The high-precision measurement of spatial clustering of emission line galaxies (ELGs) is a primary objective for upcoming cosmological spectroscopic surveys. The source of strong emission of ELGs is nebular emission from surrounding ionized gas irradiated by massive stars and such massive short-lived stars form in star-forming galaxies. As a result, ELGs are more likely to reside in newly-formed halos and this leads to a nonlinear relation between ELG number density and matter density fields. In order to estimate the covariance matrix of cosmological observables, it is essential to produce many independent realisations to simulate ELG distributions for large survey volumes. In order to efficiently generate mock ELG catalogues, we present a novel and fast scheme to populate ELGs to dark-matter only $N$-body simulations based on local density field. This method enables fast production of mock ELG catalogues suitable for upcoming spectroscopic surveys and can populate ELGs in moderately high-density regions even though the halo structure cannot be resolved due to low resolution. The simulated ELGs are more likely to be found in filamentary structures, which is consistent with results of semi-analytic and hydrodynamical simulations. Furthermore, we address the redshift-space power spectrum of simulated ELGs. The measured multipole moments clearly exhibit a weaker Finger-of-God effect due to infalling motion towards halo centre, which is predicted by the simulations.
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