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Dual AGNs are natural byproducts of hierarchical mergers of galaxies in theLambdaCDM cosmogony. Recent observations have shown that only a small fraction(~ 0.1%-1%) of AGNs at redshift z<~ 0.3 are dual with kpc-scale separations,which is rather low compared to the high merger rate of galaxies. Here weconstruct a phenomenological model to estimate the number density of dual AGNsand its evolution according to the observationally-estimated major merger ratesof galaxies and various scaling relations on the properties of galaxies andtheir central massive black holes. We show that our model reproduces theobserved frequency and separation distribution of dual AGNs provided thatsignificant nuclear activities are triggered only in gas-rich progenitorgalaxies with central massive black holes and only when the nuclei of thesegalaxies are roughly within the half-light radii of their companion galaxies.Under these constraints, the observed low dual AGN frequency is consistent withthe relatively high merger rate of galaxies and supports the hypothesis thatmajor mergers lead to AGN/QSO activities. We also predict that the number ofkpc-scale dual AGNs decreases with increasing redshift and only about0.02%--0.06% of AGNs are dual AGNs with double-peaked narrow line features atredshifts of z 0.5-1.2. Future observations of high-redshift dual AGNs wouldprovide a solid test for this prediction.

The low frequency of dual AGNs versus the high merger rate of galaxies: A phenomenological model