J-PLUS: Synthetic galaxy catalogues with emission lines for photometric surveys

We present a synthetic galaxy lightcone specially designed for narrow-band optical photometric surveys. To reduce time-discreteness effects, unlike previous works, we directly include the lightcone construction in the L-Galaxies semi-analytic model applied to the subhalo merger trees of the Millennium simulation. Additionally, we add a model for the nebular emission in star-forming regions, which is crucial for correctly predicting the narrow- and medium-band photometry of galaxies. Specifically, we consider, individually for each galaxy, the contribution of 9 different lines: Ly alpha (1216 angstrom), H beta (4861 angstrom), H alpha (6563 angstrom), [OII] (3727 angstrom, 3729 angstrom), [OIII] (4959 angstrom, 5007 angstrom), [NeIII] (3870 angstrom), [OI] (6300 angstrom), [NII] (6548 angstrom, 6583 angstrom), and [SII] (6717 angstrom, 6731 angstrom). We validate our lightcone by comparing galaxy number counts, angular clustering, and H alpha, H beta, [OII], and [OIII](5007) luminosity functions to a compilation of observations. As an application of our mock lightcones, we generated catalogues tailored for J-PLUS, a large optical galaxy survey featuring five broad-band and seven medium-band filters. We study the ability of the survey to correctly identify, with a simple three-filter method, a population of emission-line galaxies at various redshifts. We show that the 4000 angstrom break in the spectral energy distribution of galaxies can be misidentified as line emission. However, all significant excess (>0.4 mag) can be correctly and unambiguously attributed to emission-line galaxies. Our catalogues are publicly released to facilitate their use in interpreting narrow-band surveys and in quantifying the impact of line emission in broad-band photometry.