Narrow-band surveys for very high redshift Lyman-α emitters

Abstract

Context.Many current and future surveys aim to detect the highest redshift ($z \ga 7$) sources through their Lyman-$\alpha$ (Ly$\alpha$) emission, using the narrow-band imaging method. However, to date the surveys have only yielded non-detections and upper limits as no survey has reached the necessary combination of depth and area to detect these very young star forming galaxies. Aims.We aim to calculate model luminosity functions and mock surveys of Ly$\alpha$ emitters at $z \ga 7$ based on a variety of approaches calibrated and tested on observational data at lower redshifts. Methods.We calculate model luminosity functions at different redshifts based on three different approaches: a semi-analytical model based on CDM, a simple phenomenological model, and an extrapolation of observed Schechter functions at lower redshifts. The results of the first two models are compared with observations made at redshifts $z \sim 5.7$ and $z \sim 6.5$, and they are then extrapolated to higher redshift. Results.We present model luminosity functions for redshifts between z = 7-12.5 and give specific number predictions for future planned or possible narrow-band surveys for Ly$\alpha$ emitters. We also investigate what constraints future observations will be able to place on the Ly$\alpha$ luminosity function at very high redshift. Conclusions.It should be possible to observe z = 7-10 Ly$\alpha$ emitters with present or near-future instruments if enough observing time is allocated. In particular, large area surveys such as ELVIS (Emission Line galaxies with VISTA Survey) will be useful in collecting a large sample. However, to get a large enough sample to constrain well the $z \geq 10$ Ly$\alpha$ luminosity function, instruments further in the future, such as an ELT, will be necessary.