No strong radio absorption detected in the low-frequency spectra of radio-loud quasars at z > 5.6
No strong radio absorption detected in the low-frequency spectra of radio-loud quasars at z > 5.6
复制标题
在 z > 5.6 的射电强类星体的低频光谱中没有检测到强射电吸收
DOI：
10.1051/0004-6361/202347582
复制
发表时间：
2023
影响因子：
6.5
通讯作者：
Gloudemans A
中科院分区：
文献类型：
--
作者：
Gloudemans A
我们利用巨型米射电望远镜波段3、-4和-5观测（~ 300−1200 MHz），存档低频阵列（LOFAR; 144 MHz）和甚大阵列（VLA； 1.4和3 GHz）数据，展示了9颗高红移类星在5.6≤z≤6.6的低频射电频谱。我们样本中的五个类星体是最近发现的，它们代表了一些已知的低频红移最高的射电明亮类星体。通过对同步加速器自吸收（SSA）或自由-自由吸收（FFA）引起的光谱转换进行约束，建立了它们的射电光谱模型，研究了它们的射电发射机制和射电喷流的年龄。除了J0309+2717（一个atz= 6.1的耀变体），我们的类星体在144 MHz到几千兆赫之间没有显示出光谱变平的迹象，这表明在观测到的频率范围内没有强烈的SSA或FFA吸收。然而，我们发现光谱指数范围在- 1.6到0.05之间，包括3个潜在的超陡光谱类星体的发现。利用甚长基线阵列的进一步存档数据，我们确认耀变体J0309+2717的无线电频谱能量分布可能在0.6 - 2.3 GHz （90 - 330 MHz观测帧）的静帧频率上翻转，高频断裂表明无线电叶中电子种群的辐射老化。为了限制J0309+2717的吸收机制和样品中其他高z类星体的周转频率，需要低于50 MHz的超低频率数据。在低红移情况下，建立了线性射电射流尺寸与周转率之间的关系。如果这种关系在高红移时成立，那么我们样品的周转频率限制表明，射电喷流的尺寸必须比在类似红移时在其他射电明亮类星体中观察到的典型尺寸更大。为了证实这一点，需要高空间分辨率的深射电后续观测。
We present the low-frequency radio spectra of nine high-redshift quasars at 5.6 ≤z≤ 6.6 using the Giant Metre Radio Telescope band-3, -4, and -5 observations (∼300−1200 MHz), archival Low Frequency Array (LOFAR; 144 MHz), and Very Large Array (VLA; 1.4 and 3 GHz) data. Five of the quasars in our sample have been discovered recently, representing some of the highest redshift radio bright quasars known at low frequencies. We model their radio spectra to study their radio emission mechanism and age of the radio jets by constraining the spectral turnover caused by synchrotron self-absorption (SSA) or free-free absorption (FFA). Besides J0309+2717, a blazar atz= 6.1, our quasars show no sign of a spectral flattening between 144 MHz and a few gigahertz, indicating there is no strong SSA or FFA absorption in the observed frequency range. However, we find a wide range of spectral indices between −1.6 and 0.05, including the discovery of 3 potential ultra-steep spectrum quasars. Using further archival Very Long Baseline Array data, we confirm that the radio spectral energy distribution of the blazar J0309+2717 likely turns over at a rest-frame frequency of 0.6−2.3 GHz (90−330 MHz observed frame), with a high-frequency break indicative of radiative ageing of the electron population in the radio lobes. Ultra-low frequency data below 50 MHz are necessary to constrain the absorption mechanism for J0309+2717 and the turnover frequencies for the other high-zquasars in our sample. A relation between linear radio jet size and turnover frequency has been established at low redshifts. If this relation were to hold at high redshifts, the limits on the turnover frequency of our sample suggest the radio jet sizes must be more extended than the typical sizes observed in other radio-bright quasars at similar redshift. To confirm this, deep radio follow-up observations with high spatial resolution are required.