[论文解读] The ATESP 5 GHz radio survey.III 4.8, 8.6 and 19 GHz follow-up observations of radio galaxies

I. Prandoni, H. R. de Ruiter+4 more|arXiv (Cornell University)|Jan 1, 2010

Galaxies: Formation, Evolution, Phenomena参考文献 47被引用 16

一句话总结

本研究基于ATESP 5 GHz巡天中的26个亚毫毫斯蒂尔（sub-mJy）射电星系，利用澳大利亚望远镜紧凑阵列（ATCA）对4.8、8.6和19 GHz三个频段进行了准同时观测，旨在探究低光度活动星系核（AGN）的性质。射电谱表明这些系统以喷流主导，具有平坦或倒谱，排除了纯ADAF模型的可能性，但允许ADAF+喷流的场景。结果表明，这些源可能是FRII型射电星系的低光度对应体，其中部分紧凑、平坦谱源可能属于低功率BL Lacs或被遮挡的射电宁静类星体群体。

ABSTRACT

Aims. Physical and evolutionary properties of the sub-mJy radio population are not entirely known. The radio/optical analysis of the ATESP 5 GHz sample has revealed a significant class of compact flat/inverted radio-spectrum sources associated to early-type galaxies up to redshift 2. Such sources are most plausibly triggered by an AGN, but their observational properties are not entirely consistent with those of standard radio galaxy populations. In the present work we aim at a better understanding of the radio spectra of such sources and ultimately of the nature of AGNs at sub-mJy flux levels. In particular we are interested in assessing whether the AGN component of the sub-mJy population is more related to efficiently accreting systems – like radio-intermediate/quiet quasars – or to systems with low accretion rates – like e.g. FRI radio galaxies – or to low radiative efficiency accretion flows – like e.g. ADAF.
 Methods. We used the ATCA to get multi-frequency (4.8, 8.6 and 19 GHz) quasi-simultaneous observations for a representative sub-sample of ATESP radio sources associated with early-type galaxies (26 objects with S>0.6 mJy). This can give us insight into the accretion/radiative mechanism that is at work, since different regimes display different spectral signatures in the radio domain.
 Results. From the analysis of the radio spectra, we find that our sources are most probably jet-dominated systems. ADAF models are ruled out by the high frequency data, while ADAF+jet scenarios are still consistent with flat/moderately inverted-spectrum sources, but are not required to explain the data. We compared our sample with high ( GHz) frequency selected surveys, finding spectral properties very similar to the ones of much brighter (S>500 mJy) radio galaxies extracted from the Massardi et al.  (2008) sample. Linear sizes of ATESP 5 GHz sources associated with early type galaxies are also often consistent with the ones of brighter B2 and 3C radio galaxies, with possibly a very compact component that could be associated at least in part to (obscured) radio-quiet quasar-like objects and/or low power BL Lacs.

研究动机与目标

• 理解与早型星系相关的亚毫毫斯蒂尔射电源中，吸积与辐射机制的驱动机制。
• 确定亚毫毫斯蒂尔源中AGN组分是否与高吸积率系统（如射电中等/宁静类星体）相关，或与低吸积率系统（如FRI型射电星系或ADAF模型）相关。
• 评估低辐射效率吸积流（如ADAF/ADIOS）在塑造观测射电谱中的作用。
• 调查是否存在紧凑、核心主导型源，其可能代表被遮挡的射电宁静类星体或低功率BL Lacs。

提出的方法

• 对ATESP巡天中26个S > 0.6 mJy的代表性子样本源，使用澳大利亚望远镜紧凑阵列（ATCA）进行了4.8、8.6和19 GHz的准同时观测。
• 将新获得的数据与现有的1.4 GHz和5 GHz ATESP数据相结合，构建1.4–19 GHz频段的完整谱能分布。
• 通过分析射电谱指数（α，其中S ∝ ν^α）以区分光学薄（α ≈ -0.7）与光学厚（α > -0.5）的辐射成分。
• 将谱型与源大小与已知类群（B2、3C、射电宁静类星体）进行比较，以推断源的类型与演化状态。
• 评估观测谱与理论模型（纯ADAF、ADAF+喷流、喷流主导的同步辐射）的一致性。
• 利用未分辨源的线尺度上限，约束紧凑、核心主导型系统的存在。

实验结果

研究问题

• RQ1 ATESP样本中平坦/倒谱、紧凑的射电源是否与纯ADAF吸积模型一致？
• RQ2 多频射电谱是否支持观测辐射起源于喷流主导，而非热辐射或ADAF主导的辐射？
• RQ3 这些亚毫毫斯蒂尔源的谱型与尺寸特性是否与低光度FRII型射电星系或紧凑、被遮挡的类星体一致？
• RQ4 在紧凑、平坦谱源中是否存在低功率BL Lacs或射电宁静类星体群体的证据？
• RQ5 这些源的大小-光度关系与更明亮的射电星系（B2、3C）及射电宁静类星体相比如何？

主要发现

• 26个源的射电谱最合理的解释是喷流主导的辐射，平坦/倒谱表明来自喷流基底的光学厚发射，而更陡的谱则指示光学薄同步辐射。
• 纯ADAF模型被高频（19 GHz）数据排除，其显示的显著流量密度与ADAF辐射预期的陡峭、低频谱谱谱转折不符。
• ADAF+喷流情景在平坦/中等倒谱源中与数据一致，但并非解释观测所必需。
• ATESP源的谱特性与Massardi等人（2008）样本中更明亮（S > 500 mJy）的射电星系极为相似，表明ATESP源是FRII型射电星系的低光度对应体。
• 相当一部分源（d < 10 kpc）的尺寸与射电宁静类星体或低功率BL Lacs一致，尽管此类紧凑源数量较少（在有利倾角下不超过6%）。
• ATESP样本的大小-光度关系与B2和3C射电星系一致，表明FRII型大小-光度关系可外推至更低光度。