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arXiv 0704.2074astro-ph
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Sloan数字巡天低射电光度AGN中的恒星形成

Star-Formation in Low Radio Luminosity AGN from the Sloan Digital Sky Survey

W. H. de Vries, J. A. Hodge, R. H. Becker, R. L. White, D. J. Helfand

摘要:

我们研究了从斯隆数字巡天(SDSS)中选出的低光度到高光度活动银河核(AGN)的微弱射电发射。它们的无线电特性是通过从第一次调查中添加大量无线电图像剪切来推断的,因为几乎所有的源都是单独未检测到的。我们将中值射电通量密度与一系列其他样品性质相关联,包括红移中值、[OIII]光度、发射线比和4000 A断裂强度。我们探测到一个强烈的趋势,即那些正在积极进行恒星形成的源具有超过~10^28尔格/s/Hz水平的过量无线电发射,而这些源没有任何可辨别的恒星形成。此外,这种额外的无线电发射与光谱中4000 A的强度有很好的相关性,可以用来评估恒星形成成分的年龄。我们检查两个子样本,一个包含系统的发射线比最像恒星形成的系统,和一个具有特征活动星系核比的源。这一划分也区分了负责射电发射的机制(恒星形成与活动星系核)。对于这两种情况下,我们发现一个强大的,几乎相同的,[OIII]和无线电光度之间的相关性,与AGN的样品向低延伸,和恒星形成的样品向高光度。两个子样本之间的一个更清晰的分离被看作是宿主星系的中心速度色散的函数。对于具有类似红移和速度色散的系统,恒星形成子样本比射电中的活动星系核亮一个数量级。这强调了恒星形成系统中的射电辐射可以主导与活动星系核相关的辐射的概念。

英文摘要:

We investigate faint radio emission from low- to high-luminosity Active Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey (SDSS). Their radio properties are inferred by co-adding large ensembles of radio image cut-outs from the FIRST survey, as almost all of the sources are individually undetected. We correlate the median radio flux densities against a range of other sample properties, including median values for redshift, [OIII] luminosity, emission line ratios, and the strength of the 4000A break. We detect a strong trend for sources that are actively undergoing star-formation to have excess radio emission beyond the ~10^28 ergs/s/Hz level found for sources without any discernible star-formation. Furthermore, this additional radio emission correlates well with the strength of the 4000A break in the optical spectrum, and may be used to assess the age of the star-forming component. We examine two subsamples, one containing the systems with emission line ratios most like star-forming systems, and one with the sources that have characteristic AGN ratios. This division also separates the mechanism responsible for the radio emission (star-formation vs. AGN). For both cases we find a strong, almost identical, correlation between [OIII] and radio luminosity, with the AGN sample extending toward lower, and the star-formation sample toward higher luminosities. A clearer separation between the two subsamples is seen as function of the central velocity dispersion of the host galaxy. For systems with similar redshifts and velocity dispersions, the star-formation subsample is brighter than the AGN in the radio by an order of magnitude. This underlines the notion that the radio emission in star-forming systems can dominate the emission associated with the AGN.

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