[论文解读] Photometric redshifts for the CFHTLS-Wide

F. Brimioulle, M. Lerchster+3 more|ArXiv.org|Nov 19, 2008

Galaxies: Formation, Evolution, Phenomena被引用 8

一句话总结

本论文利用CFHTLS-Wide巡天中u*,g',r',i',z'测光数据，基于PHOTO-z代码，为260万颗星系构建了无偏差、高精度的光度红移星表。其σΔz/(1+z)为0.033–0.037，全样本的异常值率仅为1–2.5%，其中一个稳健子样本（6,100颗星系）的异常值率低至1%，σΔz/(1+z) = 0.037，非常适合用于弱引力透镜宇宙学研究。

ABSTRACT

We want to derive bias free, accurate photometric redshifts for those fields of the CFHTLS-Wide data which are covered in the u*, g', r', i' and z' filters and are public on January 2008. These are 37 square degrees in the W1, W3 and W4 fields with photometric data for a total of 2.597.239 galaxies. We use the photometric redshift code PHOTO-z of Bender et al. (2001). We compare our redshifts for the W1, W3 and W4 fields to about 7500 spectroscopic redshifts from the VVDS therein. For galaxies with 17.5 <= i' AB <= 22.5 the accuracies and outlier rates become sigma=0.033, eta~2 % for W1, sigma=0.037, eta~2% for W3 and sigma=0.035, eta~2.5 % outliers for W4 fields. For the total galaxy sample with about 9000 spectroscopic redshifts from VVDS, DEEP2 or SDSS we obtain a sigma=0.04 and eta~5.7% for the PHOTO-z redshifts. We consider the photometric redshifts of Erben et al. (2008) which were obtained with exactly the same photometric catalog using the BPZ-redshift code and compare them with our computed redshifts. We also merge the subsample with good photometric redshifts from PHOTO-z with that one from BPZ to obtain a sample which then contains 'secure' redshifts according to both the PHOTO-z and the BPZ codes. This sample contains about 6100 spectra and the photometric redshift qualities become sigma=0.037 and eta~1.0% for our PHOTO-z redshifts. We conclude that this work provides a bias free, low dispersion photometric redshift catalog, that we have criteria at hand to select a 'robust' subsample with fewer outliers. Such a subsample is very useful to study the redshift dependent growth of the dark matter fluctuations with weak lensing cosmic shear analyses or to investigate the redshift dependent weak lensing signal behind clusters of galaxies in the framework of dark energy equation of state constraints.

研究动机与目标

• 利用多波段测光数据，为CFHTLS-Wide巡天区域推导无偏差、高精度的光度红移。
• 通过与VVDS、DEEP2和SDSS的光谱红移进行比较，评估光度红移的可靠性。
• 识别并标记因SED不匹配或测光误差过高而可能导致红移计算不准确的星系。
• 创建一个稳健、高质量的星系子样本，异常值极少，适用于高精度宇宙学分析。
• 比较PHOTO-z代码与BPZ代码的性能，并评估红移校正方法的影响。

提出的方法

• 将PHOTO-z光度红移代码（Bender et al. 2001）应用于CFHTLS-Wide巡天的u*,g',r',i',z'测光数据。
• 使用VVDS（7,500颗星系）、DEEP2和SDSS（9,000颗星系）的光谱红移作为验证参考。
• 对光度红移误差较高或SED易与其他红移产生混淆的星系进行标记。
• 将结果与Erben et al. (2008)提供的BPZ光度红移进行比较，包括使用校正公式：z_BPZ-corr = 1.29*z_BPZ - 0.13。
• 通过在PHOTO-z和BPZ代码中均具有可靠红移的星系，定义“共同样本”。
• 通过σΔz/(1+z)和异常值率η量化红移精度，采用裁剪方法去除极端异常值。

实验结果

研究问题

• RQ1 基于CFHTLS-Wide巡天的u*,g',r',i',z'滤镜组，使用PHOTO-z代码推导的光度红移精度与可靠性如何？
• RQ2 PHOTO-z得到的光度红移与光谱红移相比，在偏差、色散和异常值率方面表现如何？
• RQ3 能否通过测光误差和SED类型标准，识别出一个异常值极少的稳健星系子样本？
• RQ4 在相同数据集中，PHOTO-z代码与BPZ代码及其校正版本相比，性能如何？
• RQ5 在多个光度红移代码之间建立共同样本，能在多大程度上提升弱引力透镜应用中的红移质量？

主要发现

• 对于i′_AB在17.5 ≤ i′_AB ≤ 22.5范围内的星系，光度红移精度为：W1场σΔz/(1+z) = 0.033，W3场为0.037，W4场为0.035。
• 异常值率η在W1和W3场约为2%，W4场为2.5%，表明所有区域均具有高度可靠性。
• 对于约9,000个光谱红移的全样本，PHOTO-z的σΔz/(1+z) = 0.04，η = 5.7%。
• 稳健子样本（在PHOTO-z和BPZ中均被认定可靠）包含约6,100颗星系，σΔz/(1+z) = 0.037，η = 1.0%。
• 在两种代码间建立的共同样本，使PHOTO-z的异常值率降至1%，BPZ的异常值率降至1.5–1.8%，且色散性能得到改善。
• PHOTO-z星表无系统偏差，适用于星系团后方的弱引力透镜宇宙剪切与剪切比值检验。