A simple prediction of the surface density of galaxies at z ≃ 6

Systematic surveys are being proposed to discover a significant number of galaxies at z ≃ 6, which is now suggested as the epoch at which the reionization era of the universe ends. To plan such surveys, we need a reasonable expectation of the surface density of high-redshift galaxies at different flux limits. Here we present a simple prediction of the surface density of 5.5 ≤ z ≤ 6.5 galaxies in the optical regime, extrapolating from what is already known about galaxies at z ≃ 3. This prediction is consistent with the results of nearly all known searches for objects at z ≃ 6, giving confidence that we may use it to plan optimal combination of survey depth and sky coverage in searching for such objects. We suggest that the most efficient strategy with existing ground-based facilities is to do medium-depth (m_AB ≃ 24.0-24.5 mag), wide-field (a couple of square degrees) survey using a wide-field camera at a 4 m class telescope. As the predicted surface density at this brightness level is very sensitive to the value of L_*, the result of such a survey can be easily used to constrain the luminosity evolution from z ≃ 3 to 6.