星系团是目前宇宙中最大的引力束缚系统，星系团结构与演化以及动力学状态的研 究在宇宙大尺度结构、星系演化等领域具有承上启下的作用。“冷核”问题是星系团研究中 一个热门问题。冷核是指在一部分X射线星系团中出现的低温高亮的核状结构。冷核星系 团在X射线表面亮度轮廓、温度轮廓、中心冷却时间、质量沉积率等多个方面具有与非冷 核星系团截然不同的观测特征。但是冷核的产生机制和维持条件以及观测判据等均未有 明确答案。

由于星系团演化过程中的并合作用会破坏星系团的原有结构，同时改变星系团内物 质的动力学状态，因此冷核与非冷核星系团的核心区ICM应该具有不同的动力学特征，这 也可以成为冷核的观测判据之一。我们试图通过确定ICM的动力学状态来探究其与星系团 结构和演化之间的关系。

本文首先介绍了一套利用X射线红移测量技术探究ICM视向运动状态的方法。基 于Chandra望远镜角秒量级的空间分辨率，我们可以分别测量星系团核心区不同区 域ICM的X射线红移，并通过红移分布来判断是否具有视向的团块运动。通过模拟并拟 合Chandra的X射线光谱，我们确定了一套完整的光谱拟合方法，该方法可以最大限度地 降低ICM多温成分对红移测量带来的影响，并且准确地限制X射线红移的误差范围。

我们将此方法应用于子弹星系团。结果表明，子弹星系团的ICM在2σ置信水平上存 在视向的团块运动，在“子弹”经过的一段路径中存在(60 ± 17) km/s/kpc的视向速度梯度。 由于子弹星系团的并合方向几乎平行于视平面，因此我们认为该运动是剧烈的并合作用 向视线方向高速挤压出一部分ICM所致。

随后，我们进一步利用此方法探究了一个星系团样本的ICM动力学状态。该样本包 含3颗典型的冷核星系团和3颗有并合过程的非冷核星系团。我们最终发现，冷核星系团 存在ICM视向团块运动的置信度仅为40 − 50%，而非冷核星系团均超过了90%。该结果说 明冷核星系团与非冷核星系团具有不同的ICM动力学状态；另一方面，该结果表明星系团 的并合作用在另一个方向上也能给ICM的动力学状态带来显著的影响，这为我们大范围搜 寻并合星系团，并由此探究星系团的演化过程提供了一种可靠的途径；该结果同时也验 证了我们的方法用于探究ICM动力学状态的可靠性。

Clusters of galaxies are the largest virialized systems in the Universe. Since clusters mark the transition between cosmic structure formation and the evolution of cosmic baryons, they occupy a pivotal position in astrophysics. The structure and evolution of clusters are very useful in cosmology and science of galaxies. Among the topics on clusters, the “cool core” problem is very popular one. Cool cores are common structures in the center of X-ray luminous clusters. The dense intra-cluster medium in cool cores emits very strong X-ray radiation through bremsstrahlung, thus forms a sharply peaked X-ray surface brightness distribution, while the temperature is lower than the virial temperature. The energy loss is so fast that the cooling time of a cool core can be shorter than the age of the cluster. These phenomena always indicate the existence of a cool core. So far there are many unknowns about “cool core”, including a lack of clear definition of a cool-core cluster.

Since merging processes between sub-halos may destroy the primordial structure of a cluster, and strongly affect the dynamic status of ICM, cool-core and non cool-core clusters should have significantly different dynamic status, which could also be a criteria of cool-cores. Thus we decide to study the dynamic status of ICM and investigate its relation between cluster structure and evolution.

This thesis starts with a strategy to search for bulk motions in the Intra Cluster Medium (ICM) of merging clusters based on Chandra CCD data. By fitting simulated Chandra X-ray spectra, we set up an appropriate strategy of spectra analysis. We thoroughly explore the effect of the unknown temperature structure along the line of sight to accurately evaluate the systematic uncertainties on the ICM redshift, which add to the statistical errors directly measured from the spectral fits.

We first apply this method to the “Bullet cluster”(1E 0657-56), which is a famous merging cluster at z=0.3. 56). We identify 23 independent regions directly on the basis of the surface brightness contours, and measure the redshift of the ICM averaged along the line of sight in each of them. We find that the redshift distribution across these regions is statistically inconsistent with the null hypothesis of a constant redshift, or no bulk motion in the ICM, with a confidence level of about 2σ. We tentatively identify the regions most likely affected by bulk motions along the direction of the merging, and find a maximum velocity gradient of about (60 ± 17) km/s/kpc along the line of sight on a scale of ∼260 kpc along the path of the “bullet”, which we interpret as the signature of significant mass of ICM pushed away along a direction perpendicular to the merging.

We then apply the method to a sample of galaxy clusters, including 3 typical cool-core clusters and 3 non cool-core clusters who are undergoing merging process. The results show that ICM in cool-core clusters only host bulk motions at 40%-50% confidence level, while in non cool-core clusters, there are bulk motions in the ICM at a confidence level of 90%. This result indicates that ICM in cool-core and non cool-core clusters have different dynamic status, and that merging in the plane of the sky can also bring prominent bulk motions in another direction. Moreover, this result verifies the reliability of our method to search for bulk motion in the ICM.