Galactic Magnetism: Extended is the new Compact

磁场是星际介质(ISM)的四个主要组成部分之一，也就是“恒星之间的物质”。其他成分是气体、尘埃和宇宙射线(高能带电粒子)。磁场被认为对星系压力平衡、恒星形成，甚至对星系本身的形成都很重要。了解我们银河系(和其他星系)内磁场的结构为我们提供了重要的约束，使我们能够建立模型，说明银河系磁场最初是如何形成的，以及它是如何演变的。 然而，与ISM的其他组成部分不同，磁场不会发出任何形式的光。因此，除非它们位于足够近的空间区域，可以用磁强计(例如，旅行者1号和2号这样的航天器)进行现场测量，否则任何类型的望远镜都无法直接探测到磁场。这使得确定磁场的属性比确定其他ISM组件更具挑战性。 相反，我们利用了这样一个事实，即磁场可以改变它们存在时产生的光的性质，它们可以影响通过它们的光，主要是通过一个称为法拉第旋转的过程。这个过程旋转输入信号的偏振角的性质，取决于信号的波长，以及沿到光源的视线上的磁场和电子密度。如果我们知道一些关于电子密度的知识，我们就可以“反向工作”来确定产生我们测量的自转的磁场分布。 我们目前所知道的关于银河系磁场的大部分信息都是利用脉冲星(爆炸的恒星)和外部星系的光作为“垂直线”源来确定的；我们能看到的源越多，我们就能拼凑出越多的信息。就像从单个像素重建图像一样，目标一直是以最高密度观察尽可能多的源的法拉第旋转。根据这些数据，我们正在拼凑出银河系磁场的故事。 然而，许多问题仍然存在。我相信，在理解银河系磁性方面的下一大步，将源自对“延展发射”数据的研究。当光穿过银河系的磁场时，我们可以使用从磁场区域发射的(极化的)扩展发射光，而不是观察(极化的)光。这些数据有可能将银河系磁场的所有碎片拼凑在一起。我们在加拿大处于独特的地位，可以从事这项工作，拥有解决这些问题所需的设施和经验。通过这项工作，我们将推开我们对银河系磁性知识的界限，并确保加拿大在宇宙磁性研究方面的世界领先地位。

Magnetic fields are one of the four primary components of the interstellar medium (ISM), or, 'the stuff between the stars'. The other components are gas, dust, and cosmic rays (highly energetic charged particles). Magnetic fields are believed to be important in galactic pressure balance, star formation, and perhaps even in the formation of galaxies themselves. Understanding the structure of magnetic fields within our Galaxy (and others) provides us with important constraints on models addressing how the Galactic magnetic field originally formed and how it is evolving. However, unlike the other constituents of the ISM, magnetic fields do not give off any form of light. Consequently, unless they are located in a region of space close enough to be measured in situ with a magnetometer (e.g., by spacecraft like Voyager 1 and 2), magnetic fields cannot be directly detected with any type of telescope. This makes determining the properties of magnetic fields more challenging than for the other ISM components. Instead, we take advantage of the fact that magnetic fields can change the properties of light that is generated within their presence and they can affect light that passes through them, primarily through a process known as Faraday rotation.This process rotates the property of 'polarisation angle' of the incoming signal, and is dependent on the wavelength of the signal, as well as the magnetic field and electron density along the line-of-sight to the source. If we know something about the electron density, we can "work backwards" to determine what the magnetic field distribution must be to produce the rotation we measure. Most of what we currently know about the Galactic magnetic field has been determined using light from pulsars (exploded stars) and external galaxies as 'plumb-line' sources; the more sources we can look at, the more information we can piece together. Much like reconstructing an image from individual pixels, the goal has been to observe the Faraday rotation of as many sources as possible, at the highest density. From these data, we are piecing together the story of the Galactic magnetic field. However, many questions still remain. I believe the next big step forward in understanding Galactic magnetism will originate from studies using data from the 'extended emission'. Rather than looking at (polarised) light as it passes through the magnetic field of our Galaxy, we can use the (polarised) extended emission light that is being emitted from regions within the magnetic field. These data have the potential to bring all of the pieces of the Galactic magnetic field puzzle together. We are uniquely positioned in Canada to do this work, with the facilities and experience needed to address these questions. By doing this work, we will push back the boundaries of our knowledge of Galactic magnetism, and secure Canada's position as a world leader in the study of Cosmic magnetism.