  Star-forming galaxies with strong nebular and collisional emission lines are
privileged target galaxies in forthcoming cosmological large galaxy redshift
surveys. We use the COSMOS2015 photometric catalog to model galaxy spectral
energy distributions and emission-line fluxes. We adopt an empirical but
physically-motivated model that uses information from the best-fitting spectral
energy distribution of stellar continuum to each galaxy. The emission-line flux
model is calibrated and validated against direct flux measurements in subsets
of galaxies that have 3D-HST or zCOSMOS-Bright spectra. We take a particular
care in modelling dust attenuation such that our model can explain both
H$\alpha$ and [OII] observed fluxes at different redshifts. We find that a
simple solution to this is to introduce a redshift evolution in the dust
attenuation fraction parameter, $f=E_{\rm star}(B-V)/E_{\rm gas}(B-V)$, as
$f(z)=0.44+0.2z$. From this catalog, we derive the H$\alpha$ and [OII]
luminosity functions up to redshifts of about 2.5 after carefully accounting
for emission line flux and redshift errors. This allows us to make predictions
for H$\alpha$ and [OII] galaxy number counts in next-generation cosmological
redshift surveys. Our modeled emission lines and spectra in the COSMOS2015
catalog shall be useful to study the target selection for planned
next-generation galaxy redshift surveys and we make them publicly available as
`EL-COSMOS' on the ASPIC database.
