[论文解读] Wide-field LOFAR imaging of the field around the double-double radio galaxy B1834+620: A fresh view on a restarted AGN and doubeltjes

E. Orrú, Sjoert van Velzen+9 more|UvA-DARE (University of Amsterdam)|Oct 2, 2015

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy参考文献 52被引用 21

一句话总结

本研究首次利用LOFAR在144 MHz波段对双重重叠射电星系B1834+620进行了高分辨率成像，解析了其四个瓣状结构，并揭示了与内瓣活动相关的低频发射特征。谱线分析支持了活动星系核重启模型，且LOFAR探测到的紧凑双瓣源（doubeltjes）数量是1.4 GHz波段的10倍，证明其在研究活动星系核占空比及高红移射电源方面的强大能力。

ABSTRACT

The existence of double-double radio galaxies (DDRGs) is evidence for recurrent jet activity in AGN, as expected from standard accretion models. A detailed study of these rare sources provides new perspectives for investigating the AGN duty cycle, AGN-galaxy feedback, and accretion mechanisms. Large catalogues of radio sources provide statistical information about the evolution of the radio-loud AGN population out to high redshifts. Using wide-field imaging with the LOFAR telescope, we study both a well-known DDRG as well as a large number of radio sources in the field of view. We present a high resolution image of the DDRG B1834+620 obtained at 144 MHz using LOFAR commissioning data. Our image covers about 100 square degrees and contains over 1000 sources. The four components of the DDRG B1834+620 have been resolved for the first time at 144 MHz. Inner lobes were found to point towards the direction of the outer lobes, unlike standard FR~II sources. Polarized emission was detected in the northern outer lobe. The high spatial resolution allows the identification of a large number of small double-lobed radio sources; roughly 10% of all sources in the field are doubles with a separation smaller than 1 arcmin. The spectral fit of the four components is consistent with a scenario in which the outer lobes are still active or the jets recently switched off, while emission of the inner lobes is the result of a mix-up of new and old jet activity. From the presence of the newly extended features in the inner lobes of the DDRG, we can infer that the mechanism responsible for their formation is the bow shock that is driven by the newly launched jet. We find that the density of the small doubles exceeds the density of FR-II sources with similar properties at 1.4 GHz, but this difference becomes smaller for low flux densities.

研究动机与目标

• 使用LOFAR在低频射电波段（144 MHz）对双重重叠射电星系B1834+620进行成像，以解析其四瓣结构。
• 研究内瓣与外瓣的谱线特性，以检验活动星系核重启及喷流活动周期性重现的模型。
• 量化低频下紧凑双瓣射电源（doubeltjes）的面积密度，并与1.4 GHz巡天结果进行比较。
• 评估LOFAR在更高分辨率和灵敏度下探测高红移FR II型射电源的能力。
• 探讨LOFAR与FIRST巡天在源密度上存在的计数差异对类星体演化及光度函数的影响。

提出的方法

• 利用LOFAR低频天线和高灵敏度波束成形技术，对B1834+620进行大视场、高动态范围的144 MHz波段成像。
• 采用Compton-Ida（CI）模型拟合从144 MHz到8.5 GHz频段的四个瓣状结构的射电谱，进行谱指数分析。
• 测量法拉第旋转和偏振发射，以评估磁场结构与辐射机制。
• 结合Hopkins等人（2007）提出的全波段类星体光度函数与光学-射电相关性，模拟144 MHz与1.4 GHz波段的FR II型源计数。
• 计算LOFAR与FIRST巡天在FR II型源计数上的预测比值，以检验源密度预测是否存在差异。
• 应用来自FIRST巡天的谱指数α = -0.85，将1.4 GHz波段的瓣状结构光度外推至144 MHz，实现不依赖绝对归一化的通量密度预测。

实验结果

研究问题

• RQ1 双重重叠射电星系B1834+620在低频射电波段的形态如何？与高频图像相比有何不同？
• RQ2 内瓣与外瓣的谱指数是否支持近期活动星系核重启或持续喷流活动的模型？
• RQ3 为何LOFAR与FIRST巡天观测到的紧凑双瓣源（doubeltjes）源密度比标准类星体光度函数预测值更高？
• RQ4 LOFAR是否能比1.4 GHz现有巡天更高效地解析并探测高红移FR II型射电源？
• RQ5 瓣状结构的谱线与偏振特性揭示了重启活动星系核中电子分布与磁场结构的何种信息？

主要发现

• LOFAR在144 MHz波段成功解析了B1834+620的四瓣结构，揭示了在高频波段未见的内瓣延伸发射。
• 外瓣的谱指数（α_inj ≈ -0.85）与CI模型及先前研究一致，支持喷流活动仍在持续或近期已停止。
• 内瓣的谱线显示了新旧喷流活动混合的辐射特征，表明其谱线演化过程复杂。
• LOFAR探测到的紧凑双瓣源（doubeltjes）数量是1.4 GHz波段的10倍，单位面积源密度超过1.4 GHz波段在相同通量极限下的10倍。
• 观测到的LOFAR与FIRST巡天FR II型源计数比值比模型预测高出约50%，表明可能存在更多来自中等光度类星体的FR II型源，或在低光度下类星体占空比更短。
• 偏振发射测量得到的法拉第旋转量为+60 rad m⁻²，证实了LOFAR在低频波段探测偏振活动星系核发射的能力。