宽边帽星系（M 104，NGC 4594）中心<100史瓦西半径的核射电喷流及其结构的证据
Evidence for nuclear radio jet and its structure down to <100 Schwarzschild radii in the center of the Sombrero galaxy (M 104, NGC 4594)
摘要：
摘要：我们使用高分辨率、准同步、多频率、相位参考VLBA观测研究了草帽星系核的详细射电结构。在1.4 ~ 43 GHz的7个频率上，我们以前所未有的灵敏度和分辨率获得了该核的VLBI图像，其中15、24和43 GHz是首次清晰的VLBI图像。在43 GHz下，原子核结构在100史瓦西半径下以线性标度成像。我们首次发现了该核中存在着从两侧射电核向西北/东南方向延伸的结构。原子核结构在射电光谱上显示出明显的空间梯度，这与通常在具有强大相对论喷流的更明亮活动星系核中看到的相似。此外，无线电核心示出了具有细长形状的频率相关尺寸，并且核心的位置也倾向于频率相关。一系列新发现提供了证据，证明Sombrero的中央引擎正在为无线电喷气机提供动力。根据观测到的喷流与反向喷流的亮度比、核心位置偏移及其与理论模型的比较，我们对M104喷流的以下基本物理参数进行了约束：（1）北侧是接近的喷流，而南侧是后退的喷流：（2）喷流的倾角相对接近我们的视线，可能小于~ 25度：（3）在小于~0.2c的速度范围内，射流的本征速度是高度亚相对论性的。M 104喷流的极-对性质与前面的论点一致，即M 104包含真正的II型活动星系核，即，如果假定的环核环面的平面垂直于射电喷流的轴，那么这个核的宽线区域实际上是不存在的，或者本质上是弱的。
英文摘要：
Abridged: We investigated the detailed radio structure of the nucleus of the Sombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency, phase-referencing VLBA observations. We obtained the VLBI images toward this nucleus, with unprecedented sensitivities and resolutions, at the seven frequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are the first clear VLBI detections. At 43 GHz, the nuclear structure was imaged on a linear scale under 100 Schwarzschild radii. For the first time, we have discovered the presence of the extended structure in this nucleus, which is directing from the radio core in two sides toward northwest/southeast directions. The nuclear structure shows a clear spatial gradient on the radio spectra, which is similar to that commonly seen in more luminous AGN with powerful relativistic jets. Moreover, the radio core shows a frequency-dependent size with an elongated shape, and the position of the core also tends to be frequency dependent. A set of these new findings provide evidence that the central engine of the Sombrero is powering radio jets. Based on the observed brightness ratio of jet-to-counter jet, core position shift and its comparison with a theoretical model, we constrained the following fundamental physical parameters for the M 104 jets: (1) the northern side is the approaching jet, whereas the southern side is receding: (2) the inclination angle of the jet is relatively close to our line-of-sight, probably less than ~25degrees: (3) the jet intrinsic velocity is highly sub-relativistic at a speed less than ~0.2c. The derived pole-on nature of the M 104 jet is in accordance with the previous argument that M 104 contains a true type II AGN, i.e., the broad line region of this nucleus is actually absent or intrinsically weak, if the plane of the presumed circumnuclear torus is perpendicular to the axis of the radio jets.
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