The Infrared Medium-deep Survey. VII. Faint Quasars at z similar to 5 in the ELAIS-N1 Field

The intergalactic medium ( IGM) at z.. 5 to 6 is largely ionized, and yet the main source for the IGM ionization in the early universe is uncertain. Of the possible contributors are faint quasars with -26. M1450. -23, but their number density is poorly constrained at z..5. In this paper, we present our survey of faint quasars at z..5 in the European Large-Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field over a survey area of 6.51 deg2 and examine if such quasars can be the dominant source of the IGM ionization. We use the deep optical/nearinfrared data of the ELAIS-N1 field as well as the additional medium-band observations to find z..5 quasars through a two-step approach using the broadband color selection, and spectral energy distribution fitting with the medium-band information included. Adopting Bayesian information criterion, we identify 10 promising quasar candidates. Spectra of three of the candidates are obtained, confirming all of them to be quasars at z..5 and supporting the reliability of the quasar selection. Using the promising candidates, we derive the z..5 quasar luminosity function at.-26...M1450...-23. The number density of faint z..5 quasars in the ELAIS-N1 field is consistent with several previous results that quasars are not the main contributors to the IGM-ionizing photons at z..5.