On the Probability Distribution of Cosmological Microlensing Optical Depths
On the Probability Distribution of Cosmological Microlensing Optical Depths
复制标题
宇宙学微透镜光学深度的概率分布
DOI：
10.1086/338426
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发表时间：
2001
期刊：
影响因子：
--
通讯作者：
E. Turner
中科院分区：
文献类型：
--
作者：
J. Wyithe;J. Wyithe;E. Turner
通常基于 Press-Gunn 近似来计算宇宙遥远源的微透镜发生概率，即透镜物体以恒定的同动密度均匀且随机地分布在介入空间中。我们在这里通过考虑星系中可能的透镜物体彼此之间的强烈空间聚类以及它们与构成巨大星系晕的暗物质团块的关联来研究更现实的宇宙学微透镜统计数据。研究了微透镜物体像星系中观测到的星光一样分布的情况，以及延伸的大质量晕本身也由能够充当微透镜的致密大质量物体组成的情况。计算微透镜光学深度 κ 沿随机选择的视线的分布，以及 κ 沿强微透镜附近视线的条件分布。还考虑了说明性的放大偏差。这些分布使我们能够计算高红移源在各种情况下被微透镜化的概率，以及发生此类微透镜事件的可能局部 κ（附近视线的平均值）。我们的总体结果是，如果星系的巨大光晕是由暗致密天体构成的话，普雷斯-冈恩近似是一个有用的数量级近似，但在更可能的情况下，只有普通的星系星群是宇宙微透镜事件的主要来源，它会严重失败，并且可能在质量上产生误导。特别是，我们发现任何时候恒星的微透镜作用仅限于高红移源的 1% 左右。此外，尽管只有一小部分高红移源被多重成像（通过星系），但这些源最有可能被恒星微透镜化。因此，恒星的微透镜效应通常在 κ 接近 1 时观察到，此时简单的孤立点质量透镜近似是不合适的。然而，如果冷暗物质晕由凝聚的物体组成，那么在任何给定时间超过 10% 的高红移源都会被微透镜化。这些源中的绝大多数不是多重成像的，并且 κ 小于 0.01。
It is conventional to calculate the probability of microlensing for a cosmologically distant source based on the Press-Gunn approximation that the lensing objects are uniformly and randomly distributed in the intervening space with a constant comoving density. We here investigate more realistic cosmological microlensing statistics by considering the strong spatial clustering of likely lensing objects with each other in galaxies and their association with the clumps of dark matter that make up the massive halos of galaxies. Both cases in which microlensing objects are distributed like the observed starlight in galaxies and ones in which the extended massive halos themselves are also composed of compact massive objects capable of acting as microlenses are investigated. The distribution of microlensing optical depth κ along randomly chosen sight lines is calculated, as is the conditional distribution of κ along sight lines near one that is strongly microlensed. Illustrative magnification biases are also considered. These distributions allow us to calculate both the probability that a high-redshift source will be microlensed in the various scenarios and the likely local κ (averaged over nearby sight lines) at which such microlensing events will occur. Our overall result is that the Press-Gunn approximation is a useful order-of-magnitude approximation if the massive halos of galaxies are made of dark compact objects, but that it fails badly and can be qualitatively misleading in the more likely case in which only the ordinary stellar populations of galaxies are the dominant source of cosmological microlensing events. In particular, we find that microlensing by stars is limited to of the order of 1% of high-redshift sources at any one time. Furthermore, even though only a small fraction of high-redshift sources are multiply imaged (by galaxies), it is these sources that are most likely to be microlensed by stars. Consequently, microlensing by stars is usually observed at κ near 1 where the simple isolated point mass lens approximation is not appropriate. However, if cold dark matter halos are composed of condensed objects, then more than 10% of high-redshift sources are microlensed at any given time. The vast majority of these sources are not multiply imaged, and have κ smaller than 0.01.