Luminous quasars at > z 5.6 can be studied in detail with the current generation of telescopes and provide us with unique information on the first gigayear of the universe. Thus far, these studies have been statistically limited by the number of quasars known at these redshifts. Such quasars are rare, and therefore, wide-field surveys are required to identify them, and multiwavelength data are required to separate them efficiently from their main contaminants, the far more numerous cool dwarfs. In this paper, we update and extend the selection for thez 6 quasars presented in Bañados et al. (2014) using the Pan-STARRS1 (PS1) survey. We present the PS1 distant quasar sample, which currently consists of 124 quasars in the redshift range   z 5.6 6.7 that satisfy our selection criteria. Of these quasars, 77 have been discovered with PS1, and 63 of them are newly identified in this paper. We present the composite spectra of the PS1 distant quasar sample. This sample spans a factor of ∼20 in luminosity and shows a variety of emission line properties. The number of quasars at > z 5.6 presented in this work almost doubles the previously known quasars at these redshifts, marking a transition phase from studies of individual sources to statistical studies of the high-redshift quasar population, which was impossible with earlier, smaller samples.

Ratios of different ions of the same element encode ionization information independently from relative abundances in quasar absorption line systems, crucial for understanding the multiphase nature and origin of absorbing gas, particularly at z > 6 where H I cannot be observed. Observational considerations have limited such studies to a small number of sightlines, with most surveys at z > 6 focused upon the statistical properties of individual ions such as Mg II or C IV. Here we compare high-and low-ionization absorption within 69 intervening systems at z > 5, including 16 systems at z > 6, from Magellan/FIRE spectra of 47 quasars together with a Keck/HIRES spectrum of the 'ultraluminous' z = 6.3 quasar SDSSJ010013.02+280225.8. The highest redshift absorbers increasingly exhibit lowionization species alone, consistent with previous single-ion surveys that show the frequency of Mg II is unchanging with redshift while C IV absorption drops markedly toward z = 6. We detect no C IV or Si IV in half of all metal-line absorbers at z > 5.7, with stacks not revealing any slightly weaker C IV just below our detection threshold, and most of the other half have N CII > N CIV . In contrast, only 20% of absorbers at 5.0-5.7 lack high-ionization gas, and a search of 25 HIRES sightlines at z ∼ 3 yielded zero such examples. We infer these low-ionization high-redshift absorption systems may be analogous to metal-poor Damped Lyman-α systems (∼ 1% of the absorber population at z ∼ 3), based on incidence rates and absolute and relative column densities. Simple photoionization models suggest that circumgalactic matter at redshift six has systematically lower chemical abundances and experiences a softer ionizing background relative to redshift three.

Cold accretion is a primary growth mechanism of simulated galaxies, yet observational evidence of "cold flows" at redshifts where they should be most efficient (z = 2-4) is scarce. In simulations, cold streams manifest as Lyman-limit absorption systems (LLSs) with low heavy-element abundances similar to those of the diffuse IGM. Here we report on an abundance survey of 17 H I-selected LLSs at z = 3.2-4.4 which exhibit no metal absorption in SDSS spectra. Using medium-resolution spectra obtained at Magellan, we derive ionization-corrected metallicities (or limits) with a Markov-Chain Monte Carlo sampling that accounts for the large uncertainty in N HI measurements typical of LLSs. The metal-poor LLS sample overlaps with the IGM in metallicity and is best described by a model where 71 +13 −11 % are drawn from the IGM chemical abundance distribution. These represent roughly half of all LLSs at these redshifts, suggesting that 28-40% of the general LLS population at z ∼ 3.7 could trace unprocessed gas. An ancillary sample of ten LLSs without any a priori metal-line selection is best fit with 48 +14 −12 % of metallicities drawn from the IGM. We compare these results with regions of a moving-mesh simulation; the simulation finds only half as many baryons in IGM-metallicity LLSs, and most of these lie beyond the virial radius of the nearest galaxy halo. A statistically significant fraction of all LLSs have low metallicity and therefore represent candidates for accreting gas; largevolume simulations can establish what fraction of these candidates actually lie near galaxies and the observational prospects for detecting the presumed hosts in emission.

We present initial results from the Cosmic Ultraviolet Baryon Survey (CUBS). CUBS is designed to map diffuse baryonic structures at redshift $z\:^{<}_{\sim }\:1$ using absorption-line spectroscopy of 15 UV-bright QSOs with matching deep galaxy survey data. CUBS QSOs are selected based on their NUV brightness to avoid biases against the presence of intervening Lyman Limit Systems (LLSs) at zabs < 1. We report five new LLSs of $\log \, N({\rm {H\,{I}}})/\rm {{\rm cm^{-2}}}\:^{>}_{\sim }\:17.2$ over a total redshift survey pathlength of Δ zLL = 9.3, and a number density of $n(z)=0.43_{-0.18}^{+0.26}$. Considering all absorbers with $\log \, N({\rm {H\,{I}}})/\rm {{\rm cm^{-2}}}>16.5$ leads to $n(z)=1.08_{-0.25}^{+0.31}$ at zabs < 1. All LLSs exhibit a multi-component structure and associated metal transitions from multiple ionization states such as C II, C III, Mg II, Si II, Si III, and O VI absorption. Differential chemical enrichment levels as well as ionization states are directly observed across individual components in three LLSs. We present deep galaxy survey data obtained using the VLT-MUSE integral field spectrograph and the Magellan Telescopes, reaching sensitivities necessary for detecting galaxies fainter than 0.1 L* at $d\:^{<}_{\sim }\:300$ physical kpc (pkpc) in all five fields. A diverse range of galaxy properties is seen around these LLSs, from a low-mass dwarf galaxy pair, a co-rotating gaseous halo/disk, a star-forming galaxy, a massive quiescent galaxy, to a galaxy group. The closest galaxies have projected distances ranging from d = 15 to 72 pkpc and intrinsic luminosities from ≈0.01 L* to ≈3 L*. Our study shows that LLSs originate in a variety of galaxy environments and trace gaseous structures with a broad range of metallicities.