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

Abstract

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 , 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 deg 2 and examine if such quasars can be the dominant source of the IGM ionization. We use the deep optical/near-infrared 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 ≲  M 1450  ≲ −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.