Exploring Reionization-era Quasars. III. Discovery of 16 Quasars at 6.4 ≲ z ≲ 6.9 with DESI Legacy Imaging Surveys and the UKIRT Hemisphere Survey and Quasar Luminosity Function at z ∼ 6.7

Abstract

This is the third paper in a series aimed at finding reionization-era quasars with the combination of DESI Legacy imaging Surveys (DELS), the Pan-STARRS1 (PS1) Survey, and near-infrared imaging surveys, such as the UKIRT Hemisphere Survey (UHS), as well as the Wide-field Infrared Survey Explorer (WISE) mid-infrared survey. In this paper, we describe the updated quasar candidate selection procedure, report the discovery of 16 quasars at 6.4 z6.9 from an area of ∼13,020 deg 2 , and present the quasar luminosity function (QLF) at z∼6.7. The measured QLF follows F µ-L L 1450 1450 2.35 () in the magnitude range −27.6<M 1450 <−25.5. We determine the quasar comoving spatial density at á ñ z =6.7 and M 1450 <−26.0 to be 0.39±0.11 Gpc −3 and find the exponential density evolution parameter to be k=−0.78±0.18 from z∼6 to z∼6.7, corresponding to a rapid decline by a factor of ∼6 per unit redshift toward earlier epochs. This indicates that the rapid decline of quasar spatial density at z>5 that was found by previous works continues to z>6, at a rate significantly faster than the average decline rate between z∼3 and 5. We measured quasar comoving emissivity at z∼6.7, which indicates that high-redshift quasars are highly unlikely to make a significant contribution to hydrogen reionization. The broad absorption line quasar fraction at z6.5 is measured to be 22%. In addition, we also report the discovery of six additional quasars at z∼6 in the Appendix.

The Evolution of the IR Luminosity Function and Dust-obscured Star Formation over the Past 13 Billion Years

“…In Figure 8 we compare the SFRD derived in this work with results from galaxy evolution models, including the predictions from the cosmological hydrodynamical IllustrisTNG simulations (Pillepich et al 2018), the SHARK semianalytic model (Pillepich et al 2018), the dashed champagne line those from the SHARK semianalytic model (Lagos et al 2018), and the dashed-dotted green line the results from the SIDES simulations (Béthermin et al 2017). Additionally, we include the density evolution of luminous quasars from Wang et al (2019a) (scaled for better visualization), which strongly resemble the shape of the SFRD from bright DSFGs with L IR > 10 12 L e illustrated by the orange dashed region. (Lagos et al 2018), and the results from the SIDES simulations (Béthermin et al 2017).…”

“…Generally speaking, and taking into account the associated uncertainties in these values, our results are in good agreement with the aforementioned studies, pointing toward a convergence between galaxy evolution models and observations. Finally, we include the density evolution of luminous (M 1450 < − 26) quasars (Wang et al 2019a) in Figure 8 (scaled for visualization). The shape of the space density of bright quasars strongly resembles that from the bright DSFGs with L IR > 10 12 L e (shaded orange region in the figure), suggesting a connection between these two populations and therefore between the onset of star formation and the growth of their massive black holes (e.g., Wall et al 2005).…”

The REQUIEM Survey. I. A Search for Extended Lyα Nebular Emission Around 31 z > 5.7 Quasars

“…Our sample consists of 31quasars in the redshift range 5.77<z<6.62 located in the southern sky (e.g., Fan et al 2001Fan et al , 2003Fan et al , 2006Willott et al 2007Willott et al , 2010Venemans et al 2013;Bañados et al 2016;Jiang et al 2016;Mazzucchelli et al 2017;Reed et al 2017;Matsuoka et al 2018;Wang et al 2019b;Yang et al 2019b). This includes all available MUSE observations of z>5.7 quasars present in the ESO Archive at the time of writing (2019 August).…”

Reality or Mirage? Observational Test and Implications for the Claimed Extremely Magnified Quasar at z = 6.3

“…We obtain N WFIRST (z > 6) ∼ 5 × 10 4 , while restricting our attention to z > 7 we obtain N WFIRST (z > 7) ∼ 3, 000. For this order-of-magnitude estimate, we employ the Jiang et al (2016) The evolution of the LF for z 6.5 is uncertain: for the purpose of this estimate, we keep the shape of the z 6.5 LF equal to the one presented by Jiang et al (2016), while we change the overall density of quasars following the prescription by Wang et al (2019b). Note that the spatial density of quasars is expected to decline by ∼ 1 order of magnitude between z = 6.5 and z = 8.…”