[论文解读] Effect of intervening Mg II systems on residual rotation measure of background QSOs

Sunil Malik, Hum Chand+1 more|arXiv (Cornell University)|Oct 28, 2017

Astronomical Observations and Instrumentation参考文献 3被引用 2

一句话总结

本研究分析了970个具有旋转测量（RM）和光学光谱的类星体，结果表明，前景Mg II吸收体显著增加了残差RM弥散（σ_rrm），表明高红移星系中存在磁化等离子体。额外标准差为40.57 ± 2.52 rad m⁻²，且与偏振的强负相关性表明，磁场强度与邻近星系中的磁场相当，可能呈随机取向。

ABSTRACT

Excess extragalactic contribution to residual rotation measure (RRM) for quasars for sightlines with intervening Mg II absorbers is a powerful tool to investigate the presence of magneto-active plasma and to estimate the strength of the magnetic field in high-redshift galaxies. We have compiled a large sample of 970 quasars for which we have RRM data as well as optical spectra to check for intervening Mg II absorbers. We found that the dispersion in RRM ($\sigma_{rrm}$) for 294 sightlines having Mg II intervening systems is 45.91$\pm2.04$ rad m$^{-2}$ as compared to its value of 21.47$\pm1.93$ rad m$^{-2}$ for the 676 sightlines. This result in an excess standard deviation ($\sigma_{rrm}^{ex}$) of 40.57$\pm2.52$ rad m$^{-2}$ among these two subsample. We have also found that the subset of sightlines with two Mg II absorbers have more $\sigma_{rrm}$ than the subset with one absorber, having values of 35.62$\pm4.63$ rad m$^{-2}$ and 28.28$\pm2.35$ rad m$^{-2}$, respectively. It is also noticed that there is an increasing trend in $\sigma_{rrm}$ for larger rest frame equivalent width ($EW_{r}$), having values 40.48$\pm 2.71$ rad m$^{-2}$ and 55.53$\pm 3.18$ rad m$^{-2}$ for $EW_{r}<$1\AA and $EW_{r}\ge$1\AA, respectively. Additionally, we found a strong anti-correlation between $\sigma_{rrm}$ and fractional polarisation ($p$) with the Pearson correlation coefficient ($ ho_{p}$) of $-0.78$ for sightlines with Mg II absorbers. This gives support to the hypothesis that intervening Mg II absorbers do have magnetic field of sufficient strength, and perhaps oriented in random directions, leading to a scatter in RRM and hence also causing a reduction in the resultant fractional polarization. All these observational evidences, allow us to infer the presence of magnetised plasma in intervening high redshift galaxies with strengths similar to that in nearby galaxies.

研究动机与目标

• 调查前景Mg II吸收体是否对背景类星体的残差旋转测量（RRM）产生额外弥散的影响。
• 利用RRM弥散作为探针，评估高红移星系中磁场的存在与强度。
• 研究Mg II吸收体特性（如等效宽度、系统数量）与RRM散射之间的关系。
• 探讨磁场对类星视线中偏振分数的影响。

提出的方法

• 收集了970个具有测量残差旋转测量（RRM）和光学光谱的类星体样本，以识别前景Mg II吸收体。
• 计算了存在与不存在Mg II吸收体的视线的RRM弥散（σ_rrm），以量化额外散射。
• 根据Mg II系统数量（一个与两个吸收体）和本征系等效宽度（EW_r <1 Å 与 ≥1 Å）对视线进行细分，以评估趋势。
• 计算RRM与偏振分数（p）之间的皮尔逊相关系数（ρ_p），以检验磁场的影响。
• 使用统计误差传播方法估算σ_rrm和额外标准差（σ_rrm^ex）的不确定性。

实验结果

研究问题

• RQ1 前景Mg II吸收体是否导致类星视线中残差RM弥散增加？
• RQ2 在存在Mg II吸收体的高红移星系中，是否存在可测量的由磁场引起的RRM弥散额外部分？
• RQ3 沿视线的Mg II吸收体数量如何影响RRM散射？
• RQ4 Mg II吸收体的本征系等效宽度是否与RRM弥散相关？
• RQ5 是否存在可检测的RRM散射与偏振分数之间的负相关性，表明磁场的影响？

主要发现

• 具有Mg II吸收体的视线的RRM弥散（σ_rrm = 45.91 ± 2.04 rad m⁻²）显著高于无Mg II吸收体的视线（21.47 ± 1.93 rad m⁻²）。
• 由Mg II吸收体引起的额外标准差（σ_rrm^ex）为40.57 ± 2.52 rad m⁻²，表明磁化等离子体有显著贡献。
• 具有两个Mg II吸收体的视线表现出更高的RRM弥散（35.62 ± 4.63 rad m⁻²），高于仅有一个吸收体的视线（28.28 ± 2.35 rad m⁻²）。
• RRM弥散随本征系等效宽度增加而上升，当EW_r ≥ 1 Å时达到55.53 ± 3.18 rad m⁻²，而EW_r < 1 Å时为40.48 ± 2.71 rad m⁻²。
• σ_rrm与偏振分数之间发现强负相关性（ρ_p = -0.78），支持随机磁场在散射RRM并降低偏振中的作用。
• 观测到的RRM与偏振趋势共同表明，前景高红移星系中存在磁化等离子体，其磁场强度与邻近星系中的磁场相当。