A Faraday rotation search for magnetic fields in quasar damped Ly alpha absorption systems
A Faraday rotation search for magnetic fields in quasar damped Ly alpha absorption systems
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DOI：
10.1086/175726
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发表时间：
1995-06
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影响因子：
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通讯作者：
A. Oren;A. Wolfe
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作者：
A. Oren;A. Wolfe
我们给出了在国家射电天文台(NRAO)甚大阵列(VLA)对61个射电明亮类星体进行的法拉第旋转测量结果。估计并减去银河系对法拉第自转的贡献，以确定每个源的河外自转测量(RRM)。已知其中11个类星体表现出衰减的Lyα吸收。将这11个震源的显著法拉第旋发率与其余50个震源进行比较，发现在99.8%的置信度水平下，法拉第旋转的发生率更高。然而，由于这是基于在衰减的Lyα样品中只检测到两次法拉第旋转，所以结果只是试验性的。如果将受阻的Lyα样品中的两个探测器挖掘到吸收系统，则由这些系统引起的旋转量约为250rad/平方米。这两个探测器是针对样品中红移最低的两个吸收体。我们发现，在受阻的Lyα样品中，由于观察到的红移RRM的1/(1+2)平方稀释，对于任何其他吸收体，250rad/sm2的旋转量都是没有被检测到的。因此，这些数据与法拉第旋转是阻尼型Lyα吸波材料的一般性质的假设是一致的，但并不能证明这一假设。我们不能证实RRMS的幅度随红移而增加的说法。相反，这些数据与红移演化是一致的。我们发现，星系旋转量(GRM)估计的不确定性是一个比以往星系外法拉第旋转研究中认识到的更严重的问题。对目前估算GRM的方法的仔细分析表明，它可以被测量到大约15-20rad/平方米的精度。以前的研究将这种不确定度低估了2倍以上。由于这种不确定度，我们怀疑与受阻的Lyα吸收系统有关的旋转测量只能在小于z约等于1的红移时被探测到。
We present the results of a Faraday rotation survey of 61 radio-bright QSOs conducted at the National Radio Astronomy Observatory (NRAO) Very Large Array (VLA). The Galactic contribution to the Faraday rotation is estimated and subtracted to determine the extragalactic rotation measure (RRM) for each source. Eleven of these QSOs are known to exhibit damped Ly alpha absorption. The rate of incidence of significant Faraday rotation of these 11 sources is compared to the remaining 50 and is found to be higher at the 99.8% confidence level. However, as this is based upon only two detections of Faraday rotation in the damped Ly alpha sample, the result is only tentative. If the two detections in the damped Ly alpha sample are dug to the absorbing systems, then the inferred rotation measure induced by these systems is roughly 250 rad/sq m. The two detections were for the two lowest redshift absorbers in the sample. We find that a rotation measure of 250 rad/sq m would have gone undetected for any other absorber in the damped Ly alpha sample due to the 1/(1 + 2) squared dilution of the observed RRM with redshift. Thus the data are consistent with, but do not prove, the hypothesis that Faraday rotation is a generic property of damped Ly alpha absorbers. We do not confirm the suggestion that the amplitude of RRMs increases with redshift. Rather, the data are consistent with no redshift evolution. We find that the uncertainty in the estimation of the Galactic rotation measure (GRM) is a more serious problem than previously realized for extra-galactic Faraday rotation studies of QSO absorbers. A careful analysis of current methods for estimating GRM indicate that it can be determined to an accuracy of about 15 - 20 rad/sq m. Previous studies underestimated this uncertainty by more than a factor of 2. Due to this uncertainty, rotation measures such as we suspect are associated with damped Ly alpha absorption systems can only be detected at redshifts less than z approximately equal 1.