman alpha Systems

Arthur M. Wolfe, Jason X. Prochaska, Regina A. Jorgenson, Marc Rafelski

备注：: Accepted for publication in the Astrophysical Journal 18 pages 14 figures
DOI：: 10.1086/588090

摘要：

我们报告的双峰阻尼Ly系统（DLAs）的证据。使用[C II] 158 μ的冷却速率lc，我们发现在lc=10^-27.4和10^-26.6 ergs s^-1 H^-1处有峰值，在lc^crit ~= lc < 10^-27.0 ergs s^-1 H^-1处有谷值。我们将样品分为lc < lc^crit的低温DLAs和lc > lc^crit的高温DLAs，发现两个子样品中的速度宽度、金属丰度、尘气比和Si II等效宽度来自同一母体的Kolmogorv-Smirnov概率都很小。所有这些量都显着较大的高冷却人口，而H I柱密度是不可区分的两个群体。我们发现，加热的X射线和FUV背景辐射是不够的，以平衡任何人口的冷却速率。相反，DLA气体被局部辐射场加热。在哈勃超深空中，微弱的扩展天体的罕见外观排除了原位恒星形成作为高冷人口的主导恒星形成模式，但与原位恒星形成作为低冷人口的主导模式兼容。在高温DLAs中的恒星形成可能出现在Lyman Break星系中。我们调查是否这些属性的DLAs是类似的双峰附近星系的属性。使用Si II等效宽度作为质量指标，我们构建了金属丰度，LC和面积SFR与质量指标的二元分布。初步证据发现的相关性和平行序列，这表明DLAs和附近的星系之间的相似性。我们认为跃迁质量模型为我们发现的双峰态提供了一个合理的解释，因此，当前星系中的双峰态可能起源于DLAs。

英文摘要：

We report evidence for a bimodality in damped Ly systems (DLAs). Using [C II] 158 mu cooling rates, lc, we find a distribution with peaks at lc=10^-27.4 and 10^-26.6 ergs s^-1 H^-1 separated by a trough at lc^crit ~= lc < 10^-27.0 ergs s^-1 H^-1. We divide the sample into low cool DLAs with lc < lc^crit and high cool DLAs with lc > lc^crit and find the Kolmogorv-Smirnov probabilities that velocity width, metallicity, dust-to-gas ratio, and Si II equivalent width in the two subsamples are drawn from the same parent population are small. All these quantities are significantly larger in the high cool population, while the H I column densities are indistinguishable in the two populations. We find that heating by X-ray and FUV background radiation is insufficient to balance the cooling rates of either population. Rather, the DLA gas is heated by local radiation fields. The rare appearance of faint, extended objects in the Hubble Ultra Deep Field rules out in situ star formation as the dominant star-formation mode for the high cool population, but is compatible with in situ star formation as the dominant mode for the low cool population. Star formation in the high cool DLAs likely arises in Lyman Break galaxies. We investigate whether these properties of DLAs are analogous to the bimodal properties of nearby galaxies. Using Si II equivalent width as a mass indicator, we construct bivariate distributions of metallicity, lc, and areal SFR versus the mass indicators. Tentative evidence is found for correlations and parallel sequences, which suggest similarities between DLAs and nearby galaxies. We suggest that the transition-mass model provides a plausible scenario for the bimodality we have found. As a result, the bimodality in current galaxies may have originated in DLAs.

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