2020ApJ...904..111K

Faint z ∼ 5 quasars with M₁₄₅₀ ∼ -23 mag are known to be potentially important contributors to the ultraviolet ionizing background in the postreionization era. However, their number density has not been well determined, making it difficult to assess their role in the early ionization of the intergalactic medium (IGM). In this work, we present the updated results of our z ∼ 5 quasar survey using the Infrared Medium-deep Survey (IMS), a near-infrared imaging survey covering an area of 85 deg². From our spectroscopic observations with the Gemini Multi-Object Spectrograph on the Gemini-South 8 m telescope, we discovered eight new quasars at z ∼ 5 with -26.1 <= M₁₄₅₀ <= -23.3. Combining our IMS faint quasars (M₁₄₅₀ > -27 mag) with the brighter Sloan Digital Sky Survey quasars (M₁₄₅₀ < -27 mag), we derive the z ∼ 5 quasar luminosity function (QLF) without any fixed parameters down to the magnitude limit of M₁₄₅₀ = -23 mag. We find that the faint-end slope of the QLF is very flat (α=-1.2_–0.6^+1.4), with a characteristic luminosity of M₁₄₅₀^* =-25.8_–1.1^+1.4 mag. The number density of z ∼ 5 quasars from the QLF gives an ionizing emissivity at 912 Å of ε₉₁₂ = (3.7-7.1) x 10²³ erg s^–1 Hz^–1 Mpc^–3 and an ionizing photon density of {dot}n_ion=(3.0–5.7)×10⁴⁹ Mpc^–3 s^–1. These results imply that quasars are responsible for only 10%-20% (up to 50% even in the extreme case) of the photons required to completely ionize the IGM at z ∼ 5, disfavoring the idea that quasars alone could have ionized the IGM at z ∼ 5.