双活动星系核寄主星系的光谱合成分析

doi:10.3964/j.issn.1000-0593(2020)01-0290-04

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Abstract The merger of massive galaxies can also invoke a series of phenomena such as the starburst and SMBH activities, besides galaxies pairing and dual AGNs. When two galaxies merger to kiloparsec (kpc) scale and begin to co-rotate with each other, and the adjacent narrow line regions are sensed by a single spectrograph slit or fiber, double-peaked emission lines would appear in the integrated spectra. In this paper, starting from this observation characteristic, we systematically search for dual AGN candidates whose spectra display double-peaked narrow emission line features in the Data Release 4 of LAMOST survey (LAMOST DR4). The profiles of emission lines of AGN spectra are composed of several different dynamic components, which can be divided into three types including narrow line components (Hβ, [OⅢ], Hα and [NⅡ]), wings of [OⅢ] lines, and broad Balmer emission lines. Based on LAMOST DR4 extragalactic spectra, we apply a set of search process, after the initial screening (involving the signal-to-noise ratio, the equivalent width and redshift cuts) and visual check and selection, we establish a sample with double-peaked narrow emission lines, which meets with our criteria of fluxes, full-width-half-maximum and velocity gap between two narrow components in their multi-gaussians fitting. Based on the Baldwin-Phillips-Terlevich (BPT) diagnosis, we finally detect 28 dual AGN candidates. In order to obtain more accurate and find the common characteristics of this kind of dual AGN candidates, we re-correct the spectral flux of these 28 objects with a low-order polynomial and combine them with the traditional interpolation and median method to obtain a high signal-to-noise ratio spectrum. We also take a composite spectra constructed from single type Ⅱ AGNs in LAMOST DR4 as the control sample. The software STARLIGHT is carried out to fit these two spectra, and the contributions of stars of different ages and metallicities are quantified for more direct comprehension. We find that dual AGN candidates will host more contributions from the middle age and the old populations than the single AGN composite, indicating a more intense central black hole activity. As for the metallicity effects, the dominant stellar population of single AGN is the population with solar metallicity Z_⊙, while the spectra from dual AGN candidates show a significant contribution from populations with metallicities 0.2 Z_⊙ and 2.5 Z_⊙, showing a heterogeneous feature and revealing a more complicated star formation history encodes in these possible dual AGNs than it exists in corresponding regular sample. A power-law is considered when we conduct the fitting, and the contributions of this featureless continuum component in single AGN account for 8.2%, which is significantly higher than that in the dual AGN candidates.

Key words：LAMOST; Dual AGNs; Spectral synthesis; Stellar population analysis

Received: 2018-11-04 Accepted: 2019-03-20

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)01-0290-04 OR https://www.gpxygpfx.com/EN/Y2020/V40/I01/290