利用合成观测星系评估从同步辐射得到的均分磁场强度的不可靠性
The fallibility of equipartition magnetic field strengths from synchrotron emission using synthetically observed galaxies
AI总结：
本文通过类银河系磁流体模拟的合成射电观测，检验宇宙射线与磁场能量均分假设，发现其无法再现射电轮廓并会显著高估磁场强度。
摘要：
要理解磁场在星系形成过程中所扮演的角色，就需要有推断河外磁场性质的准确方法。射电同步辐射一直是推断星系范围内磁场强度最有前景的途径，而其应用依赖一个核心假设：星系宇宙射线与磁场处于能量均分状态。在本研究中，我们利用对一个类银河系高分辨率磁流体动力学模拟进行的灵活合成观测，检验真正的均分是否能够再现星系的射电观测结果，并研究在改变宇宙射线性质和密度分布时，该假设对磁场强度推断的影响。我们发现，强加均分条件（无论尺度长度如何）所得出的宇宙射线电子密度，既无法产生旋涡星系中通常观测到的射电强度轮廓的幅值，也无法再现其形状。相反，基于观测动机设定的、宇宙射线电子在整个星系中的平滑分布，却能与观测结果显著吻合。我们进一步证明，对这些模拟观测假设均分，会导致对磁场强度的显著高估。这种误估随宇宙射线电子密度、宇宙射线能谱幂律指数和星系环境而变化，在旋臂间区域加剧，而在恒星形成区域减弱。我们的结果提醒人们在观测中采用均分假设时应保持谨慎，并表明需要更多理论和数值工作，以利用即将到来的新一代射电观测；这些观测有望彻底革新我们对天体物理磁场的理解。
英文摘要：
Understanding the role that magnetic fields play on the stage of galaxy formation requires accurate methods for inferring the properties of extragalactic magnetic fields. Radio synchrotron emission has been the most promising avenue to infer magnetic field strengths across galaxies, with the application of a central assumption: that galactic cosmic rays are in energy equipartition with the magnetic field. In this work, we leverage flexible synthetic observations of a high-resolution magnetohydrodynamic simulation of a Milky Way-like galaxy to review whether true equipartition is capable of reproducing radio observations of galaxies, and investigate its impact on the inference of magnetic field strengths when varying the properties and density distribution of the cosmic rays. We find that imposing equipartition (regardless of scale length) results in cosmic ray electron densities that are unable to generate either the amplitude or the shape of the radio intensity profiles typically observed in spiral galaxies. Instead, observationally motivated smooth distributions of cosmic ray electrons across the galaxy provide a remarkable match to observations. We further demonstrate that assuming equipartition with those mock observations can lead to significant overestimation of the magnetic field strength. This misestimation varies with cosmic ray electron densities, cosmic ray spectrum power-law index, and galactic environment, aggravated in inter-arm regions and attenuated in star-forming regions. Our results promote caution when assuming equipartition in observations, and suggest that additional theoretical and numerical work is required to leverage the upcoming generation of radio observations poised to revolutionize our understanding of astrophysical magnetic fields.
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