The bright end of the z ~ 7 UV luminosity function from a wide and deep HAWK-I survey

Aims: We perform a deep search for galaxies in the redshift range 6.5 ≤ z ≤ 7.5 to measure the evolution of the number density of luminous galaxies in this redshift range and derive useful constraints on the evolution of their luminosity function.
Methods: We present here the second half of an ESO Large Programme, which exploits the unique combination of area and sensitivity provided in the near-IR by the camera Hawk-I at the VLT. We have obtained ~ 30 observing hours with Hawk-I in the Y-band of two high galactic latitude fields. We combined the Y-band data with deep J and K Hawk-I observations, as well as FORS1/FORS2 U, B, V, R, I, and Z observations to select z-drop galaxies with Z-Y>1, no optical detection, and flat Y-J and Y-K colour terms.
Results: We detect eight high-quality candidates in the magnitude range Y = 25.5-26.5 that we add to the z-drop candidates selected in two Hawk-I pointings over the GOODS-South field. We use this full sample of 15 objects found in ~161 arcmin² of our survey to constrain the average physical properties and the evolution of the number density of z ~ 7 LBGs. A stacking analysis yields a best-fit SED with photometric redshift z = 6.85_-0.15^+0.20 and an E(B-V) = 0.05_-0.05^+0.15. We compute a binned estimate of the z ~ 7 LF and explore the effects of photometric scatter and model uncertainties on the statistical constraints. After accounting for the expected incompleteness through MonteCarlo simulations, we strengthen our previous finding that a Schechter luminosity function constant from z = 6 to z = 7 is ruled out at a ⪆99% confidence level, even including the effects of cosmic variance. For galaxies brighter than M₁₅₀₀ = -19.0, we derive a luminosity density ρ_UV = 1.5_-0.8^+2.1 × 10²⁵ erg s^-1 Hz^-1 Mpc^-3, implying a decrease by a factor 3.5 from z = 6 to z ≃ 6.8. We find that under standard assumptions, the emission rate of ionizing photons coming from UV bright galaxies is lower by at least a factor of two than the value required for reionization. Finally, we exploit deep Hawk-I J and K band observations to derive an upper limit on the number density of M₁₅₀₀ ⪉ -22.0 LBGs at z ~ 8 (Y-dropouts).