Predicting emission line fluxes and number counts of distant galaxies
  for cosmological surveys
  We estimate the number counts of line emitters at high redshift and their
evolution with cosmic time based on a combination of photometry and
spectroscopy. We predict the H$\alpha$, H$\beta$, [OII], and [OIII] line fluxes
for more than $35,000$ galaxies down to stellar masses of $\sim10^9$
$M_{\odot}$ in the COSMOS and GOODS-S fields, applying standard conversions and
exploiting the spectroscopic coverage of the FMOS-COSMOS survey at $z\sim1.55$
to calibrate the predictions. We calculate the number counts of H$\alpha$,
[OII], and [OIII] emitters down to fluxes of $1\times10^{-17}$ erg cm$^{-2}$
s$^{-1}$ in the range $1.4 < z < 1.8$ covered by the FMOS-COSMOS survey. We
model the time evolution of the differential and cumulative H$\alpha$ counts,
steeply declining at the brightest fluxes. We expect $\sim9,300-9,700$ and
$\sim2,300-2,900$ galaxies deg$^{-2}$ for fluxes $\geq1\times10^{-16}$ and
$\geq2\times10^{-16}$ erg cm$^{-2}$ s$^{-1}$ over the range $0.9<z<1.8$. We
show that the observed evolution of the Main Sequence of galaxies with redshift
is enough to reproduce the observed counts variation at $0.2<z<2.5$. We
characterize the physical properties of the H$\alpha$ emitters with fluxes
$\geq2\times10^{-16}$ erg cm$^{-2}$ s$^{-1}$, including their stellar masses,
UV sizes, [NII]/H$\alpha$ ratios, and H$\alpha$ equivalent widths. An aperture
of $R\sim R_{\rm e}\sim0.5$" maximizes the signal-to-noise ratio for a
detection, while causing a factor of $\sim2\times$ flux losses, influencing the
recoverable number counts, if neglected. Our approach, based on deep and large
photometric datasets, reduces the uncertainties on the number counts due to the
selection and spectroscopic samplings, while exploring low fluxes. We publicly
release the line flux predictions for the explored photometric samples.
