  Upcoming spectroscopic redshift surveys use emission line galaxies (ELGs) to
trace the three-dimensional matter distributions with wider area coverage in
the deeper Universe. Since the halos hosting ELGs are young and undergo infall
towards more massive halos along filamentary structures, contrary to a widely
employed luminous red galaxy sample, the dynamics specific to ELGs should be
taken into account to refine the theoretical modelling at non-linear scales. In
this paper, we scrutinise the halo occupation distribution (HOD) and clustering
properties of ELGs by utilising IllustrisTNG galaxy formation hydrodynamical
simulations. Leveraging stellar population synthesis technique coupled with the
photo-ionization model, we compute line intensities of simulated galaxies and
construct mock H$\alpha$ and [OII] ELG catalogues. The line luminosity
functions and the relation between the star formation rate and line intensity
are well consistent with observational estimates. Next, we measure the HOD and
demonstrate that there is a distinct population for the central HOD, which
corresponds to low-mass infalling halos. We then perform the statistical
inference of HOD parameters from the projected correlation function. Our
analysis indicates that the inferred HODs significantly deviate from the HOD
measured directly from simulations although the best-fit model yields a good
fit to the projected correlation function. It implies that the information
content of the projected correlation function is not adequate to constrain HOD
models correctly and thus, it is important to employ mock ELG catalogues to
calibrate the functional form of HOD models and add prior information on HOD
parameters to robustly determine the HOD.
