[论文解读] FIRST-based survey of Compact Steep Spectrum sources, III. MERLIN and VLBI observations of subarcsecond-scale objects

A. Marecki, M. Kunert‐Bajraszewska+1 more|ArXiv.org|Dec 1, 2005

Galaxies: Formation, Evolution, Phenomena参考文献 42被引用 9

一句话总结

本研究利用MERLIN、VLBA和EVN观测，调查了19个角直径小于1角秒的弱紧凑陡谱（CSS）射电源，发现许多源近期已停止活动，形成‘死亡’源。作者提出，提前终止的核活动可解释紧凑源的过量存在及其不对称形态，这是由于光传播时间效应所致，其中1123+340在星系团环境中展现出保存完好的遗迹结构。

ABSTRACT

According to a generally accepted paradigm, small intrinsic sizes of Compact Steep Spectrum (CSS) radio sources are a direct consequence of their youth, but in later stages of their evolution they are believed to become large-scale sources. However, this notion was established mainly for strong CSS sources. In this series of papers we test this paradigm on 60 weaker objects selected from the VLA FIRST survey. They have 5-GHz flux densities in the range 150 < S_{5GHz} < 550 mJy and steep spectra in the range 0.365 \le ν\le 5 GHz. The present paper is focused on sources that fulfill the above criteria and have angular sizes in the range 0.2\arcsec - 1\arcsec. Observations of 19 such sources were obtained using MERLIN in "snapshot" mode at 5 GHz. They are presented along with 1.7-GHz VLBA and 5-GHz EVN follow-up snapshot observations made for the majority of them. For one of the sources in this subsample, 1123+340, a full-track 5-GHz EVN observation was also carried out. This study provides an important element to the standard theory of CSS sources, namely that in a number of them the activity of their host galaxies probably switched off quite recently and their further growth has been stopped because of that. In the case of 1123+340, the relic of a compact "dead source" is particularly well preserved by the presence of intracluster medium of the putative cluster of galaxies surrounding it. The observed overabundance of compact sources can readily be explained in the framework of the scenario of "premature" cessation of the activity of the host galaxy nucleus. It could also explain the relatively low radio flux densities of many such sources and, in a few cases, their peculiar, asymmetric morphologies. We propose a new interpretation of such asymmetries based on the light-travel time argument.

研究动机与目标

• 测试60个较弱源（5-GHz流量密度为150–550 mJy）的紧凑陡谱（CSS）源年轻假说。
• 利用高分辨率射电干涉测量，研究19个亚角秒尺度CSS源的形态多样性与演化状态。
• 确定观测到的弱CSS源的紧凑性与不对称性是否源于AGN活动的近期停止，而非持续的年轻状态或环境阻碍。
• 评估光传播时间在产生原本对称但正在消亡的射电源中表观不对称性中的作用。
• 评估弱CSS源中缺乏FR I形态是否支持仅在更大尺度或不同条件下才会发生FR II到FR I的过渡。

提出的方法

• 对19个角直径在约0.2"至1"之间的弱CSS源进行了5-GHz MERLIN '快照'观测。
• 对大多数源进行了后续的1.7-GHz VLBA和5-GHz EVN观测，以在更高分辨率下探测喷流核与瓣状结构。
• 对源1123+340执行了完整的5-GHz EVN观测，以研究其详细结构与遗迹瓣状结构。
• 分析射电图像中的形态类型与不对称性，以推断演化状态与方向。
• 使用光传播时间论据，将表观瓣状结构不对称性解释为近期熄灭源中信号传播延迟的结果。
• 将源的性质与宇宙学模型（H₀ = 100 km s⁻¹ Mpc⁻¹，q₀ = 0.5）进行比较，并在红移未知时假设z = 1.25以估算线性尺寸。

实验结果

研究问题

• RQ1 角直径小于亚角秒的弱CSS源是否显示出近期活动停止的迹象，从而挑战所有源均为年轻且活跃的假设？
• RQ2 紧凑源瓣状结构中观测到的不对称性是否可由光传播时间效应解释，而非ISM或喷流功率的内在不对称性？
• RQ3 1123+340中的遗迹结构是否与‘死亡’源在致密星系团内介质中被保存的假设一致？
• RQ4 为何观测到的弱CSS源中均未出现FR I形态，这又对FR II到FR I的过渡意味着什么？
• RQ5 AGN活动的停止在多大程度上可解释巡天中紧凑源的过量存在？

主要发现

• 三个源（包括1123+340）显示出近期核活动停止的明确迹象，支持‘过早熄灭’的假设。
• 1123+340的遗迹瓣状结构保存完好，可能归因于潜在星系团内介质的约束，表明其处于致密且具有约束性的环境之中。
• 多个源中表观瓣状结构的不对称性被解释为光传播时间效应的结果，而非结构上的内在差异，时间尺度约为10⁵年。
• 年轻假说依然成立，但需关键补充：部分CSS源的AGN活动可能具有间歇性并过早结束，从而解释其低流量密度与紧凑尺寸。
• 样本中无任何源表现出FR I形态，表明此类结构可能仅在更大尺度或不同环境条件下形成。
• 观测到的形态多样性与本质上小而紧凑的源随机取向一致，束流效应相比ISM不对称性仅起次要作用。