Minor Contribution of Quasars to Ionizing Photon Budget at z~6: Update on Quasar Luminosity Function at the Faint-end with Subaru/Suprime-Cam

Abstract

We constrain the quasar contribution to cosmic reionization based on our deep optical survey of z~6 quasars down to z_R=24.15 using Subaru/Suprime-Cam in three UKIDSS-DXS fields covering 6.5 deg^2. In Kashikawa et al. (2015), we select 17 quasar candidates and report our initial discovery of two low-luminosity quasars (M_1450~ -23) from seven targets, one of which might be a Lyman alpha emitting galaxy. From an additional optical spectroscopy, none of the four candidates out of the remaining ten turn out to be genuine quasars. Moreover, the deeper optical photometry provided by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) shows that, unlike the two already-known quasars, the i-z and z-y colors of the last six candidates are consistent with M- or L-type brown dwarfs. Therefore, the quasar luminosity function (QLF) in the previous paper is confirmed. Compiling QLF measurements from the literature over a wide magnitude range, including an extremely faint AGN candidate from Parsa et al. (2017}, to fit them with a double power-law, we find that the best-fit faint-end slope is alpha=-2.04^+0.33_-0.18 (-1.98^+0.48_-0.21) and characteristic magnitude is M_1450^*=-25.8^+1.1_-1.9 (-25.7^+1.0_-1.8) in the case of two (one) quasar detection. Our result suggests that, if the QLF is integrated down to M_1450=-18, quasars produce ~1-12% of the ionizing photons required to ionize the whole universe at z~6 with 2sigma confidence level, assuming that the escape fraction is f_esc=1 and the IGM clumpy factor is C=3. Even when the systematic uncertainties are taken into account, our result supports the scenario that quasars are the minor contributors of reionization.

8 pages, 3 figures, ApJL accepted

References in corpus (12)

• A luminous quasar at a redshift of z = 7.085
• Observational constraints on Cosmic Reionization
• An ultra-luminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30
• Evidence of patchy hydrogen reionization from an extreme Ly$\alpha$ trough below redshift six
• The Pan-STARRS1 distant z>5.6 quasar survey: more than 100 quasars within the first Gyr of the universe
• The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization
• The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7
• Faint AGNs at z>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe
• The first ultraviolet quasar stacked spectrum at z=2.4 from WFC3
• The Subaru high-z quasar survey: discovery of faint z~6 quasars
• The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion
• Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys

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