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Register a new userJoint Survey Processing I: Compact oddballs in the COSMOS field -- low-luminosity Quasars at z > 6?
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The faint-end slope of the quasar luminosity function at z~6 and its implication on the role of quasars in reionizing the intergalactic medium at early times has been an outstanding problem for some time. The identification of faint high-redshift quasars with luminosities of <1e44.5 erg/s is challenging. They are rare (few per square degree) and the separation of these unresolved quasars from late-type stars and compact star-forming galaxies is difficult from ground-based observations alone. In addition, source confusion becomes significant at >25mag, with ~30% of sources having their flux contaminated by foreground objects when the seeing resolution is ~0.7. We mitigate these issues by performing a pixel-level joint processing of ground and space-based data from Subaru/HSC and HST/ACS. We create a deconfused catalog over the 1.64 square-degrees of the COSMOS field, after accounting for spatial varying PSFs and astrometric differences between the two datasets. We identify twelve low-luminosity (M_UV ~ -21 mag) z>6 quasar candidates through (i) their red color measured between ACS/F814W and HSC/i-band and (ii) their compactness in the space-based data. We estimate that late-type stars could contribute up to 50% to our sample. Our constraints on the faint end of the quasar luminosity function at z~6.4 suggests a negligibly small contribution to reionization compared to the star-forming galaxy population. The confirmation of our candidates and the evolution of number density with redshift could provide better insights into how supermassive galaxies grew in the first billion years of cosmic time.
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The aim of this paper is to introduce the WIRCam Ultra Deep Survey (WUDS), a near-IR photometric public survey carried out at the CFH Telescope in the field of the CFHTLS-D3 field (Groth Strip). WUDS includes four near-IR bands (Y, J, H and K_s) over a field of view of ~400 arcmin^2. The typical depth of WUDS data reaches between ~26.8 in Y and J, and ~26 in H and K_s (AB, 3 sigma in 1.3 arcsec aperture). The area and depth of this survey were specifically tailored to set strong constraints on the cosmic star formation rate and the luminosity function brighter or around L* in the z~6-10 redshift domain, although these data are also useful for a variety of extragalactic projects.This first paper is intended to present the properties of WUDS: catalog building, completeness and depth, number counts, photometric redshifts, and global properties of the galaxy population. We have also concentrated on the study of galaxy samples at z~[4.5-7] in this field. UV luminosity functions were derived at z~5 and z~6 taking advantage from the fact that WUDS covers a particularly interesting regime at intermediate luminosities, which allows a combined determination of M* and Phi* with increased accuracy. Our results on the luminosity function are consistent with a small evolution of both M* and Phi* between z=5 and z=6, irrespective of the method used to derive them, either photometric redshifts applied to blindly-selected dropout samples or the classical Lyman Break Galaxy color-preselected samples. Our results lend support to higher Phi* determinations at z=6 than usually reported. The selection and combined analysis of different galaxy samples at z>7 will be presented in a forthcoming paper. WUDS is intended to provide a robust database in the near-IR for the selection of targets for detailed spectroscopic studies, in particular for the EMIR/GTC GOYA Survey (Abridged)
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 measurements of the size of the quasar proximity zone ($R_p$) for eleven low-luminosity ($-26.16leq M_{1450}leq-22.83$) quasars at $zsim6$, discovered by the Subaru High-$z$ Exploration of Low-Luminosity Quasars project (SHELLQs). Our faint quasar sample expands the $R_p$ measurement down to $M_{1450}=-22.83$ mag, where more common quasar populations dominate at the epoch. We restrict the sample to quasars whose systemic redshifts have been precisely measured by [CII] 158 $mu$m or MgII $lambda$2798 emission lines. We also update the $R_p$ measurements for 26 luminous quasars presented in Eilers et al. (2017)(arXiv:1703.02539) by using the latest systemic redshift results. The luminosity dependence on $R_p$ is found to be consistent with the theoretical prediction assuming highly ionized intergalactic medium. We find a shallow redshift evolution of the luminosity-corrected $R_p$, $R_{p,{rm corr}}^{-25}$ ($R_{p, rm corr}^{-25}propto(1+z)^{-3.79pm1.72}$) over $5.8lesssim z lesssim6.6$. This trend is steeper than that of Eilers et al. (2017) but significantly shallower than those of the earlier studies. Our results suggest that $R_{p,rm corr}$ is insensitive to the neutral fraction of the universe at $zsim6$. Four quasars show exceptionally small $R_{p,rm corr}^{-25}$ ($ lesssim0.90$ proper Mpc), which could be the result of their young age ($<10^4$ yr) in the reionization epoch, though statistics is still small.
We present the discovery of one or two extremely faint z~6 quasars in 6.5 deg^2 utilizing a unique capability of the wide-field imaging of the Subaru/Suprime-Cam. The quasar selection was made in (i-z_B) and (z_B-z_R) colors, where z_B and z_R are bandpasses with central wavelengths of 8842A and 9841A, respectively. The color selection can effectively isolate quasars at z~6 from M/L/T dwarfs without the J-band photometry down to z_R<24.0, which is 3.5 mag. deeper than SDSS. We have selected 17 promising quasar candidates. The follow-up spectroscopy for seven targets identified one apparent quasar at z=6.156 with M_1450=-23.10. We also identified one possible quasar at z=6.041 with a faint continuum of M_1450=-22.58 and a narrow Lyman-alpha emission with HWHM=427 km/s, which cannot be distinguished from Lyman-alpha emitters. We derive the quasar luminosity function at z~6 by combining our faint quasar sample with the bright quasar samples by SDSS and CFHQS. Including our data points invokes a higher number density in the faintest bin of the quasar luminosity function than the previous estimate employed. This suggests a steeper faint-end slope than lower-z, though it is yet uncertain based on a small number of spectroscopically identified faint quasars and several quasar candidates are still remain to be diagnosed. The steepening of the quasar luminosity function at the faint-end does increase the expected emission rate of the ionizing photon, however, it only changes by a factor of ~2-6. This was found to be still insufficient for the required photon budget of reionization at z~6.
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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