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Register a new userExploring Reionization-Era Quasars III: Discovery of 16 Quasars at $6.4lesssim z lesssim 6.9$ with DESI Legacy Imaging Surveys and UKIRT Hemisphere Survey and Quasar Luminosity Function at $zsim6.7$
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This is the third paper in a series aims at finding reionzation-era quasars with the combination of DESI Legacy imaging Surveys (DELS) and near-infrared imaging surveys, such as the UKIRT Hemisphere Survey (UHS), as well as the Wide-field Infrared Survey Explore ($WISE$) mid-infrared survey. In this paper, we describe the updated quasar candidate selection procedure, report the discovery of 16 quasars at $6.4lesssim z lesssim6.9$ from area of $sim$13,020 deg$^2$, and present the quasar luminosity function (QLF) at $zsim6.7$. The measured QLF follows $Phi(L_{1450})propto L_{1450}^{-2.35}$ in the magnitude range $27.6<M_{1450}<-25.5$. We determine the quasar comoving spatial density at $langle z rangle$=6.7 and $M_{1450}<-26.0$ to be $rm 0.39pm0.11 Gpc^{-3}$ and find that the exponential density evolution parameter to be $k=-0.78pm0.18$ from $zsim6$ to $zsim6.7$, corresponding to a rapid decline by a factor of $sim 6$ per unit redshift towards earlier epoch, a rate significantly faster than that at $zsim 3- 5$. The cosmic time between $zsim6$ and $zsim6.7$ is only 121 Myrs. The quasar density declined by a factor of more than three within such short time requires that SMBHs must grow rapidly or they are less radiatively efficient at higher redshifts. We measured quasar comoving emissivity at $zsim6.7$ which indicate that high redshift quasars are highly unlikely to make a significant contribution to hydrogen reionization. The broad absorption line (BAL) quasar fraction at $zgtrsim6.5$ is measured to be $gtrsim$22%. In addition, we also report the discovery of additional five quasars at $zsim6$ in the appendix.
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We present the first discoveries from a survey of $zgtrsim6$ quasars using imaging data from the DECam Legacy Survey (DECaLS) in the optical, the UKIRT Deep Infrared Sky Survey (UKIDSS) and a preliminary version of the UKIRT Hemisphere Survey (UHS) in the near-IR, and ALLWISE in the mid-IR. DECaLS will image 9000 deg$^2$ of sky down to $z_{rm AB}sim23.0$, and UKIDSS and UHS, which will map the northern sky at $0<DEC<+60^{circ}$, reaching $J_{rm VEGA}sim19.6$ (5-$sigma$). The combination of these datasets allows us to discover quasars at redshift $zgtrsim7$ and to conduct a complete census of the faint quasar population at $zgtrsim6$. In this paper, we report on the selection method of our search, and on the initial discoveries of two new, faint $zgtrsim6$ quasars and one new $z=6.63$ quasar in our pilot spectroscopic observations. The two new $zsim6$ quasars are at $z=6.07$ and $z=6.17$ with absolute magnitudes at rest-frame wavelength 1450 AA being $M_{1450}=-25.83$ and $M_{1450}=-25.76$, respectively. These discoveries suggest that we can find quasars close to or fainter than the break magnitude of the Quasar Luminosity Function (QLF) at $zgtrsim6$. The new $z=6.63$ quasar has an absolute magnitude of $M_{1450}=-25.95$. This demonstrates the potential of using the combined DECaLS and UKIDSS/UHS datasets to find $zgtrsim7$ quasars. Extrapolating from previous QLF measurements, we predict that these combined datasets will yield $sim200$ $zsim6$ quasars to $z_{rm AB} < 21.5$, $sim1{,}000$ $zsim6$ quasars to $z_{rm AB}<23$, and $sim 30$ quasars at $z>6.5$ to $J_{rm VEGA}<19.5$.
We report discovery of 41 new high-z quasars and luminous galaxies, which were spectroscopically identified at 5.7 < z < 6.9. This is the fourth in a series of papers from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project, based on the deep multi-band imaging data collected by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. We selected the photometric candidates by a Bayesian probabilistic algorithm, and then carried out follow-up spectroscopy with the Gran Telescopio Canarias and the Subaru Telescope. Combined with the sample presented in the previous papers, we have now spectroscopically identified 137 extremely-red HSC sources over about 650 deg2, which include 64 high-z quasars, 24 high-z luminous galaxies, 6 [O III] emitters at z ~ 0.8, and 43 Galactic cool dwarfs (low-mass stars and brown dwarfs). The new quasars span the luminosity range from M1450 ~ -26 to -22 mag, and continue to populate a few magnitude lower luminosities than have been probed by previous wide-field surveys. In a companion paper, we derive the quasar luminosity function at z ~ 6 over an unprecedentedly wide range of M1450 ~ -28 to -21 mag, exploiting the SHELLQs and other survey outcomes.
This is the fourth paper in a series of publications aiming at discovering quasars at the epoch of reionization. In this paper, we expand our search for $zsim 7$ quasars to the footprint of the Dark Energy Survey (DES) Data Release One (DR1), covering $sim 5000$ deg$^2$ of new area. We select $zsim 7$ quasar candidates using deep optical, near-infrared (near-IR) and mid-IR photometric data from the DES DR1, the VISTA Hemisphere Survey (VHS), the VISTA Kilo-degree Infrared Galaxy (VIKING) survey, the UKIRT InfraRed Deep Sky Surveys -- Large Area Survey (ULAS) and the unblurred coadds from the Wide-field Infrared Survey Explore ($WISE$) images (unWISE). The inclusion of DES and unWISE photometry allows the search to reach $sim$ 1 magnitude fainter, comparing to our $z gtrsim 6.5$ quasar survey in the northern sky (Wang et al. 2018). We report the initial discovery and spectroscopic confirmation of six new luminous quasars at $z>6.4$, including an object at $z=7.02$, the fourth quasar yet known at $z>7$, from a small fraction of candidates observed thus far. Based on the recent measurement of $z sim 6.7 $ quasar luminosity function using the quasar sample from our survey in the northern sky, we estimate that there will be $gtrsim$ 55 quasars at $z > 6.5$ at $M_{1450} < -24.5$ in the full DES footprint.
We present new measurements of the quasar luminosity function (LF) at $z sim 6$, over an unprecedentedly wide range of the rest-frame ultraviolet luminosity $M_{1450}$ from $-30$ to $-22$ mag. This is the fifth in a series of publications from the Subaru High-$z$ Exploration of Low-Luminosity Quasars (SHELLQs) project, which exploits the deep multi-band imaging data produced by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The LF was calculated with a complete sample of 110 quasars at $5.7 le z le 6.5$, which includes 48 SHELLQs quasars discovered over 650 deg$^2$, and 63 brighter quasars discovered by the Sloan Digital Sky Survey and the Canada-France-Hawaii Quasar Survey (including one overlapping object). This is the largest sample of $z sim 6$ quasars with a well-defined selection function constructed to date, and has allowed us to detect significant flattening of the LF at its faint end. A double power-law function fit to the sample yields a faint-end slope $alpha = -1.23^{+0.44}_{-0.34}$, a bright-end slope $beta = -2.73^{+0.23}_{-0.31}$, a break magnitude $M_{1450}^* = -24.90^{+0.75}_{-0.90}$, and a characteristic space density $Phi^* = 10.9^{+10.0}_{-6.8}$ Gpc$^{-3}$ mag$^{-1}$. Integrating this best-fit model over the range $-18 < M_{1450} < -30$ mag, quasars emit ionizing photons at the rate of $dot{n}_{rm ion} = 10^{48.8 pm 0.1}$ s$^{-1}$ Mpc$^{-3}$ at $z = 6.0$. This is less than 10 % of the critical rate necessary to keep the intergalactic medium ionized, which indicates that quasars are not a major contributor to cosmic reionization.
We present discovery imaging and spectroscopy for nine new z ~ 6 quasars found in the Canada-France High-z Quasar Survey (CFHQS) bringing the total number of CFHQS quasars to 19. By combining the CFHQS with the more luminous SDSS sample we are able to derive the quasar luminosity function from a sample of 40 quasars at redshifts 5.74 < z < 6.42. Our binned luminosity function shows a slightly lower normalisation and flatter slope than found in previous work. The binned data also suggest a break in the luminosity function at M_1450 approx -25. A double power law maximum likelihood fit to the data is consistent with the binned results. The luminosity function is strongly constrained (1 sigma uncertainty < 0.1 dex) over the range -27.5 < M_1450 < -24.7. The best-fit parameters are Phi(M_1450^*) = 1.14 x 10^-8 Mpc^-3 mag^-1, break magnitude M_1450^* = -25.13 and bright end slope beta = -2.81. However the covariance between beta and M_1450^* prevents strong constraints being placed on either parameter. For a break magnitude in the range -26 < M_1450^* < -24 we find -3.8 < beta < -2.3 at 95% confidence. We calculate the z = 6 quasar intergalactic ionizing flux and show it is between 20 and 100 times lower than that necessary for reionization. Finally, we use the luminosity function to predict how many higher redshift quasars may be discovered in future near-IR imaging surveys.
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