An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30Xue-Bing Wu1,2, Feige Wang1,2, Xiaohui Fan2,3, Weimin Yi4,5,6, Wenwen Zuo7, Fuyan Bian8, Linhua Jiang2, Ian D. McGreer3,Ran Wang2, Jinyi Yang1,2, Qian Yang1,2, David Thompson9 & Yuri Beletsky10
So far, roughly40quasarswith redshifts greater thanz5 6havebeendiscovered1–8. Eachquasar contains a black holewith amass of aboutone billion solarmasses (109M[)2,6,7,9–13. The existence of such blackholes when the Universe was less than one billion years old presentssubstantial challenges to theoriesof the formationandgrowthofblackholes and the coevolution of black holes and galaxies14.Herewe reportthediscoveryofanultraluminousquasar, SDSSJ010013.021280225.8,at redshift z5 6.30. It has an optical and near-infrared luminosity afewtimesgreater than thoseofpreviouslyknownz.6quasars.Onthebasis of thedeepabsorption trough15 on the blue sideof the Lyman-aemission line in the spectrum,we estimate the proper size of the ion-izedproximity zoneassociatedwith thequasar tobeabout26millionlight years, larger than found with other z. 6.1 quasars with lowerluminosities16.Weestimate (on thebasisof anear-infrared spectrum)that the blackholehas amass of 1.23 1010M[, which is consistentwith the 1.33 1010 M[ derived by assuming an Eddington-limitedaccretion rate.High-redshift quasars have been efficiently selected using a combi-
nation of optical and near-infrared colours3,4. We have carried out asystematic survey of quasars at z. 5 using photometry from the SloanDigital Sky Survey (SDSS)17, the twoMicronAll Sky Survey (2MASS)18
and theWide-field Infrared Survey Explorer (WISE)19, resulting in thediscoveryof a significant populationof luminoushigh-redshift quasars.SDSS J010013.021280225.8 (hereafter J010012802) was selected as ahigh-redshift quasar candidate owing to its red optical colour (withSDSSABmagnitudes iAB5 20.846 0.06 and zAB5 18.336 0.03) anda photometric redshift of z< 6.3. It has bright detections in the 2MASSJ, H and Ks bands with Vega magnitudes of 17.006 0.20, 15.986 0.19and 15.206 0.16, respectively; it is also strongly detected inWISE,withVega magnitudes in W1 to W4 bands of 14.456 0.03, 13.636 0.03,11.716 0.21 and 8.986 0.44, respectively (see Extended Data Figs 1and 2 for images in different bands). Its colour in the two bluestWISEbands,W1 andW2, clearly differentiates it from the bulk of stars in ourGalaxy20. The object was within the SDSS-III imaging area. It is close tothe colour selection boundary of SDSS z< 6 quasars1, butwas assignedto low priority earlier because of its relatively red zAB2 J colour and itsbright apparent magnitudes. It is undetected in both radio and X-raybands by the wide-area, shallow survey instruments.Initial optical spectroscopy on J010012802 was carried out on 29
December 2013 with the Lijiang 2.4-m telescope in China. The low-resolution spectrum clearly shows a sharp break at about 8,800 A, con-sistent with a quasar at a redshift beyond 6.2. Two subsequent opticalspectroscopic observations were conducted on 9 and 24 January 2014respectively with the 6.5-mMultipleMirror Telescope (MMT) and thetwin 8.4-m mirror Large Binocular Telescope (LBT) in the USA. TheLyman-a (Lya) line shown in the spectra confirms that J010012802 isa quasar at a redshift of 6.306 0.01 (see Fig. 1 andMethods for details).
Weuse themultiwavelengthphotometry to estimate theoptical lumi-nosity at rest-framewavelength 3,000 A (L3,000),which is consistentwiththat obtained fromK-band spectroscopy (see below). The latter gives amore reliable value of (3.156 0.47)3 1047 erg s21, adopting aLCDMcosmologywithHubble constantH05 70 km s21Mpc21, matter den-sity parameterVM5 0.30 anddark energydensityparameterVL5 0.7.Assuminganempirical conversion factor fromthe luminosity at 3,000 Ato the bolometric luminosity21, this gives Lbol5 5.153 L3,0005 1.6231048 erg s215 4.293 1014 L[ (where L[ is the solar luminosity). Weobtain a similar resultwhen estimating the bolometric luminosity fromthe Galactic extinction corrected absolute magnitude at rest-frame1,450 A, which is M1450,AB5229.266 0.20. The luminosity of this
1Department of Astronomy, School of Physics, Peking University, Beijing 100871, China. 2Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China. 3Steward Observatory,University of Arizona, Tucson, Arizona 85721-0065, USA. 4Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China. 5University of Chinese Academy of Sciences, Beijing 100049,China. 6Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650011, China. 7Shanghai Astronomical Observatory, Chinese Academy of Sciences,Shanghai 200030, China. 8Mount Stromlo Observatory, Research School of Astronomy and Astrophysics, Australian National University, Weston Creek, Australian Capital Territory 2611, Australia. 9LargeBinocular Telescope Observatory, University of Arizona, Tucson, Arizona 85721, USA. 10Las Campanas Observatory, Carnegie Institution of Washington, Colina el Pino, Casilla 601, La Serena, Chile.
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Figure 1 | The optical spectra of J010012802. From top to bottom, spectrataken with the Lijiang 2.4-m telescope, the MMT and the LBT (in red, blueand black colours), respectively. For clarity, two spectra are offset upward byone and two vertical units. Although the spectral resolution varies from verylow to medium, in all spectra the Lya emission line, with a rest-framewavelength of 1,216 A, is redshifted to around 8,900 A, giving a redshift of 6.30.J010012802 is a weak-line quasarwith continuum luminosity about four timeshigher than that of SDSS J114815251 (in green on the same flux scale)1,which was previously the most luminous high-redshift quasar known atz5 6.42.
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quasar is roughly 4 times greater than that of the luminous z5 6.42quasar1 SDSS J114815251, and 7 times greater than that of the mostdistant known quasar6 ULAS J112010641 (z5 7.085); it is the mostluminous quasar known at z. 6 (see Extended Data Fig. 3).The rest-frame equivalent width of the Lya1NV emission lines as
measured from the LBT spectrum is roughly 10 A, suggesting thatJ010012802 is probably a weak-line quasar (WLQ)22. The fraction ofWLQs is higher among the z< 6 quasars compared to those at lowerredshift8, and ahighdetection rate of strongmillimetre dust continuumin z< 6WLQs points to active star formation in these objects23. Givenits extreme luminosity, J010012802 will be helpful in the study of theevolutionary stage of WLQs by future (sub)millimetre observations,though the origin of theweak ultraviolet emission line feature ofWLQsis still uncertain.The LBT spectrum of J010012802 (Fig. 1) exhibits a deep Gunn–
Peterson absorption trough15 blueward of the Lya emission. The trans-mission spectrum (assuming an intrinsic power-law continuum24 ofFl / l21.5, where Fl is the flux density at wavelength l) is shown inFig. 2.CompleteGunn–Petersonabsorption canalsobe seen in theLya,Lyb and Lyc transitions. Statistically significant transmission peaks aredetected at z5 5.99 in both the Lya andLyb troughs, and an additionaltransmissionpeak is detected at z5 5.84 in theLyb trough.The2s lowerlimit on the Lya Gunn–Peterson optical depth (ta) at z5 6.00–6.15 ista. 5.5 and the 2s lower limit for Lyb is tb. 6, corresponding to anequivalent ta. 13.5, following the conversion in literature16. The char-acteristics of the intergalactic medium (IGM) transmission along theline of sight of J010012802, including the deep Lya and Lyb troughs,and the narrow, unresolved transmission peaks, are similar to thoseobserved in SDSS J114815251, andare consistentwith the rapid increase
in the IGMneutral fraction at z. 5.5 observed in a large sample of SDSSquasars16. The size evolution of the quasar proximity zone, which ishighly ionized by quasar ultraviolet photons, can also be used to con-strain the IGMneutral fraction.The size of theproximity zone is definedby the point where the transmitted flux first drops by a significantamount to below 10% (ignoring small absorption leaks) of the quasarextrapolated continuum emission after the spectrum is smoothed to aresolution of 20 A (ref. 16).As shown inFig. 2, J010012802 has amuchlargerproperproximity zone (7.96 0.8Mpc; 1Mpc is about 3.26millionlight years) than that of other SDSSquasars16,25 at z. 6.1; its largeprox-imity zone size is expected from the higher level of photo-ionizationdominated by quasar radiation.We obtained the near-infrared J,H,K-band spectra with Gemini and
Magellan telescopes on 6 August and 7 October 2014, respectively (seeMethods for details). Figure 3 shows the combined optical/near-infraredspectrum of J010012802 and the results of fitting the Mg II emis-sion line. The Mg II full-width at half-maximum (FWHM) is 5,1306150km s21, and the continuum luminosity at the rest-framewavelengthof 3,000 A is (3.156 0.47)3 1047 erg s21. After applying a virial black-holemass estimator based on theMg II line26, we estimate its black-holemass to be (1.246 0.19)3 1010M[. Theuncertaintyofblack-holemassdoes not include the systematic uncertainty of virial black-hole massestimation, which could be up to a factor of three27. Assuming that thisquasar is accreting at the Eddington accretion rate and the bolometricluminosity is close to the Eddington luminosity (LEdd5 1.33 1038
(M/M[)), similar to other z. 6 quasars11, leads to a black-hole mass of
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Figure 2 | Transmission in absorption troughs and the proximity zone forJ010012802. a, b, Transmission in Lya and Lyb absorption troughs(respectively a, red; b, blue) were calculated by dividing the spectrum by apower-law continuum24, Fl / l21.5. The shaded band in both panels shows1s standard deviation. The Lya and Lyb absorption redshifts are given byl/lLya(Lyb)2 1,where lLya5 1,216 A andlLyb5 1,026 A. The optical spectrumexhibits a deep Gunn–Peterson trough and a significant transmission peak atz5 5.99. c, Transmission in the proximity zone. The proper proximity zonefor J010012802 (in black) extends to 7.96 0.8Mpc, a much larger value thanthose of other z. 6.1 quasars, including 4.96 0.6Mpc for J114815251 (ingreen), consistent with its higher ultraviolet luminosity. The transmission incwas calculated by dividing themeasured spectrumby a power-law continuumFl / l21.5 plus two Gaussian fittings of Lya and NV lines. The horizontaldotted line and the twodashed lines denote transmission values of 0, 0.1 and 1.0respectively, while the vertical dashed line denotes the proper proximity zonesize of 0.
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Figure 3 | The combined optical/near-infrared spectrum of J010012802and the fitting of the Mg II line. Main panel, the black line shows the LBToptical spectrum and the red line shows the combined Magellan and Gemininear-infrared J,H,K-band spectra (from left to right, respectively). The gapsbetween J and H and between H and K bands are ignored due to the low skytransparency there. The magenta line shows the noise spectrum. The mainemission lines Lya, C IV and Mg II are labelled. The details of the absorptionlines are described in Extended Data Fig. 4. Inset, fits of the Mg II line (withFWHM of 5,1306 150 km s21) and surrounding Fe II emissions. The green,cyan and blue solid lines show the power law (PL), Fe II andMg II components.The black dashed line shows the sum of these components in comparison withthe observed spectrum, denoted by the red line. The black-hole mass isestimated to be (1.246 0.19)3 1010 M[.
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1.33 1010M[ for J010012802.Therefore, ourobservations strongly indi-cate that J010012802 harbours a black hole ofmass about 1.23 1010M[,the first such system known at z. 6, though black holes of such a sizehave been found in local giant elliptical galaxies28 and low-redshiftquasars21.Although gravitational lensing is a possible explanation for the high
luminosity of J010012802, we do not expect a large lensingmagnifica-tion.AnLBTK-band imagewith seeingof 0.40 shows amorphology fullyconsistent with a single point source (Extended Data Fig. 2); and thelarge size of the quasar proximity zone further supports a high ultra-violet luminosity consistentwith the expected photoionization scaling29.However, absorption features at different redshift have been identifiedfrom its near-infrared spectroscopy (Extended Data Fig. 4), implyingthe existence of abundant intervening materials along the line of sight.J010012802 is the only known quasar with a bolometric luminosity
higher than 1048 erg s21 and a black-holemass larger than 53 109M[at z$ 6. It is also close to being the most luminous quasar with themost massive black hole at any redshift (Fig. 4). The discovery of thissingle ultraluminous quasarwithin the entire SDSS footprint (,13,000degrees2) is broadly consistentwith the extrapolation of the SDSS z< 6quasar luminosity function16. Thenumber density of suchobjectswouldset strong constraints on the early growth of supermassive black holesand the evolutionof thehigh-redshift quasarblack-holemass function5,11.In addition to ULAS J112010641 with a 23 109 M[ black hole6,13 atz5 7.085, and a recently discovered z5 6.889 quasar with a black holeof 2.13 109M[ (ref. 13), J010012802with a 1.23 1010M[ black holeat z5 6.30presents thenextmost significant challenge to theEddington-limited growthof blackholes in the earlyUniverse11,14. Its existence alsostrengthens the claim that supermassive black holes in the early Uni-verse probably grew much more quickly than their host galaxies, asargued from amolecular gas study of z< 6 quasars30. Therefore, as themost luminous quasar known to date at z. 6, J010012802 will be a
unique resource for the future study of mass assembly and galaxy for-mation around themostmassive black holes at the end of the epoch ofcosmic reionization16.
Online Content Methods, along with any additional Extended Data display itemsandSourceData, are available in theonline versionof thepaper; referencesuniqueto these sections appear only in the online paper.
Received 17 June 2014; accepted 15 January 2015.
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19. Wright, E. L. et al. The Wide-field Infrared Survey Explorer (WISE): missiondescription and initial on-orbit performance. Astron. J. 140, 1868–1881(2010).
20. Wu, X.-B. et al. SDSS quasars in the WISE preliminary data release andquasar candidate selection with optical/infrared colours. Astron. J. 144, 49–59(2012).
21. Shen, Y. et al. A catalog of quasar properties from Sloan Digital Sky Survey datarelease 7. Astrophys. J. 194 (suppl.), 45–65 (2011).
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Supplementary Information is available in the online version of the paper.
Acknowledgements X.-B.W. thanks the NSFC (grant nos 11033001 and 11373008),the Strategic Priority Research Program ‘The Emergence of Cosmological Structures’of the Chinese Academy of Sciences (grant no. XDB09000000), and the National KeyBasic Research Program of China (grant no. 2014CB845700) for support. X.F., R.W.and I.D.M. thank theUSNSF (grant nosAST08-06861andAST11-07682) for support.R.W. thanks the NSFC (grant no. 11443002) for support. We acknowledge the supportof the staff of the Lijiang2.4-m telescope. Funding for the telescopewasprovidedby theChinese Academy of Sciences and the People’s Government of Yunnan Province. This
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Figure 4 | Distribution of quasar bolometric luminosities, Lbol, and black-hole masses, MBH, estimated from the Mg II lines. The red circle at top rightrepresents J010012802. The small blue squares denote SDSS high-redshiftquasars2,10,12, and the large blue square represents J114815251. The greentriangles denote CFHQS high-redshift quasars11,12. The purple star denotesULAS J112010641 at z5 7.085 (ref. 6). Black contours (which indicate 1s to5s significance from inner to outer) and grey dots denote SDSS low-redshiftquasars21 (with broad absorption line quasars excluded). Error bars representthe 1s standard deviation, and the mean error bar for low-redshift quasarsis presented in the bottom-right corner. The dashed lines denote the luminosityin different fractions of the Eddington luminosity, LEdd. Note that the black-hole mass and bolometric luminosity are calculated using the same methodand the same cosmology model as in the present Letter, and the systematicuncertainties (not included in the error bars) of virial black-hole masses couldbe up to a factor of three27.
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research uses data obtained through the Telescope Access Program (TAP), which hasbeen funded by the Strategic Priority Research Program ‘The Emergence ofCosmological Structures’ (grant no. XDB09000000), National AstronomicalObservatories, ChineseAcademyofSciences, and theSpecialFund forAstronomy fromthe Ministry of Finance of China. We thank D. Osip for help with Magellan/FIREspectroscopy, and Y.-L. Ai, L. C. Ho, Y. Shen and J.-G. Wang for suggestions about dataanalyses. We acknowledge the use of SDSS, 2MASS and WISE data, and of the MMT,LBT,Gemini andMagellan telescopes;detailedacknowledgments of these facilities canbe found in Supplementary Information.
Author Contributions X.-B.W., F.W. and X.F. planned the study, and wrote the draftversion of the paper. All other co-authors contributed extensively and equally to theobservations, data analyses and writing of the manuscript.
Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Readers are welcome to comment on the online version of the paper.Correspondence and requests for materials should be addressed toX.-B.W. ([email protected]).
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METHODSThe optical spectroscopy on J010012802 was first carried out on 29 December2013 with the Yunnan Fainter Object Spectrograph and Camera (YFOSC) of theLijiang 2.4-m telescope in China. We used a very low resolution grism (G12, at adispersion of 18 A perpixel) and took 3,000 s exposureon this target. The spectrumclearly showsa sharpbreak at about 8,800 A andno significant emissions blueward,consistent with a quasar spectrum at a redshift beyond 6.2. To confirm this discov-ery, two subsequent optical spectroscopic observations were obtained on 9 and 24January 2014with the 6.5-mMultipleMirrorTelescope (MMT)and the twin 8.4-mmirrorLargeBinocularTelescope (LBT) in theUSA, respectively.The lowtomediumresolution spectra, obtainedwith 1,200 s exposure using theMMTRedChannel (at adispersionof 3.6 A per pixel) and2,400 s exposurewith the LBTMulti-ObjectDoubleCCDSpectrographs/Imagers (MODS)31 (at a dispersion of 1.8 A per pixel) respec-tively, explicitly confirm that SDSS J010012802 is a quasar at redshift 6.306 0.01(obtained by the Lya line).The near-infrared K-band spectroscopy on J010012802 was carried out with
LBT/LUCI-1 on 2 January 2014. Owing to the short exposure time (15min), thespectrumisofmodest signal-to-noise ratio (S/N).Although theMg II linewas clearlydetected, the noisy LBT spectrum did not allow us to accurately measure the linewidth. To improve the quality of the near-infrared spectrum, we obtained J,H,K-band spectroscopy with Gemini/GNIRS and Magellan/FIRE on 6 August and7 October 2014, respectively. The exposure time was 3,600 s for GNIRS and 3,635 sfor FIRE.TheFIRE spectrumhashigher S/N (about 30 inKband) andhigher spectralresolution (R5 l/Dl< 6,000) than the GNIRS spectrum (with S/N of about 10 inK band and R< 1,800). In order to achieve the best spectral quality, we combinedthe FIRE and the GNIRS spectra, and scaled the combined spectrum according toits 2MASS J,H,Ks-bandmagnitudes. TheMg II line shown in theK-band spectrumgives the same redshift as that given by the Lya line in the optical spectrum. Thehigh-quality J,H,K-band spectra also clearly display abundant absorption features,which have been identified as being from intervening or associated systems withredshifts from 2.33 to 6.14 (Extended Data Fig. 4).After redshift andGalactic extinction corrections, the rest-frameH- andK-band
spectrum isdecomposed into a pseudo-continuumand theMg II emission line.Thepseudo-continuum consists of a power-law continuum and Fe II emissions, and
is fitted within the rest-frame wavelength range between 2,000 A and 3,200 A byexcluding the boundary regionbetweenHandKbandswhere the sky transparencyis lower. An Fe II template32,33 is adopted for the fitting of Fe II emissions. TheMg II
emission line is fittedwith twobroadGaussian components. The fourMg II absorp-tion linesnear the redderpart of theMg II emission line are also fitted as fourGaussianlines in order to remove their effects on the fittings ofMg II and Fe II emission lines.The overall FWHMof theMg II emission line is,5,130 kms21with anuncertaintyof 150 kms21. The continuumhas a slope of21.43 and the continuum luminosityat the rest-framewavelength of 3,000 A (L3,000) is (3.156 0.47)3 1047 erg s21. TheFe II to Mg II line ratio is 2.566 0.18, which is consistent with the mean value ofother z. 6 quasars12,13. After applying a virial black-holemass estimator based onthe Mg II line26, we estimate its black-hole mass to be (1.246 0.19)3 1010 M[.Although the systematic uncertainty of virial black-holemass estimation can be upa factor of three27, our result still strongly indicates that J010012802 hosts a centralblack hole with mass close to 1.23 1010 M[. This is also well consistent with ablack-hole mass obtained by assuming an Eddington luminosity of J010012802,which leads to a mass of 1.33 1010 M[. Considering the contribution of Balmercontinuum, as done for other z. 6 quasars12,13, leads to a decrease of L3,000 to(2.906 0.44)3 1047 erg s21, an increase of FWHMofMg II to 5,3006 200 km s21
and yields a black-hole mass of (1.266 0.21)3 1010 M[. Therefore, the effect ofconsidering Balmer continuum is insignificant for the black-hole mass measure-ment of J010012802. In addition, if we adopt a different virial black-hole massscaling relation34, the black-hole mass changes to (1.0760.14)3 1010M[, whichis still well consistent with the result we obtained above.Sample size. No statistical methods were used to predetermine sample size.
31. Pogge, R.W. et al. Themulti-object double spectrographs for the Large BinocularTelescope. Proc. SPIE 7735, 9–16 (2010).
32. Vestergaard,M. &Wilkes, B. J. An empirical ultraviolet template for iron emissionin quasars as derived from I Zwicky 1. Astrophys. J. Suppl. Ser. 134, 1–33 (2001).
33. Salviander, S.et al.Theblack holemass-galaxybulge relationship forQSOs in theSloan Digital Sky Survey data release 3. Astrophys. J. 662, 131–144 (2007).
34. Vestergaard,M. & Osmer, P. S. Mass functions of the active black holes in distantquasars from the Large Bright Quasar Survey, the Bright Quasar Survey, and thecolor-selected sample of the SDSS Fall Equatorial Stripe. Astrophys. J. 699,800–816 (2009).
Extended Data Figure 1 | Images of J010012802 in SDSS, 2MASS andWISE bands. J010012802 is undetected in SDSS u,g,r bands (top row) but isrelatively bright in other bands (lower three rows). It is consistent with a point
source in the bands with high signal-to-noise detections. The size is 193 19 forall images. The green circle represents an angular size of 100 in each image.
Extended Data Figure 2 | The LBT K-band image of J010012802. The sizeis 1003 100. The horizontal and vertical axes denote the offsets in rightascension (DRA) and in declination (DDec.). The image, with seeing of 0.40,shows a morphology fully consistent with a point source.
Extended Data Figure 3 | The rest-frame spectral energy distributions ofJ010012802, J114815251 and ULAS J112010641. The redshifts of thesethree quasars are 6.30, 6.42 and 7.085, respectively. The luminosity ofJ010012802 in theultraviolet/optical bands is about four times higher than that
of J114815251, and seven times higher than that of ULAS J112010641. Thephotometric data are from literature for J114815251 and J112010641. Theerror bars show the 1s standard deviation.
Extended Data Figure 4 | The major absorption features identified fromoptical and near-infrared spectroscopy of J010012802. Most of them arefromMg II, C IV and Fe II. The labels fromA toH correspond to the redshifts of
absorption materials at 6.14, 6.11, 5.32, 5.11, 4.52, 4.22, 3.34 and 2.33,respectively. Studies of intervening and associated absorption systems will bediscussed elsewhere.
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