{'fraction_non_alphanumeric': 0.05738900854582089, 'fraction_numerical': 0.053428750086847776, 'mean_word_length': 3.8318227593152066, 'pattern_counts': {'":': 0, '<': 35, '<?xml version=': 0, '>': 20, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
The identification of z-dropouts in Pan-STARRS1: three quasars at 6.5

Luminous distant quasars are unique probes of the high redshift intergalactic medium (IGM) and of the growth of massive galaxies and black holes in the early universe. Absorption due to neutral Hydrogen in the IGM makes quasars beyond a redshift of z~6.5 very faint in the optical $z$-band, thus locating quasars at higher redshifts require large surveys that are sensitive above 1 micron. We report the discovery of three new z>6.5 quasars, corresponding to an age of the universe of<850 Myr, selected as z-band dropouts in the Pan-STARRS1 survey. This increases the number of known z>6.5 quasars from 4 to 7. The quasars have redshifts of z=6.50, 6.52, and 6.66, and include the brightest z-dropout quasar reported to date, PSO J036.5078+03.0498 with M_1450=-27.4. We obtained near-infrared spectroscopy for the quasars and from the MgII line we estimate that the central black holes have masses between 5x10^8 and 4x10^9 M_sun, and are accreting close to the Eddington limit (L_Bol/L_Edd=0.13-1.2). We investigate the ionized regions around the quasars and find near zone radii of R_NZ=1.5-5.2 proper Mpc, confirming the trend of decreasing near zone sizes with increasing redshift found for quasars at 5.7<z<6.4. By combining R_NZ of the PS1 quasars with those of 5.7<z<7.1 quasars in the literature, we derive a luminosity corrected redshift evolution of R_NZ,corrected=(7.2+/-0.2)-(6.1+/-0.7)x(z-6) Mpc. However, the large spread in R_NZ in the new quasars implies a wide range in quasar ages and/or a large variation in the neutral Hydrogen fraction along different lines of sight.

. We obtained near-infrared spectroscopy for the quasars, and from the Mg II line, we estimate that the central black holes have masses between 5 × 10 8 and 4 × 10 9  M and are accreting close to the Eddington limit ( = -L L 0.13 1.2

Bol Edd
). We investigate the ionized regions around the quasars and find near-zone radii of = -R 1.5 5.2 NZ proper Mpc, confirming the trend of decreasing near-zone sizes with increasing redshift found for quasars at < < z 5.7 6.4. By combining R NZ of the PS1 quasars with those of < < z 5.7 7.1 quasars in the literature, we derive a luminosity-corrected redshift evolution of =  -´-R z (7.2 0.2) (6.1 0.7) ( 6)

INTRODUCTION
Quasars are the most luminous non-transient objects known. Their high luminosity makes quasars ideal to probe the universe at early cosmic times. Since distant (  z 5.7) luminous quasars are rare, with an estimated source density of ∼1 Gpc −3 (e.g., Fan et al. 2004;Willott et al. 2010b), surveys covering a large area of the sky are required to uncover the distant quasar population. Over the last 15 years more than 70 quasars with redshifts between < < z 5.5 6.5 have been discovered in various surveys (e.g., Fan et al. 2006;Jiang et al. 2008;Mortlock et al. 2009;Willott et al. 2010a;Morganson et al. 2012;Bañados et al. 2014). Most of these quasars have been found by looking for sources with a large break between the optical i and z bands (e.g., Fan et al. 2006), the so-called i-band dropouts or i-dropouts. To find quasars beyondz 6.5, wide-field surveys with coverage beyond ∼1 μm are needed.
Currently, four quasars above > z 6.5 have been discovered in near-infrared surveys. Mortlock et al. (2011) presented a quasar at z = 7.1 discovered in the UK infrared Telescope Infrared Deep Sky Survey (UKIDSS; Lawrence et al. 2007), while Venemans et al. (2013) reported three quasars at < < z 6.6 6.9 from the Visible and Infrared Survey Telescope Kilo-Degree Infrared Galaxy (VIKING) survey. Detailed studies of these four > z 6.5 quasars have given insight into the properties of the universe less than a gigayear after the Big Bang. For example, the optical spectrum of the z = 7.1 quasar places constraints on the fraction of neutral hydrogen (Mortlock et al. 2011;Bolton et al. 2011), while Simcoe et al. (2012 use near-infrared spectroscopy to put limits on the metal enrichment ("metallicity") of the intergalactic medium (IGM) up toz 7. Furthermore, these quasars set a lower limit to the number density of supermassive black holes at > z 6.5: mans et al. 2013;De Rosa et al. 2014).
The limitation of both UKIDSS and VIKING is that they cover only small fractions of the extragalactic sky: UKIDSS has imaged ∼4000 deg 2 , while VIKING will cover an area of 1500 deg 2 . Overcoming this limitation, the Panoramic Survey Telescope & Rapid Response System 1 (Pan-STARRS1 or PS1; Kaiser et al. 2002Kaiser et al. , 2010 has imaged the whole sky above a declination of - 30 for about four years in five filters (g P1 , r P1 , i P1 , z P1 , and y P1 ; Stubbs et al. 2010;Tonry et al. 2012). The inclusion of the y P1 filter (central wavelength l = 9620 c Å, FWHM = 890 Å; Tonry et al. 2012) enables the search for luminous quasars at > z 6.5 over more than 20,000 deg 2 of extragalactic sky by selecting sources with a red z P1 -y P1 color (z-dropouts; see Figure 1). In this paper, we present the first results from our ongoing search for > z 6.5 quasars in the PS1 data.
All magnitudes are given in the AB system, and we adopt a cosmology with

The Pan-STARRS1 3π Survey
To search for z-dropouts, we made use of the first internal release of stacked PS1 imaging data (PV1). We selected our initial > z 6.5 quasar candidates as objects with a signal-tonoise ratio (S/N) > 7 yP1 and a non-detection in the g P1 , r P1 , and i P1 bands (i.e., S/N <3). We further required a break between the z P1 and y P1 bands (see also Figure 1 in Bañados et al. 2014): To avoid extended sources, we demanded that the difference between the y P1 -band PSF and aperture magnitudes (y ext ) be consistent within 0.3 mag. This value was determined by comparing y ext with spectroscopically confirmed stars and galaxies from the SDSS-III database 13 . Setting < y 0.3 ext selected the vast majority of stars (>85%), while rejecting a large fraction (>94%) of galaxies. Finally, we removed sources that were marked as likely spurious in the catalogs (see Bañados et al. 2014) or that had less than 85% of the expected flux in the z P1 or y P1 images on valid pixels. For bright sources (y P1 < 19.5), we applied the same criteria, but we relaxed our limits in the g P1 , r P1 , and i P1 bands, requiring S/ N < 5. The total number of z-dropouts selected from the PS1 catalogs after removing objects in the plane of the Milky Way and M31 ( <  b 20 and  < 7 R.A. <  14 ;  < 37 decl. <  43 ) was 328,372 (of which 13,093 had y P1 < 19.5).

Public Infrared Surveys
To extend and verify the photometry of the quasar candidates selected from the PS1 catalogs, we first matched the sources with several public infrared surveys.
UKIDSS: The PS1 candidates were matched with the nearinfrared data of the UKIDSS survey (Lawrence et al. 2007). The UKIDSS Large Area Survey (LAS) provides Y, J, H, and K imaging over ∼4000 deg 2 . We matched the PS1 z-dropout list with the catalogs from UKIDSS data release 14 10, using a search radius of 2″ .0. We identified objects as foreground interlopers if they had a -> Y J 0.6 or -> y J 1 P1 (which is typical for cool dwarfs; see, e.g., Best et al. 2013) and removed them from our candidate lists.
VHS: For objects in the area < 150 R.A. <  240 and - < 20 decl. <  0 , aperture photometry was performed on the Y, J, H, and K s images of the VISTA Hemisphere Survey  Objects with an S/N >3 in W1 and W2 were assigned a higher priority if their colors fulfilled the additional criteria: , which is loosely based on the PS1-WISE colors of brown dwarfs (e.g., Best et al. 2013). Objects with an S/N <3 in W1 or W2 were assigned an intermediate priority, and the remaining candidates were given a low priority.
For the~13,000 objects with a match in at least one of the above surveys, we performed forced photometry on the PS1 images to confirm the colors and non-detections (see Bañados et al. 2014). After visually inspecting the remaining ∼1000 candidates, we selected the best ∼500 objects that were our main targets for follow-up observations.

FOLLOW-UP OBSERVATIONS
To confirm the colors of the possible quasars and to remove lower-redshift interlopers, we imaged 194 z-dropout candidates during five observing runs. We obtained optical and infrared images between 2014 January 24 and 2014 August 13 with the MPG 2.2 m/GROND (Greiner et al. 2008), NTT/EFOSC2 (Buzzoni et al. 1984), NTT/SofI (Moorwood et al. 1998), and the Calar Alto 3.5 m/Omega2000 (Bailer-Jones et al. 2000); see Table 1 for the details of the observations.
Candidates were considered foreground interlopers if they had -> y J 1 P1 (see Section 2.2). Sources with a -<y J 1 P1 or undetected in J were rejected on the basis that they could be moving, varying, or a spurious object in the PS1 catalog. We reobserved objects with with the NTT in the filters I N and Z N . Only three sources remained undetected in I N or were red with  - . These objects were targets for spectroscopy. We obtained optical and near-infrared spectroscopy of all the three candidates that had good quasar colors after the follow-up imaging. We carried out spectroscopic observations between 2014 April 26 and 2014 December 6 using the following instruments: VLT/FORS2 (Appenzeller et al. 1998 Table 1. We reduced the data following standard reduction steps (e.g., Venemans et al. 2013;Bañados et al. 2014). We show the merged spectra in Figure 1. 4. THREE QUASARS AT > z 6.5 All three z-dropouts for which we obtained optical spectroscopy showed a strong continuum break in their optical spectrum ( Figure 1) and were identified as quasars at redshifts < < z 6.5 6.7. We fitted the continuum with three components: a power law with slope β ( l l b f ), a Balmer continuum, and an Fe II template (see, e.g., De Rosa et al. 2014). In all cases, the Balmer continuum was found to be negligible at the wavelengths we considered (  l 3000 rest Å). A single-Gaussian fit of the emission lines (most prominently C IV and Mg II) provided a sufficiently good model of the line profiles given the S/N of our spectra. Only in the spectrum of the brightest quasar, PSO J036.5078 + 03.0498, were we able to constrain on the Fe II emission (Section 4.2). The near-infrared spectra of the other two quasars did not have sufficient S/N. This did not significantly affect the fit of the Mg II lines in these quasars. The redshifts were determined by the peak of the Mg II line. Other bright emission lines, such as Si IV λ 1397 and C IV λ 1549, are blueshifted by 300-2000 km s −1 with respect to Mg II. Such shifts are similar to those measured in spectra of other distant luminous quasars (e.g., Richards et al. 2002;De Rosa et al. 2014).
We estimated black hole masses using the local scaling relation based on the Mg II line (Equation (1) in Vestergaard & Osmer 2009), which has a systematic uncertainty of a factor of ∼3. The black hole mass uncertainties quoted in Sections 4.1-4.3 and in Table 2 represent only statistical errors.
We computed bolometric luminosities by applying the bolometric correction obtained by Shen et al. (2008) to the monochromatic luminosity density measured at 3000 Å. The Eddington luminosity is defined as  Table 2. Below, we describe the new quasars in more detail.

PSO J167.6415-13.4960
The quasar PSO J167.6415-13.4960 (hereafter P167-13) was discovered based on forced photometry on VHS images at the positions of PS1 candidates. Our FORS2 discovery spectrum revealed a source with a strong continuum decrement around 9100 Å, and we identified the object as a quasar with a redshift ofz 6.52. From the near-infrared spectrum, we derive a redshift =  z 6.508 0.001    . The quality of the infrared spectrum is not sufficient to constrain the Fe II emission to better than s 2 . We measure Fe II/Mg II = 3.4 ± 1.7, fully consistent with previously discovered quasars at similar redshifts (e.g., . This signals an increase in f HI close to quasars at higher redshifts, although it is not straightforward to translate a change in R NZ to a change in f HI (e.g., Bolton & Haehnelt 2007).
We measured the near zones following the method described in Fan et al. (2006) and also employed by Carilli et al. (2010). The results are shown in Figure 2(a). We derive near-zone radii of 1.5, 3.1, and 5.2 Mpc (proper) for P167-13, P036 + 03, and P338 + 29, respectively. The uncertainty in the computed near zone (including the uncertainty in the quasar's systemic redshift derived from Mg II) is about 0.7 Mpc (Carilli et al. 2010 , 2.8, and 6.9 Mpc, respectively. In Figure 2(b), we compare the near zones of the PS1 quasars with those of < < z 5.7 7.1 quasars from the literature. The PS1 quasars roughly follow the trend of smaller near zones at higher redshifts. A weighted linear fit results in a relation =  -´-R z (7.2 0.2) (6.1 0.7) ( 6) NZ,corrected Mpc. Interpreting the decrease in R NZ as an increase in f HI , µ + --R z f (1 ) NZ 1 HI 1 3 (e.g., Fan et al. 2006; but see Bolton & Haehnelt 2007), the decrease in R NZ,corrected by a factor of 6.5 between z = 6 and z = 7 implies an increase in the neutral fraction of a factor of ∼180. Combined with a measured »´f 2 10 HI 4 atz 6 (e.g., Fan et al. 2006), this suggests » f 0.04 HI at z = 7, confirming the rapid evolution of f HI at > z 6 (e.g., Fan et al. 2006;Bolton et al. 2011). The large spread (a factor of ∼3) in (corrected) near-zone sizes between individual quasars indicates a wide range in quasar ages and/or a large variation in f HI along different lines of sight.

SUMMARY
We identified three new quasars at redshifts < < z 6.5 6.7 in PS1, nearly doubling the number of known > z 6.5 quasars from four to seven. The newly discovered quasars have a wide range of properties ( Table 2) . The brightest of the PS1 quasars is the most luminous quasar discovered at > z 6.5 so far (Figure 3), with a luminosity at 1450 Å close to  Mortlock et al. (2011) and were updated when more accurate redshifts have become available (Willott et al. 2010a;Venemans et al. 2012;Wang et al. 2013;Willott et al. 2013). The green stars illustrate the near-zone radii of the PS1 > z 6.5 presented here. The black line and shaded region show the weighted linear fit of =  -´-R z (7.2 0.2) (6.1 0.7) ( 6) NZ,corrected . The dashed line shows the weighted fit derived by Carilli et al. (2010) for quasars at  z 6.4.