Study of X-ray emission from nearby normal galaxies using eROSITA All-Sky Survey data

星系的X射线观测使我们能够研究在恒星生命结束时形成的致密物体的数量以及弥漫的星际气体，这些气体主要由恒星风和超新星电离和加热。由于大多数恒星是在多个系统中形成的，致密天体通常有一颗伴星星星，致密天体从伴星中吸积物质。吸积中子星和黑洞是正常星系中最明亮的恒星X射线源，被称为X射线双星（XRB）。由于双星的演化和致密天体的形成依赖于恒星的星族分布，XRB是研究星系中恒星的星族分布和星星形成历史的理想示踪剂。大质量恒星和超新星的强烈星风在星际介质和被驱逐的恒星物质中产生强烈的激波，在星际介质中形成弥漫的热电离相。由于热的低密度等离子体的冷却效率非常低，它将持续很长一段时间，并且可以在整个星系上观察到漫射X射线。在过去的几十年中，X射线观测使我们能够限制正常星系中的发射分量。在邻近星系中，源的布居和热气体已被详细研究，而来自更远星系的积分辐射的研究表明，积分X射线光度L_X与恒星形成率（SFR）之间存在关系，也分别对XRB和漫射热气体存在关系。此外，L_X/SFR关系还依赖于金属丰度，从而也依赖于星系的年龄或距离。因此，对更远距离星系的X射线研究将有助于我们提高对宇宙中星系演化的理解。到目前为止，由于望远镜在较低X射线能量下的相对低灵敏度，对弥漫的热气体的研究只能在最近的星系中进行。由于eROSITA在低能量波段的高灵敏度及其全天空覆盖，我们现在拥有理想的X射线望远镜来研究正常星系完整样本的漫射X射线发射。在我们对极近星系（D < 10 Mpc）的漫射X射线辐射研究的基础上，我们将利用eROSITA全天巡天数据研究星系中漫射热气体的X射线辐射。首先，我们将研究D ~ 20 Mpc的星系中热扩散气体的辐射，从而可以分辨星系中的辐射。我们将使用基于XMM-牛顿或钱德拉研究非常接近的星系的结果的发射模型来分析光谱。在第二步中，我们将使用这些模型来研究更远星系在不同时间间隔（高达~200 Mpc）的叠加光谱以及X射线光度与SFR、距离或金属丰度之间的关系。在最后一步，我们还将对未来的X射线任务进行模拟，以估计类似研究的预期样本量和深度。

X-ray observations of galaxies allow us to study the population of compact objects formed at the end of the stellar life and the diffuse interstellar gas, which has been ionised and heated mainly by stellar winds and supernovae. As most of the stars are formed in multiple systems, compact objects often have a companion star, from which the compact object accretes matter. Accreting neutron stars and black holes are the brightest stellar X-ray sources in normal galaxies and are called X-ray binaries (XRBs). Since binary evolution and formation of compact objects depend on the underlying stellar population, XRBs are ideal tracers of stellar population and star formation histories in galaxies. Strong stellar winds of massive stars and supernovae cause strong shock waves in the interstellar medium (ISM) and the expelled stellar material, which will form the diffuse hot ionised phase in the ISM. Since the cooling of the hot low-density plasma is very inefficient, it will persist for a long time, and diffuse X-rays can be observed over the entire galaxy. X-ray observations in the last decades have allowed us to constrain the emission components in normal galaxies. In nearby galaxies, the source populations and the hot gas have been studied in detail, while studies of the integrated emission from further distant galaxies have shown that there is a relation between the integrated X-ray luminosity L_X and the star-formation rate (SFR), also separately for the XRBs and the diffuse hot gas. Moreover, the L_X/SFR relations also depend on the metallicity, and hence also on the age or on the distance of galaxies. Therefore, X-ray study of galaxies at higher distances will help us to improve our understanding of the evolution of galaxies in the Universe. The study of the diffuse hot gas has so far only been possible in the closest galaxies due to the relativity low sensitivity of the telescopes at lower X-ray energies. Thanks to eROSITA's high sensitivity in the lower-energy band and its all-sky coverage, we now have the ideal X-ray telescope to study the diffuse X-ray emission from a complete sample of normal galaxies. Based on our studies of the diffuse X-ray emission in very nearby galaxies (D < 10 Mpc), we will study the X-ray emission of the diffuse hot gas in galaxies using eROSITA All-Sky Survey data. First, we will study the emission of the hot diffuse gas in galaxies up to D ~ 20 Mpc, in which we can resolve the emission in the galaxies. We will analyse the spectrum using emission models based on the results of our XMM-Newton or Chandra study of the very nearby galaxies. In the second step, we will use the models to study the stacked spectra of further distant galaxies in different intervals (up to ~200 Mpc) and the relation between the X-ray luminosity and SFR, distance, or metallicity. In the last step, we will also perform simulations for future X-ray missions to estimate the expected sample size and depth of similar studies.