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Galaxy setsdistance markA galaxy has smashd h cod fo h mos disan objc v obsvd.Th objc shds ligh on h nau of h soucs ha sid lcons fomhydogn aoms duing h ionizaion och. See Letter p.940

M i c h e l e T r e n T i

On page 940 of this issue, Lehnert et al.1descrbe follow-up spectroscopcobservatons that dentfy one galaxy,

on a patch of sky called the Hubble Ultra DeepFeld, as the most dstant astronomcal objectknown so far. The galaxy is located more than4 bllon parsecsfrom Earth, at a redshftz= 8.56. It breaks the previous redshift record,z= 8.2, held by a gamma-ray burst a one-off

powerful cosmc exploson that faded wthna few hours of ts peak lumnosty2,3. Prob-ably composed of about one bllon stars4, thegalaxy formed wthn 600 mllon years of theBig Bang, and will continueshining atthe levelwe see todayfor several tens of mllon yearsto come.

UDFy-38135539, as the galaxy is designated,was ntally classfed as a canddate galaxy ata redshft larger than 8 on the bass of mag-ng observatons5,6 made at optcal and near-nfrared wavelengths wth the Hubble SpaceTelescopes Wde Feld Camera 3 (Fg. 1). Infact, because of the absorpton of photons by

neutral hydrogen atoms n the ntergalactc

medium between the source and the observer,a galaxy at redshft zhas no observed flux atwavelengths shorter than the redshifted hydro-gen Lyman- line limit, which is 0.1216(1 +z)micrometres.

Imaging in multiple spectral bands is there-fore routinely used to identify candidate galax-ies at high redshift through the detection of theLyman- limit. But because the filters that aregenerally used have a broad passband, the loca-tion of the Lyman- limit is uncertain, and the

redshft estmate only approxmate (typcallythe error on the redshift estimate is about 0.5).In addition, there can be other sources suchas brown-dwarf stars and low-redshft galax-es wth unusual propertes that mmc thetypical broadband properties of high-redshiftobjects. Spectroscopc observatons are thusrequred to confrm the redshft estmatesobtained from imaging.

Lehnert et al.1 used the SINFONI IntegralFeld Unt Spectrograph on the Very LargeTelescope in Chile to observe UDFy-38135539for a total of 16 hours. From the resultngspectrum, they detected, wth robust statst-

cal confdence (6 sgma), an emsson lne at

can be produced from a three-dmensonallattce of slca nanospheres 40 nanometresn dameter. To explan ths, the authors sug-gest that partal cleavage of the lattce mghtoccur because of the great strain exerted on thelattce as gold structures grow nsde t. Thspartial cleavage creates a two-dimensional crackthat acts as a template for the growth of nano-

metre-thick plates of gold. Hexagonal arrays ofdmples form on both sdes of the gold nano-plate, caused by mprnts of the spheres thatmake up the surfaces of the crack. The authorswent on to construct herarchcal assemblesn whch gold nanopartcles were postonedin the dimples of the plates, so generating two-dimensional periodic arrays of nanoparticles.

Kuroda and Kuroda obtained a microscopymage of a gold nanoplate stll attached to asilica template (Fig. 1c), which shows dimpleson the exposed gold surface whose arrange-ment corresponds well with that of the under-lyng slca spheres. The authors nterpret

ths as evdence that partal cleavage of thecrystallne lattce of slca nanospheresoccurred smultaneously wth the growth ofthe nanoplate, rulng out the possblty thatthe plate could have formed elsewhere andthen moved to its present position. But anotherpossblty s that gold s deposted along pre-formed, planar cracks in the crystalline latticethat can easly develop when the slca nano-sphere assembly s dred before ts use as atemplate. The mechansm responsble for theformation of cracks in the template lattice, andthus for the development of surface-patternednanoplates, deserves further investigation.

A similar example of template fragmentationduring synthesis has previously been observed7in the formation of gold nanostructures withina three-dmensonal lattce constructed fromron nanospheres assembled on a magnetcstrrer bar. As the volume of gold n the nter-sttal space of the lattce grew, the magnetcattracton between the ron nanospheresgradually weakened untl the template lattcespontaneously dsassembled, leavng behndbranched gold nanostructures.

The spontaneous cleavage and/or ds-assembly of a colloidal lattice template duringthe replication process of a template-mediatedsynthess offers an ntrgung way of prepar-

ng unconventonal metal nanostructures.Manpulaton of the stress and stran n thelattce may enable one to control the surfacetopography of a replcated nanostructure, orto generate nanostructures that have new mor-phologies. Smart templates, whose structuresare remodelled or disassembled spontaneouslyand advantageously durng synthess, couldbe used for preparng metal nanostructuresfor applcatons ncludng catalyss, sensng,imaging and device fabrication. The syntheticstrategy could also be extended to other mater-ials, such as alloys, semiconductors, polymers,carbon and composites.

Leavng asde any smart features, collodal

lattice templates will also be useful for a rangeof fundamental studies examining the nuclea-ton and growth of nanostructures. Metalnanostructures prepared from solutons aregenerally considered to grow by the addition ofatoms to seed structures that form naturally insolution, and usually exhibit polyhedral shapessuch as a truncated octahedron. Ansotropc

growth of metal nanostructures n soluton soften explaned by the preferental growth ofseeds along specfc drectons, whch can beguded by the presence of cappng agents orpolymerc stablzers1. The chemcal reduc-ton of a metal salt nsde a collodal lattcetemplate as used by Kuroda and Kuroda2 presents an nterestng system for study-ng the nucleaton and growth of metal nano-structures n hghly confned spaces. In suchsystems, metal nanostructures may follow adifferent growth pathway from those observedn soluton. A deeper understandng of themechansms underlyng the nucleaton and

growth of metal nanostructures wthn andbeyond the confnement of a collodal lattce

template might make it possible to design andsynthesize nanostructures more complex thananything currently available.

Younan Xia is in the Department ofBiomedical Engineering, WashingtonUniversity, St Louis, Missouri 63130,USA. Byungkwon Lim is in the School of

Advanced Materials Science and Engineering,Sungkyunkwan University, Suwon 440-746,South Korea.e-mails: yxia@biomed.wustl.edu;blim@skku.edu

1. Xia, Y., Xiong, Y., Lim, B. & Skrabalak, S. E.Angew.Chem. Int. Edn48,60103 (2009).

2. Kuroda, Y. & Kuroda, K.Angew. Chem. Int. Edn49,69936997 (2010).

3. Martin, C. R.Acc. Chem. Res.28,6168 (1995).4. Velev, O. D., Tessier, P. M., Lenhoff, A. M. & Kaler,

E. W. Nature401, 548 (1999).5. Johnson, S. A., Ollivier, P. J. & Mallouk, T. E. Science

283, 963965 (1999).6. Stein, A. & Schroden, R. C.Curr. Opin. Solid State

Mater. Sci.5, 553564 (2001).

7. Li, Z., Li, W., Camargo, P. H. C. & Xia, Y.Angew. Chem.Int. Edn47,96539656 (2008).

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1.1616 m, whch they attrbute to hydrogenLyman- emsson from the galaxy. From thewavelength of the lne, they derved a redshftfor the galaxy ofz= 8.56. Because the flux ofthe emsson lne s near the detecton lmtof SINFONI, the authors performed severalstatstcal tests to demonstrate that the lnehas an astrophyscal orgn. However, there s

a very small possblty that the lne has beenmsattrbuted to hydrogen Lyman- ems-son. Lehnert et al. exclude only at 99.9%confdence the hypothess that the lne s anunresolved oxygen ii doublet at 0.3727 m whch would mply a redshft z= 2.12 forthe galaxy.

Lehnert and colleagues result1 representsa fundamental leap forward n observatonalcosmology. Not only s UDFy-38135539 thehghest-redshft object and galaxy dentfedso far (the previous record holder7 for a galaxyredshft was z= 6.96), t s also the frst galaxyknown to have lved fully wthn the epoch of

reonzaton, whch occurred wthn 600 ml-lon years of the Bg Bang. Durng ths epoch,radiation from the very first objects in the Uni-verse s thought to have strpped electrons offhydrogen atoms created durng the Bg Bang.Lehnert et al. infer the existence of a bubble ofonzed hydrogen gas, wth a radus of at least1 Mpc, surroundng UDFy-38135539. Suchbubbles allow the Lyman- photons sufficienttime to escape and redshift away from absorp-ton frequences before reachng a neutralintergalactic medium.

The radius of the bubble is larger than the onethe galaxy could have carved out by itself on thebasis of its luminosity. This indirectly confirmsthat onzng sources n the epoch of reonza-tion are primarily small-mass galaxies that areexpected to cluster around bigger siblings suchas UDFy-38135539 (refs 8, 9). But this inferenceon the nature of the onzng sources s nd-rect and based on a sngle source. Alternatveexplanations include a bubble of non-sphericalgeometry that s elongated along the lne ofsght. The escape of Lyman- radaton couldalso be facilitated by the presence of large-scalegalactc-gas outflows (wth veloctes of up to500 km s1) that shift the frequency of Lyman-photons at the time of emission10,11.

The success of the authors study1 opens up

exctng prospects for spectroscopy of hgh-redshft objects. UDFy-38135539 s a farlytypcal galaxy for ts epoch, beng the thrdmost luminous of five possible objects at a red-shft larger than 8 wthn the small area of theHubble Ultra Deep Feld5. Brghter, and thuseaser to follow up spectroscopcally, cand-dates at a redshift of about 8 will also probablybe dentfed by the Hubble CANDELS sur-vey12, whch has just started. However, futureobservatons wll not necessarly be straght-forward. First, not every galaxy at a redshift ofabout 8 that is as luminous as UDFy-38135539has an equally strong Lyman- emission. Sec-

ond, Lehnert and colleagues estimate that there

s only a 50% chance of detectng a Lyman-emission line similar to that of UDFy-38135539from ground-based observatons. Ths sbecause the nose level n the observatons swavelength-dependent owing to the presenceof atmospheric emission lines.

Later n ths decade, the next generatonof ground-based, 30-metre-class telescopesand the James Webb Space Telescope (JWST)wll overcome many of these observatonalchallenges. For example, the JWST NIRSpecspectrograph will be able to detect the Lyman-emsson from galaxes at z= 89 wth lessthan 10,000 seconds of sky-exposure tme,and at a hgher sgnal-to-nose rato than theSINFONI spectrograph, without being affectedby noise introduced by Earths atmosphere. Butastronomers should not have to wat for newtelescopes to study the topology and nature ofthe sources responsble for cosmc reonza-

ton. As Lehnert et al. have demonstrated, t

s well worth pursung spectroscopc dent-fcaton of hgh-redshft objects wth currentground-based instruments.

Michele Trenti is in the Center for Astrophysicsand Space Astronomy, University of Colorado,

Boulder, Colorado 80309-0389, USA.e-mail: michele.trenti@colorado.edu

1. Lehnert, M. D. et al. Nature 467, 940942 (2010).2. Tanvir, N. R. et al.Nature461, 12541257 (2009).3. Salvaterra, R. et al.Nature461, 12581260 (2009).4. Stark, D. P. et al.Astrophys. J.697, 14931511

(2009).5. Bouwens, R. J. et al.Astrophys. J.709, L133L137

(2010).6. McLure, R. J. et al.Mon. Not. R. Astron. Soc.403,

960983 (2010).7. Iye, M. et al.Nature443, 186188 (2006).8. Zahn, O. et al. Astrophys. J.654, 1226 (2007).9. Trenti, M. et al.Astrophys. J.714, L202L207 (2010).10. Steidel, C. C.Astrophys. J.717, 289322 (2010).11. Dijkstra, M. & Wyithe, J. S. B. Mon. Not. R. Astron. Soc.

doi:10.1111/j.1365-2966.2010.17112.x (2010).

12. www.stsci.edu/cgi-bin/get-proposal-info?12060

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B O U W E N S ( U C O / L I C K O B S E R V & L E I D E N U N I V ) / H U D F 0 9 T E A M

Figure 1 | The most distant galaxy yet. a, The Hubble Ultra Deep Field, the deepest image of theUniverse ever taken, in near-infrared light. b, Composite near-infrared Hubble image5 of galaxy UDFy-38135539, which was identified by Lehnert et al.1 on the basis of follow-up spectroscopic observations asthe most distant object ever observed. c, Simulated spectrum of the galaxy, assuming that it has a stellarpopulation with an age of 100 million years. From the exact location of the hydrogen Lyman- emissionline in the observed spectrum (see Fig. 1 on page 941), the authors1 derive a redshift of 8.56 for the galaxy.

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