Binary supermassive black holes (SMBHs) are natural outcomes of galaxy mergers, since almost all massive galaxies are believed to host a central SMBH. In the gas-rich case, the strong tidal interactions caused by galaxy mergers can trigger the active galactic nucleus (AGN) by sending a large amount of gas to the central regions. A dual AGN could emerge if the two merging SMBHs are both simultaneously accreting gas in a gas-rich major merger. Finding dual AGNs, especially those exhibiting two black holes on a kiloparsec scale, will provide important clues to understand the relation between the AGN activity and the galaxy evolution and AGN physics.

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Over the past few years, hundreds of dual AGNs of separations greater than 10 kpc have been discovered. However, such systems represent only the earliest stage of the binary SMBH evolution. No more than few dozens of close dual AGNs (separations between 1 and 15 kpc) are found both from either systematic searches or by serendipitous discoveries. However, most of them only show two prominent nuclei either in the radio, X-ray or the optical band, the properties of the host galaxies are poorly known. The latter are extremely important for probing the relation between the galaxy evolution and nucleus activity.

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The previous systematic search for dual AGNs is largely based on the follow-up of the double-peaked AGN sample elected from the SDSS data. However, most of the double-peaked AGNs are originated from complicate mechanisms (e.g. outflow, rotating disk/BLR/NLR) rather than dual AGNs. Thus most of the current systematic searches are in very low efficiency on the level of few per cents. To overcome this problem, we have developed an innovative method of systematically searching and identifying dual-AGN systems amongst kilo-parsec scale merging galaxies. In doing so, we first selects galaxies that are potentially undergoing a merging process from the SDSS imaging survey – galaxies exhibiting two optical cores separated by less than 8 arcseconds. We then cross-matches the selected galaxies with the FIRST radio sources in order to exclude physically unrelated pairs. Only systems with a FIRST radio detection and one of the cores that has been previously identified as an AGN by spectroscopy are selected for followup spectroscopy. A total of 185 dual-AGN candidates are selected in this way.

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To identify real dual AGN from the 185 candidates, we have started a project - A Systematic Search for Dual AGN in Merging Galaxies (Astro-Daring) using the YFOSC (long-slit spectrograph) mounted on LiJiang 2.4-m telescope of Yunnan observatory. To date, one-third of these candidates have been observed and 17 sources are confirmed to be real dual AGNs. The results show that our innovative selection method is very efficient.