从完整的射电样本对z-alpha相关性的新认识
New Insights on the z-alpha Correlation from Complete Radio Samples
摘要：
观测到的射电光谱指数（alpha）和红移（z）之间的相关性一直被用作定位高z射电星系的方法。我们使用9个高度光谱完整的无线电样本，选择在不同的频率和流量限制，以确定这种方法的效率，并比较一贯观察到的α，光度，线性尺寸和红移之间的相关性。我们观察到z和alpha之间的弱相关性，即使去除Malmquist偏差也仍然存在。这种相关性的强度取决于k校正和样本选择频率，以及测量α的频率，只有当分析仅限于扩展射电星系时，才能看到高频和低频选择样本的一致结果。许多最高红移的射电星系都非常紧凑，并且经常显示出负弯曲或峰值频谱，因此低频射电频谱作为一个整体应该被研究;这是LOFAR至关重要的事情。我们量化的效率和完整性，用于选择高Z射电星系的各种技术。应用于低频选定样本的陡谱切割可以使高z源的分数增加一倍以上，但代价是排除了原始样本中存在的一半以上的高z源。角度大小切割是一种几乎与陡谱切割同样有效的方法，适用于高频和低频选定样本。在多波长数据中，首先选择红外微弱射电源仍然是迄今为止选择高Z源的最有效方法。我们提出了一个简单的方法来选择高Z射电星系，纯粹的基础上结合他们的无线电特性的α和角大小，与K波段的星等，如果可用。[删节]
英文摘要：
The existence of a correlation between observed radio spectral index (alpha) and redshift (z) has long been used as a method for locating high-z radio galaxies. We use 9 highly spectroscopically complete radio samples, selected at different frequencies and flux limits, to determine the efficiency of this method, and compare consistently observed correlations between alpha, luminosity, linear size, and redshift. We observe a weak correlation between z and alpha which remains even when Malmquist bias is removed. The strength of this correlation depends on both the k-correction and sample selection frequency, in addition to the frequency at which alpha is measured, and consistent results for both high and low frequency selected samples are only seen if analysis is restricted to just extended radio galaxies. Many of the highest redshift radio galaxies are very compact and often display a negatively curved or peaked spectrum, and therefore the low-frequency radio spectrum as a whole should be studied; this is something for which the LOFAR will be crucial. We quantify both the efficiency and the completeness of various techniques used to select high-z radio galaxies. A steep-spectrum cut applied to low-frequency selected samples can more than double the fraction of high-z sources, but at a cost of excluding over half of the high-z sources present in the original sample. An angular size cut is an almost as equally effective method as a steep-spectrum cut, and works for both high and low frequency selected samples. In multi-wavelength data, selection first of infrared-faint radio sources remains by far the most efficient method of selecting high-z sources. We present a simple method for selecting high-z radio galaxies, based purely on combining their radio properties of alpha and angular size, with the addition of the K-band magnitude if available.[abridged]
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