活动星系核FeⅡ发发射线的研究进展

Progress on FeⅡ Emission Lines of Active Galactic Nuclei

FeⅡ发射线是活动星系核光谱从紫外到光学波段的一个重要特征,其不仅与活动星系核的一些基本物理问题紧密相关,而且在宇宙学上有着重要的应用。虽然对FeⅡ发射线的观测和理论研究已有好几十年,但许多问题仍未有定论。主要对FeⅡ发射线近年的研究进展进行综述,包括其与本征向量Ⅰ的关系、它的起源、激发机制、发射区的运动学特征及在宇宙学中的应用等几方面,并指出了一些研究方向。

As an important signature of many active galactic nuclei(AGNs), the FeⅡ emission lines are presented in the AGN spectra from the ultraviolet to the near-infrared. They are not only closely related to several basic issues in AGN, but also have important implication in cosmology. In this paper, the research progress of FeⅡ emission lines in recent years is reviewed and some research directions that still under debate are pointed out, which can be summarized into the following points.First, there is evidence that the basic physical driver of the Eigenvector Ⅰ is the Eddington ratio(L/LEdd ), but why the FeⅡ strength increasing with L/LEdd is not well understood.Second, we review the location of FeⅡ emission region. Firstly, systematic investigation of the width of FeⅡ emission lines and other emission lines in Type Ⅰ quasars indicates their origin from the intermediate line region. Secondly, although the AGN samples suffer from bias, reverberation-mapping(RM) shows that the location of the FeⅡ emission region is hard to be distinguished from the Hβ-emitting gas in most of RM AGNs. Thirdly, systematic observations of the narrow optical FeⅡ-emitting gas indicate an origin of the innermost regions of the narrow-line region on physical scales smaller than the obscuring torus.Third, there are growing evidences that there are several mechanisms responsible for the excitation of FeⅡ emission, but it is not yet fully confirmed which one plays an important and even dominant role.Fourth, systematic redshift of the optical FeⅡ emission lines relative to either the local source rest frame or broad-line Hβ was found, which serves as evidence for inflow of FeⅡemission region. However, follow-up studies showed that this redshift may not be significant enough. Thus, it is still under debate whether the kinematics of FeⅡ emission region is virialized or dominated by inflow.Finally, we review some progresses in the cosmology implications of FeⅡ emission lines.By correcting the systematic dependence of the FeⅡ/MgⅡ intensity ratio on L/LEdd , it is reported that α-enrichment by mass loss from metal-poor intermediate-mass stars may occur at z ≈ 2 or earlier.