标题： 限制z>5.5处类星体的射电明亮分数 显示英文标题

标题： Constraining the radio-loud fraction of quasars at z>5.5

摘要： 射电明亮的活动星系核在z~2-4范围内通常位于密集环境中，其宿主星系是这些红移下最巨大的系统，为星系演化提供了关键见解。在最高可到达的红移（z~6）处发现射电明亮的类星体对于研究它们在更早宇宙时期的特点和环境非常重要。它们还将作为射电调查的背景源，用于研究再电离时代之后的星系际介质中的HI 21 cm吸收。目前，只有五个射电明亮（$R=f_{\nu,5{\rm GHz}}/f_{\nu,4400\AA}>10$）的类星体在z~6范围内已知。在本文中，我们通过交叉匹配光学Pan-STARRS1和射电FIRST调查来寻找5.5 < z < 7.2的类星体。射电信息有助于识别被典型颜色选择方法遗漏的类星体。虽然我们在这些调查的灵敏度下没有找到6.4 < z < 7.2的好的类星体候选者，但我们发现了两个新的z~6的射电明亮类星体。此外，我们还发现了两个之前未被认定为射电明亮的z~6射电明亮类星体，几乎使当前的z~6样本翻倍。我们展示了对于z>5.5的类星体来说，拥有红外光度测量的重要性，以稳健地将它们分类为射电安静或射电明亮。基于此，我们将先前被认为是射电明亮的类星体J0203+0012（z=5.72）重新分类为射电安静。利用文献中的可用数据，我们使用Kaplan--Meier估计器限制了z~6时类星体的射电明亮比例，为$8.1^{+5.0}_{-3.2}\%$。这一结果与射电明亮比例随红移没有变化一致，这与一些低红移研究提出的结论相反。 显示英文摘要

摘要： Radio-loud Active Galactic Nuclei at z~2-4 are typically located in dense environments and their host galaxies are among the most massive systems at those redshifts, providing key insights for galaxy evolution. Finding radio-loud quasars at the highest accessible redshifts (z~6) is important to study their properties and environments at even earlier cosmic time. They would also serve as background sources for radio surveys intended to study the intergalactic medium beyond the epoch of reionization in HI 21 cm absorption. Currently, only five radio-loud ($R=f_{\nu,5{\rm GHz}}/f_{\nu,4400\AA}>10$) quasars are known at z~6. In this paper we search for 5.5 < z < 7.2 quasars by cross-matching the optical Pan-STARRS1 and radio FIRST surveys. The radio information allows identification of quasars missed by typical color-based selections. While we find no good 6.4 < z <7.2 quasar candidates at the sensitivities of these surveys, we discover two new radio-loud quasars at z~6. Furthermore, we identify two additional z~6 radio-loud quasars which were not previously known to be radio-loud, nearly doubling the current z~6 sample. We show the importance of having infrared photometry for z>5.5 quasars to robustly classify them as radio-quiet or radio-loud. Based on this, we reclassify the quasar J0203+0012 (z=5.72), previously considered radio-loud, to be radio-quiet. Using the available data in the literature, we constrain the radio-loud fraction of quasars at z~6, using the Kaplan--Meier estimator, to be $8.1^{+5.0}_{-3.2}\%$. This result is consistent with there being no evolution of the radio-loud fraction with redshift, in contrast to what has been suggested by some studies at lower redshifts.