Low frequency study of two clusters of galaxies: A2744 and A2219
Low frequency study of two clusters of galaxies: A2744 and A2219
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两个星系团的低频研究：A2744 和 A2219
DOI：
10.1051/0004-6361:20066118
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发表时间：
2007
影响因子：
6.5
通讯作者：
Inaf Trieste
中科院分区：
文献类型：
--
作者：
E. Orrú;M. Murgia;L. Feretti;F. Govoni;G. Brunetti;G. Giovannini;M. Girardi;G. B. M. D. D. Fisica;U. Cagliari;I. Cagliari;I. D. Radioastronomia;D. A. U. Bologna;D. Astronomia;U. Trieste;Inaf Trieste
目标。光谱指数图像可以用来约束射电晕和遗迹中相对论性电子的能谱和磁场分布，为了解它们的形成、演化以及与星团合并过程的联系提供有用的信息。方法。我们展示了两个星系团的低频图像：A2744和A2219，其中检测到宽漫射辐射。在325兆赫的频率上用甚大阵列进行观测。对于这两个星团，深甚大阵列1.4 GHz观测是可用的。结合325 MHz和1.4 GHz波段的数据，得到了分辨率为1´的漫射辐射的光谱指数图像和亮度径向分布。结果。在距离星团中心1 Mpc的范围内，A2744的方位角平均光谱指数一直保持在接近$\alpha\simeq$ 1的水平。然而，光谱指数图像显示存在局部区域，其中无线电频谱与平均值显着不同。观测到的光谱指数变化范围从最小的$\alpha \simeq$ 0.7±0.1到最大的$\alpha \simeq$ 1.5±0.2。通过光谱指数与x射线数据的比较，首次发现射电晕的平谱区有较高的温度。在A2219的情况下，星团中心区域的射电辐射主要是由一系列离散源混合而成。星团中心区域的方位平均无线电频谱为$\alpha\simeq$ 0.8，而射电晕则接近$\alpha\simeq$ 1。325mhz图像的有限灵敏度不允许我们检测到1.4 GHz的所有射电晕结构，因此没有对该星团的光谱指数点对点变化的约束。
Aims. Spectral index images can be used to constraint the energy spectrum of relativistic electrons and magnetic field distribution in radio halos and relics, providing useful information to understand their formation, evolution and connection to cluster merger processes. Methods. We present low-frequency images of the two clusters of galaxies: A2744 and A2219, in which a wide diffuse emission is detected. Observations were made with the Very Large Array at the frequency of 325 MHz. For both clusters deep Very Large Array 1.4 GHz observations are available. Combining the 325 MHz and 1.4 GHz data, we obtained the spectral index images and the brightness radial profiles of the diffuse radio emission with a resolution of ~1´. Results. The azimuthally averaged spectral index in A2744 is constant to a value close to $\alpha\simeq$ 1 up to a distance of 1 Mpc from the cluster center. However, the spectral index image shows the presence of localized regions in which the radio spectrum is significantly different from the average. The observed spectral index variations range from a minimum of $\alpha \simeq$ 0.7 ± 0.1 to a maximum $\alpha \simeq$ 1.5 ± 0.2. From the comparison of the spectral index with the X-ray data it is found for the first time that the flat spectrum regions of the radio halo tend to have higher temperature. In the case of A2219, the radio emission in the central regions of the cluster is dominated by a blend of discrete sources. The azimuthally averaged radio spectrum is $\alpha\simeq$ 0.8 in the central region of the cluster and is close to $\alpha\simeq$ 1 in the radio halo. The limited sensitivity of the 325 MHz image does not allow us to detect all the radio halo structure seen at 1.4 GHz and therefore no constraints on the point-to-point variations of the spectral index have been obtained for this cluster.