活动星系核外流的产生机制..

The Driven Mechanisms of Active Galactic Nuclei Outflows

越来越多不同波段的观测结果证实活动星系核外流的广泛存在,并揭示了它们的物理性质。研究活动星系核外流有两个目的:一方面有助于理解黑洞吸积的物理过程、活动星系核的结构;另一方面,这些外流对于活动星系核寄主星系的演化很可能起着关键性的作用。简要介绍了活动星系核外流的多波段观测结果,着重综述了目前主流的几个解释外流形成的理论模型,包括辐射驱动、磁场驱动以及热驱动。

In this paper, the main driven mechanisms of outflows in active galactic nuclei （AGN） are reviewed. The most famous evidences of AGN outflows are the broad absorption lines （BALs） observed in the ultraviolet （UV） spectra of quasars. The ＂warm absorber＂ observed in the soft X-ray band in Seyfert galaxy is another evidence. The warm absorber and UV BAL outflows have similar outflow properties, they may have the same origin. Recently, Tombesi et al. found the ultra-fast outflows （UFOs） in Fe absorption lines. The velocities of the UFOs are faster than 10 000 kin. s-1 and can be as high as 0.4c. There are three main kinds of driven mechanisms： thermal driven, radi-ation driven and magnetic driven. In the disk-wind picture, the gas at the outer region of accretion disk could be photo-ionization heated and Compton heated above to the local virial temperature, then thermal outflow occurs. At smaller radii, radiation force or magnetic force would dominate the forces that resist the gravity. If the gas is highly ionized and the luminosity approaches Eddington luminos- ity, Compton scattering would be important to drive outflow. If the gas is only moderately ionized, line absorption opacity becomes dominated and line-driven outflow appears. However, the radiation- driven mechanism only works for luminous quasars. The most cited magnetic-driven mechanism is the one raised by Blandford ＆ Payne. The gas material is first lifted out from the disk by local radiation force or buoyancy force, then the gas is frozen into the magnetic lines which anchor into the corona and corotate with the disk. The frozen gas would be threw out along the magnetic lines by centrifugal force. But a large scale open magnetic field is needed in this mechanism, and it can not determine the mass loss rate of the outflow. In the recent work of Yuan et al. （2012）, however, the large scale open magnetic field is not needed to produce outflows. Actually, the AGN outflows cannot be explained by just one driving mechanism. Future works will focus on the hybrid driven mechanism in which two or more mechanisms are involved. Based on a synthesis of multiwavelength research programs, Tombesi et al. （2013） proposed a stratified outflow model for Seyfert galaxies. The main idea is that the faster outflow with higher column density and higher ionization state launches at smaller radii, while the slower outflow with smaller column density and lower ionization state launches at larger radii. When the observer ob- serves along different line-of-sight, they will see different types of outflows. Outflow is an important component of AGN feedback. Especially the UFOs, the high kinetic luminosity contained in the outflow could affect the evolution of the host galaxies. The outflows produced several million years ago may be responsible for the formation of Fermi Bubble recently observed at the Galactic center.