  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 short-lived stars 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. To this end, we present a
novel and fast scheme to populate ELGs in dark-matter only $N$-body simulations
based on local density field. This method enables fast production of mock ELG
catalogues suitable for verifying analysis methods and quantifying
observational systematics in 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 power spectrum of simulated ELGs is
consistent with results of hydrodynamical simulations up to fairly small scales
($\lesssim 1 h \, \mathrm{Mpc}^{-1}$), and 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
of simulated ELGs clearly exhibit a weaker Finger-of-God effect than those of
matter due to infalling motions towards halo centre, rather than random virial
motions inside halos.
