arXiv:0802.2506v2 [astro-ph] 13 May 2008 atmr,M 21218. MD Baltimore, Germany. Garching, D-85748 2, Valpara´ıso, Breta˜na Chile. 1111, Gran Chile. para´ıso, 22, Av. Santiago 306, Casilla Chile, Cat´olica de dad uln9 Ireland. 9, Dublin edn Netherlands. Leiden, h asasml n ulu fteeris galaxies earliest to the insights of buildup significant and provided assembly searches, high-redshift mass recently such the of have forefront the which at been has 97,Israel. 69978, nteuies.The galaxies universe. earliest the the of in characterization the is cosmology /ern 1-i,206 ard Spain Madrid, 28006, 113-bis, C/Serrano ona ai,C 95616. CA Davis, fornia, rpyis nvriyo aionaSnaCu,SnaCru Santa Cruz, Santa California 95064. of University trophysics, iy 40NrhCalsSre,Blioe D21218. MD Baltimore, Street, Charles North 3400 sity, h e rplinLbrtr,Clfri nttt fTec of 1407. Institute contract California NASA Laboratory, under Propulsion Jet observa the the on with Based Res made NAS5-26555. for tions contract Universities of NASA Association under the Astronomy by operated is which Telescope Space 12 11 10 9 8 7 6 5 4 3 2 1 n ftems motn rnir fobservational of frontiers important most the of One h srpyia ora,686764 08My10 L May using 2008 typeset 678:647–654, Preprint Journal, Astrophysical The eatet eAtoo´ayAto´sc,PnicaUniv Astrof´ısica, Astronom´ıa Pontificia y de Departmento eateto hsclSine,Dbi iyUniversity, City Dublin Sciences, Physical of Department ednOsraoy ednUiest,Psbs91,2300 9513, Postbus University, Leiden Observatory, Leiden colo hsc n srnm,TlAi nvriy e Av Tel University, Aviv Tel Astronomy, and Physics of School nttt eMtmaia ´sc udmna (CSIC), Fundamental F´ısica y Matem´aticas de Instituto eateto hsc,1SilsAeu,Uiest fCali of University Avenue, Shields 1 Physics, of Department C/ikOsraoy eateto srnm n As- and Astronomy of Department Observatory, UCO/Lick eateto hsc n srnm,JhsHpisUniver- Hopkins Johns Astronomy, and Physics of Department ae nosrain aewt h NASA/ESA the with made observations on Based ..Bradley L.D. pc eecp cec nttt,30 a atnDrive, Martin San 3700 Institute, Science Telescope Space Karl-Schwarzschild-Stra Observatory, Southern Val-European de Astronom´ıa, Universidad F´ısica y de Departamento h nerdselrae ugs omto esitof redshift formation a suggest ages stellar inferred The rgts bevd ihyreliable highly observed, brightest ujc headings: Subject ednn with reddening hntebrightest-known the than z ass(1 masses size of at blue bevd(esd antd f2. B(8 AB 24.7 of magnitude (lensed) observed eraoal tb trfrigglx at galaxy star-forming a by fit reasonably be aeafrSres(ACS) Surveys for Camera rpriswt oe aayat galaxy model a with properties aaycutrAel18 ( 1689 Abell cluster galaxy rgtsrnl esdLmnbekglx LG addt at candidate (LBG) galaxy break Lyman lensed strongly bright strong ICVR FAVR RGTSRNL ESDGLX CANDIDATE GALAXY LENSED STRONGLY BRIGHT VERY A OF DISCOVERY ∼ Using z 7 J ∼ banda h pc eecp cec Institute, Science Telescope Space the at obtained , .Franx M. . 110 .Tesuc-ln erjcinsosta h trformation star the that shows deprojection source-plane The 6. . z 7 850 ubeSaeTlsoe(HST) Telescope Space Hubble . 0 c h etfi tla ouainsnhssmdl il median a yield models synthesis population stellar best-fit The pc. 300 − pte pc Telescope Space Spitzer ,tecniaehsa nrni antd of magnitude intrinsic an has candidate the 6, . 2 6 1. ..Bouwens R.J. , H − − A 160 INTRODUCTION T J 3 E 7 110 . tl mltajv 08/22/09 v. emulateapj style X A ..Frye B.L. , ubeSaeTelescope Space Hubble 9) and V omlg:osrain aais vlto aais formation galaxies: — evolution galaxies: — observations cosmology: × high-redshift ra fa es . a n scmltl neetd( undetected completely is and mag 2.2 least at of break ≤ 10 H 0 160 . 9 .Teepoete r eeal iia otoeo Bsfudat found LBGs of those to similar generally are properties These 3. M 3 z − ⊙ z ..Ford H.C. , 850 eevd20 etme 1 cetd20 aur 8 January 2008 accepted 21; September 2007 Received 8 g tla gs45 ages stellar , [4 .Infante L. , 0 = 475 hc soeae by operated is which , . dootglx.We orce o h lse anfiainof magnification cluster the for corrected When galaxy. -dropout 5 , µm . 8.Teglx addt,wihw ee oa 18-D,shows A1689-zD1, as to refer we which candidate, galaxy The 18). r 625 > z oos r xcl h rpriso an of properties the exactly are colors, ] < z , i 7 2 775 and ..Illingworth G.D. , . il atclrypo t ( fits poor particularly yield 4 9 aaycniaefudt date. to found candidate galaxy 0 ( hnology ac in earch .Motta V. , HST Hubble and , ,CA z, σ ABSTRACT − Spitzer ersi- RA nteNICMOS the in ) sse z iv 2 y,sa-omto rates star-formation Myr, 320 ) - - 7 = z 850 . RCiaig erpr h icvr favery a of discovery the report we imaging, IRAC 6 10 aa hs rpris obndwt h very the with combined properties, These data. trfrigglxe thg esit( al. low-luminosity redshift et Bouwens that high suggest at al. 2006; et indications galaxies 2004; Stanway Early star-forming 2005; al. Illingworth et & al. Bouwens Bunker 2006). et Yan important 2004; 2004; 2005; this al. Windhorst of et see & 2005 Giavalisco (Yan history also al. et luminosity star-formation epoch but Mobasher the 2006; characterize and 2005; the to 2008, density beginning Illingworth regarding are & al. and Bouwens et 2007 al. al. object) Bouwens et et al. Richard Egami 2006; et 2007; al. Chary 2005; et al. al. Kneib era al. et et 2004b; et Stark Iye Stern 2002; the 2005; 2006; al. al. probing al. et et et (Hu Bouwens now Mobasher reionization are 2004; cosmic searches of galaxy redshift own ta.20) u hsnest evrfidby verified universe 2004; at be galaxies the to redshifts; Windhorst needs higher & reionizing at z Yan this galaxies but fainter in 2006), studying 2003; al. role et Bouwens Bremer significant & (Lehnert a play esituies nupeeetddti.Telarge The high- telescopes” “cosmic the detail. nature’s observe by unprecedented provided to in magnifications opportunity universe an redshift provide clusters esrmnsidct htrinzto ea be- began reionization that tween indicate measurements ( intogsi DSqaassgetreionization suggest quasars by SDSS absorp- complete in Gunn-Peterson was troughs of measurements tion while 2007), 03 ukre l 04 a idos 2004; Windhorst & Yan Recent 2004; Bremer 2006). & (Lehnert al. al. et et reionization Bouwens Bunker cosmic 2003; to formation ± z .Rosati P. , & toggaiainllne rdcdb asv galaxy massive by produced lenses gravitational Strong 0 z & H ersn h urn ihrdhf frontier. high-redshift current the represent 7 . ( 4 ∼ 160 3 6, z ..Jee M.J. , 8 χ ν 2 − 27 = t ∼ H 1 = 0( 10 . 11 160 z ..White R.L. , 8 . . ∼ 0 ) tepst erdc these reproduce to Attempts 1). B raotan about or AB, 1 t adadis and band . − 5Gr n h otiuino star of contribution the and Gyr) 95 . 7 4 χ socrigi opc knots compact in occurring is .Ben N. , . 5(pre ta.20;Pg tal. et Page 2007; al. et (Spergel 15 ntefil ftemassive the of field the in 6 0 ν 2 z . 3Gr.A69z1i the is A1689-zD1 Gyr). 63 ≥ z < ∼ ∼ . 5.A69z1hsan has A1689-zD1 25). 1 7 Fne l 06.High- 2006). al. et (Fan 6 7 esito .,stellar 7.6, of redshift ´ σ ıtez . 12 . B n a only can and LBG 6 in ) 6 ∼ .Zheng W. , M 5 1 ⊙ ..Broadhurst T.J. , . HST a brighter mag 3 WMAP yr L AT − galaxies: — ∗ 1 Advanced z aayat galaxy n low and , z ∼ 2 ≈ > z pia depth optical 5 ∼ 7 − . 9 6 . 6. 1 a 3 )likely 6) 6 , 2 BRADLEY ET AL.
′ ′ Fig. 1.— ACS color image (3.4 × 3.4 ) of Abell 1689. The regions surveyed by our followup NICMOS J110 and H160 images are illustrated by the blue and orange outlines, respectively. A1689-zD1 is located at J2000 coordinates α = 13h11m29.96s, δ = −1◦19′18.7′′ and is denoted by the red circle. The white contours represent the z = 7.6 critical curves (µ ≥ 200). increase both the flux and size of background sources. at z 7.6 in the field of the massive cluster Abell 1689. ∼ These gains make it possible to detect faint high-redshift We adopt a cosmology with Ωm = 0.3, ΩΛ = 0.7, and −1 −1 galaxies (Kneib et al. 1996; Pell´oet al. 1999; Ellis et al. H0 =70 km s Mpc . This provides an angular scale 2001; Hu et al. 2002; Kneib et al. 2004; Egami et al. of 5.0 kpc arcsec−1 at z =7.6. All magnitudes are given 2005; Frye et al. 2007) without requiring a huge invest- in the AB photometric system (Oke 1974). ment of observing time, such as that dedicated to the Hubble Ultra Deep Field (Beckwith et al. 2006). The 2. OBSERVATIONS AND PHOTOMETRY key to making the subsequent analyses viable is hav- 2.1. HST ACS and NICMOS Data ing detailed cluster magnification maps that constrain the “optics” of the “cosmic telescope”. Our team has Abell 1689 is a well-studied and massive galaxy clus- ter at z = 0.183. With an unequaled Einstein radius been among the first to overcome this difficulty by us- ′′ ing multiband HST ACS data to perform detailed stud- of 50 , it is the strongest gravitational lens known (Broadhurst∼ et al. 2005). We observed the central 3.4′ ies of massive rich galaxy clusters, including Abell 1689 ′ (Broadhurst et al. 2005; Zekser et al. 2006, Coe et al. 3.4 of Abell 1689 with a single pointing of the ACS × 2008, in preparation) and CL0024+17 (Jee et al. 2007), Wide Field Camera (WFC) in 2002 June (see Fig. 1). to a precision useful for studying the properties of the The observations consisted of 20 orbits divided among background galaxy population. four broadband filters: F475W (g475; 9500 s), F625W In this paper, we present the discovery of a bright (r625; 9500 s), F775W (i775; 11800 s), and F850LP (z850; strongly lensed Lyman break galaxy (LBG) candidate 16600 s). In 2005 May, we followed up our ACS observa- tions with 18 orbits of Near Infrared Camera and Multi- STRONGLY LENSED LYMAN BREAK GALAXY CANDIDATE 3
TABLE 1 Photometry Summary
Measurement g475 r625 i775 z850 J110 H160 [3.6 µm] [4.5 µm] [5.8 µm] [8.0 µm] A1689-zD1 Observed Magnitude > 28.3 > 27.8 > 27.8 > 27.5 25.3 (0.1) 24.7 (0.1) 24.2 (0.3) 23.9 (0.3) > 23.9 > 23.4 A1689-zD1 Intrinsic Magnitude > 30.7 > 30.2 > 30.2 > 29.9 27.7 (0.1) 27.1 (0.1) 26.6 (0.3) 26.3 (0.3) > 26.3 > 25.8 Companion Magnitudea 27.4 (0.1) 26.4 (0.1) 26.1 (0.1) 25.9 (0.1) 25.0 (0.1) 24.3 (0.1) 25.0 (0.3) 24.3 (0.3) > 23.9 > 23.4
Note. — Errors (1 σ) are given in parenthesis. Intrinsic values were calculated assuming a cluster magnification of µ = 9.3 (see § 3). The [5.8 µm] and [8.0 µm] magnitudes are 1 and 2 σ upper limits, respectively. Because Abell 1689 lies in the plane of the ecliptic, the zodiacal light background is high and increases in the [5.8 µm] and [8.0 µm] bands, limiting our ability to detect the object in these two bands. a We include the photometry of this nearby source because it overlaps A1689-zD1 in the IRAC images and may therefore contaminate mildly our flux measurement of A1689-zD1.
◦ Fig. 2.— Cutout images (P.A. = 115 ) from the HST ACS (g475, r625, i775, z850, and “Det” – “detection”), HST NICMOS (J110 and H160), and Spitzer IRAC (3.6, 4.5, 5.8, and 8.0 µm) data centered on A1689-zD1. The source is undetected (< 1 σ) in the ACS data, including the 20-orbit combined “detection” image, and also not formally detected at 5.8 and 8.0 µm. The fluctuations that are present in the 5.8 and 8.0 µm images near the position of A1689-zD1 are consistent with being noise. The cutout images are 5′′ × 5′′, corresponding to 25 kpc on a side at z = 7.6.
Object Spectrometer (NICMOS)/NIC3 F110W (J110) a spurious detection. imaging to search for z850-dropout candidates. The ob- We present in Figure 3 the observed and intrinsic (lens- ′ ′ servations cover the central 2.5 2.5 high-magnification corrected) H160 magnitudes of A1689-zD1 relative to region of the cluster with a 9-pointing× 3 3 mosaic other z 7 8 galaxy candidates (Bouwens et al. 2008). (5376 s per NIC3 field). × While the∼ observed− (uncorrected) brightness of A1689- The bright LBG candidate, which we refer to as A1689- zD1 is 1.3 mag brighter than the brightest z 7 8 ∼ ∼ − zD1, was discovered by comparing the ACS/WFC z850 candidate, its intrinsic (lens-corrected) magnitude is very and NICMOS/NIC3 J110 data. The galaxy has an ob- similar to that of other z850-dropout galaxies. served magnitude of 25.3 0.1 in the NICMOS J band ± 110 and a strong z850 J110 break of > 2.2 mag (see Ta- 2.2. Spitzer IRAC Data ble 1). With a magnification− of µ 9.3, A1689-zD1 is strongly lensed by the foreground cluster≈ and is 1.3 mag Further evidence supporting the interpretation of ∼ A1689-zD1 as a z 7.6 LBG comes from archival Spitzer brighter (as observed) than the brightest-known z850- ∼ dropout (z 7 8) galaxy (Bouwens & Illingworth 2006; IRAC data (GO 20439, PI: Egami). In 2006, ultradeep Bouwens et∼ al.− 2008). IRAC imaging was obtained for Abell 1689 in the 3.6, A1689-zD1 is completely undetected (< 1 σ) in the 4.5, 5.8, and 8.0 µm bands (11.1 hours per band). Our LBG candidate is detected as a point-source in the IRAC ACS g475, r625, i775, and z850 data as well as in the 20- orbit ACS rms-weighted “detection” image constructed 3.6 and 4.5 µm bands. by co-adding all of the ACS images (see Fig. 2). The 1 σ The large size of the IRAC point-spread function (PSF; FWHM 2′′) poses a difficulty in that neighboring detection limits for this galaxy are 28.3, 27.8, 27.8, and ∼ 27.5 in the g , r , i , and z bands, respectively. objects may significantly contaminate the photometry. 475 625 775 850 In our case, we find that A1689-zD1 is blended with a To verify that A1689-zD1 was not a low-redshift red- ′′ dened interloper, we acquired a single-orbit NIC3 F160W nearby foreground object, which is separated by 1.5 . As seen in Figure 2, the foreground galaxy is rather∼ blue (H160) image of the source in 2007 June. Combining these data with the earlier NICMOS J data, we mea- and we estimate its photometric redshift (Ben´ıtez 2000) 110 is 2.2. To correct for the contamination of nearby sure a J110 H160 color of 0.6 0.2. This is exactly ∼ what one would− expect for an LBG± at z 7.4 7.7 objects, we convolved the NICMOS H160 detections of (see, e.g., Fig. 1 in the Supplementary Information∼ − of A1689-zD1 and its neighbors with the IRAC PSF for Bouwens & Illingworth 2006), but much bluer than a a given band. The H160 PSF-convolved objects were reddened object at lower redshift. Note that the strong then simultaneously fitted to the IRAC data, allowing z J break and the blue color redward of the the relative flux scalings as free parameters. The fitted 850 − 110 neighbors, which contributed 32% to the original flux, break are precisely the two criteria used to define high- ∼ redshift LBGs. Together this information provides com- were then subtracted from the IRAC data, leaving the pelling evidence that our source is a star-forming galaxy LBG source. We estimate that the neighbor subtraction at z 7.4 7.7. induces an additional 0.2 mag of uncertainty to the ∼ − IRAC photometry. ∼ A1689-zD1 is not detected in the individual J110 and H dither exposures ( 640 sec, 4 dithers per orbit), To minimize background noise and to further limit flux 160 ∼ contamination from nearby objects, we performed the but there also does not appear to be any artifacts (e.g., ′′ cosmic rays) at the object position. Given the similar Spitzer IRAC photometry using a 2.5 diameter aper- ture. Assuming a stellar profile to correct the fluxes for morphology in both the J110 and H160 images, which were taken about two years apart, and also that A1689- light falling outside the aperture, we applied aperture corrections of 0.56 and 0.60 mag to the [3.6 µm] and zD1 was located at different detector positions in the J110 and H images, we are confident that A1689-zD1 is not [4.5 µm] bands, respectively. We measure observed mag- 160 nitudes of 24.2 and 23.9 in the [3.6 µm] and [4.5 µm] 4 BRADLEY ET AL.
1.00 Jy) µ ( ν Z = 0.02 (Solar Metallicity) 0.10 SSP z=7.4 1.6 × 109 Msun 45 Myr τ × 9 Observed F 90 z=7.5 2.7 10 Msun 136 Myr CSF z=7.6 3.6 × 109 Msun 320 Myr
0.01
1.00 Jy) µ ( Fig. 3.— Histogram of H160 magnitudes for z850-dropout (z ∼ ν Z = 0.0004 (Subsolar Metallicity) 7−8) galaxies (Bouwens et al. 2008). We denote both the observed 0.10 (uncorrected) and intrinsic (lens-corrected) magnitudes of A1689- SSP z=7.6 2.1 × 109 Msun 72 Myr τ × 9 zD1. While the observed brightness of A1689-zD1 is ∼ 1.3 mag Observed F 90 z=7.7 2.8 10 Msun 157 Myr brighter than the brightest z ∼ 7 − 8 candidate, its intrinsic (lens- CSF z=7.6 3.9 × 109 Msun 320 Myr corrected) magnitude is very similar to that of other z850-dropout galaxies. 0.01 1•104 2•104 3•104 4•104 5•104 bands, respectively. The [5.8 µm] and [8.0 µm] upper Observed Wavelength (Å) limits are 23.9 (1 σ) and 23.4 (2 σ), respectively. The Fig. 4.— Best-fit stellar population models to the broadband broadband photometry of A1689-zD1 is summarized in photometry of our z ∼ 7.6 candidate. Solar metallicity models are shown in the top panel, with the subsolar (Z = Z⊙/50) models Table 1 and the HST /ACS, HST /NICMOS, and Spitzer in the bottom panel. The vertical bars denote the 1 σ flux uncer- IRAC cutout images are shown in Figure 2. tainties and the horizontal bars represent the width of the filter bandpass. The optical ACS non-detections are shown as 1 σ upper 3. SOURCE MAGNIFICATION limits. It is valuable to calculate the intrinsic properties of A1689-zD1 by using strong lensing models available the stellar age to be less than the age of the universe at for Abell 1689 (e.g, Broadhurst et al. 2005, Coe et al. the fit redshift (e.g., 0.7 Gyr at z 7.5). We consid- 2008, in preparation). For an improved version of the ered several star-formation histories∼ (SFH) including a Broadhurst et al. (2005) strong lensing model and a red- simple (single-burst) stellar population (SSP), an expo- shift of z 7.6 (see 4), we estimate a magnifica- nentially declining star-formation rate (SFR; τ models) tion µ of ∼9.3, which appears§ to be roughly consistent with e-folding times between 10 and 100 Myr, and con- ∼ −1 with other models in the literature (Halkola et al. 2006; stant SFR (CSF) models with SFRs up to 30 M⊙ yr . Limousin et al. 2007, Coe et al. 2008, in preparation). The best-fit stellar population models are shown in The improved Broadhurst lensing model is based on 42 Figure 4 and the parameters are given in Table 2. For sets of multiple images (12 more than Broadhurst et al. both solar and subsolar metallicities, we find acceptable 2005), with several new counterimages close to the clus- fits for the models. The best-fit redshifts are in the range ter center. Based on the mean slope of the radial mass from 7.4
TABLE 2 Best-fit Stellar Population Model Results
Massc Aged SFR a b 9 −1 2 SFH zphot zform (10 M⊙) (Myr) (M⊙ yr ) AV χν
Solar Metallicity (Z = 0.02) SSP 7.4 ± 0.3 7.8 1.6 ± 0.7 45 0 0.0 ± 0.2 1.4 τ90 7.5 ± 0.3 9.7 2.7 ± 0.4 136 3.2 0.0 ± 0.2 1.2 CSF 7.6 ± 0.4 63.4 3.6 ± 1.4 320 6.9 0.1 ± 0.2 0.9
Subsolar Metallicity (Z = 0.0004) SSP 7.6 ± 0.4 8.3 2.1 ± 0.9 72 0 0.0 ± 0.2 1.1 τ90 7.7 ± 0.3 10.5 2.8 ± 1.1 157 2.4 0.0 ± 0.2 0.7 CSF 7.6 ± 0.4 63.4 3.9 ± 1.6 320 7.6 0.3 ± 0.2 0.6
a Star-formation history (SFH): simple (single-burst) stellar population (SSP), ex- ponentially declining SFR (τ models) where the subscript represents the e-folding time in Myr, or constant SFR (CSF) models. b Formation redshift calculated from the fitted redshift and (unweighted) stellar age. c Best-fit stellar mass, corrected by the magnification at the fitted zphot redshift. d SFR-weighted mean stellar age (cf. F¨orster Schreiber et al. 2004). unable to reproduce the strong z J break of 2.2 5. SOURCE-PLANE DEPROJECTION 850 − 110 mag observed for A1689-zD1. 5.1. Morphology The detailed Abell 1689 cluster deflection map (see 3) § allows us to deproject the NICMOS J110 and H160 im- 1.00 ages of A1689-zD1 to the source plane at z 7.6. The substantial magnification of the source provides∼ us with a unique opportunity to examine the morphology of a Jy) µ
( z 7.6 galaxy at very high resolution. The deprojected ν ∼ Z = 0.02 (Solar Metallicity) images of A1689-zD1 in the J110 and H160 bands are pre- 0.10 × 9 SSP z=1.7 0.9 10 Msun 202 Myr AV = 1.3 sented in Figure 8. The galaxy shows an extended mor- Observed F τ z=1.6 0.7 × 109 Msun 421 Myr A = 1.0 90 V phology in both the image and source planes, spanning τ z=7.5 2.7 × 109 Msun 133 Myr A = 0.0 90 V 0.4′′ (2.0 kpc) in the source plane. The remarkable ∼ consistency of the morphology in both the J110 and H160
0.01 bands as well as the > 8 σ detection in each band pro- 1•104 2•104 3•104 4•104 5•104 Observed Wavelength (Å) vides strong evidence that the source is real and not a spurious detection. Fig. 5.— Fits to the observed photometry with stellar population In both images, A1689-zD1 appears to be comprised of models at z < 4 (red, green) and z > 4 (blue). Note that the low-redshift SSP (χν = 27) and τ90 (χν = 25) solutions do not at least two bright, marginally resolved knots connected agree with the i775 and z850 upper limits and cannot reproduce by a lower-luminosity region. The brighter knot is lo- the strong z850 − J110 break, even with ≥ 1.0 mag of extinction. Also note that because the break in the SED occurs well within 2.0 the J110 band, the measured flux at 1.1 µm should be appreciably τ Model Salpeter IMF below the continuum level. We find that the z ∼ 7.5 fit is quite 90 Myr reasonable (χν = 1.1). 1.5 For our preferred high-redshift solutions, we find stellar 9
masses of (1.6 3.6) 10 M⊙ for the solar metallicity V 1.0 − × 9 A models and slightly higher masses of (2.1 3.9) 10 M⊙ for the subsolar metallicity models. In− a similar× fash- ion, the subsolar metallicity models generally produce 0.5 slightly older ages (72 320 Myr) than the solar metal- licity models (45 320− Myr). These stellar mass and age − estimates are in the range of those previously estimated 0.0 0 2 4 6 8 10 by Labb´eet al. (2006) for a HUDF-selected z850-dropout Redshift
(z 7) sample. The derived instantaneous SFRs show 2 ∼ −1 Fig. 6.— Likelihood plot showing the χ values of the best- modest star formation with rates up to 7.6 M⊙ yr . ν fitting τ90 solar-metallicity models as a function of redshift and V 2 Most of the models require little reddening, with A reddening. Black corresponds to the lowest χν value (1.1), the gray ≤ 2 0.3, which is in agreement with results for z 6 LBGs contours at z ∼ 1.3 − 2.2 correspond to 25 ≤ χν < 30, and white ∼ 2 (Yan et al. 2005; Dow-Hygelund et al. 2005; Eyles et al. to χν ≥ 30. The grayscale colors are displayed with a square-root 2 2006). While we assumed a Salpeter IMF, fitting models stretch to compress the dynamic range of χν values. Clearly, the best low-redshift solutions (1.5 . z . 2.0) provide a much poorer using a Chabrier (2003) IMF yields 1.5 times lower 2 stellar masses and roughly similar ages.∼ fit (χν ≥ 25) to the data than the high-redshift (z > 6.5) solutions. 6 BRADLEY ET AL.
−1.1±0.3 4 varies as (1 + z) (Bouwens et al. 2006, see also 100 Myr SSP Model Salpeter IMF Ferguson et al. 2004; Bouwens et al. 2004a). A1689-zD1
AV = 0.0 A = 1.0 has a delensed H160 magnitude of 27.1, which corre- V ∗ 3 AV = 2.0 A = 3.0 V sponds to 0.3Lz=3 (Steidel et al. 1999). Scaling the AV = 4.0 ∗ A = 5.0 ∼ V mean sizes ( 1.3 kpc) of 0.3Lz=3 LBGs at z 3.8
AB A1689−zD1 ) (Bouwens et∼ al. 2004a), we∼ would expect a mean∼ half- 110 2
− J light radius at z 7.6 of 0.7 0.1 kpc. While this is
850 ∼ ± (z slightly smaller than the half-light radius of A1689-zD1,
1 there is at least a 20% dispersion in galaxy sizes about the mean size at any± given redshift. 5.3. Counterimages 0
0 2 4 6 8 We also used the improved Broadhurst et al. (2005) Redshift Abell 1689 strong lensing model (see 3) to look for § Fig. 7.— Plot showing (z850 − J110)AB color as a function of potential counterimages of A1689-zD1. We re-projected redshift for a range of reddened (0 ≤ AV ≤ 5) 100 Myr SSP solar the source image at z 7.6 back to the image plane at metallicity models. Even with extremely large values of the red- z 0.18 and found that∼ the model does not predict any dening, the low-redshift models are unable to reproduce the strong counterimages.∼ For a simple isothermal sphere model z850 − J110 break of 2.2 mag observed for A1689-zD1. Because we must fit the entire SED of the galaxy, the reddening cannot be with an Einstein radius of RE, one would expect that increased arbitrarily to fit only the z850 −J110 break. For example, an object at radius r would have a counterimage on the reddening values of AV & 1.5 (dashed lines) produce fits that are opposite side of the cluster at a distance of 2RE r. inconsistent with the observed H160−[4.5 µm] color of A1689-zD1. − The gray box shows the redshift range where the stellar population However, because of the complexity of the Abell 1689 models are consistent with the z850 − J110 color. lensing model, not every background object is guaranteed to produce a counterimage. cated to the southwest of the secondary knot and is sep- arated from it by 0.26′′ (1.3 kpc). Performing fits to 6. ULTRAVIOLET (UV) LUMINOSITY FUNCTION (LF) the individual knots∼ with GALFIT (Peng et al. 2002), we In theory our discovery of one very bright z850-dropout find that most of the luminosity of A1689-zD1 is emitted 2 in the 6 arcmin J110-band coverage around Abell 1689 from the knots. The brighter (fainter) knot contributes should∼ allow us to estimate the volume density of UV- 2/3 ( 1/3) of the total luminosity of A1689-zD1. The bright galaxies at z 7. To explore this further, we ∼J H∼ colors of the two knots are roughly consis- ∼ 110 − 160 generated mock images based on the UV LF, sizes, and tent, with values of 0.5 0.2and 0.8 0.2 for the brighter colors of LBGs at z 6 found by Bouwens et al. (2007) and fainter knots, respectively.± The± knots are compact ∼ ′′ and Bouwens et al. (2006) and magnified them accord- and have half-light radii . 0.06 (0.3 kpc) in the source ing to the Broadhurst et al. (2005) Abell 1689 deflection plane. map. Unfortunately, from these no-evolution simulations The concentration of a significant fraction of the to- we estimate that we would expect to find only 0.2 tal flux in compact star-forming knots is consistent galaxies over our search area. This suggests that∼ there with findings for starburst galaxies over a wide range is very little that we could learn about the UV LF from of redshifts (e.g., Meurer et al. 1995; Law et al. 2007; our z850-dropout survey around Abell 1689, and that, in Smail et al. 2007; Overzier et al. 2007) and as shown fact, we may have been quite fortunate to find the bright quite dramatically in the z = 4.92 lensed galaxy pair candidate A1689-zD1. Even though the cluster magnifi- in CL 1358+62 (Franx et al. 1997). The knots in A1689- cation increases the typical depth by 2 3 mag, the zD1 are likely separate star-forming regions within the effective source-plane area that is surveyed∼ − decreases in- galaxy, but they could conceivably be interpreted as versely proportional to the magnification factor. The small star-forming galaxies merging at high redshift. very small number of expected candidates follows from . 2 5.2. the tiny area ( 0.5 arcmin ) that the J110-band cover- Half-Light Radius age probes in the source plane at z 7. We calculated the half-light radius of A1689-zD1 by If the effective slope of the LF≥ was large enough defining the circular aperture containing half the total (d(log10 N)/dm & 0.4; Broadhurst et al. 1995), we would Kron flux (Kron 1980). For A1689-zD1 we measure a expect the greater depth to more than make up for half-light radius of 0.48′′, which translates to 0.19′′ the smaller search area. The only place the effective (0.96 kpc) in the source-plane at z 7.6. The size∼ trans- slope would be greater than 0.4 is for luminosities lation was derived by comparing the∼ size of the object above L∗, unless we assume that∼ the LF at z 7 8 along its major axis in both the image and source planes. is steeper than at z 6 (i.e. α 1.74,∼ as− de- As demonstrated by the PSF ellipses in Figure 8, the termined by Bouwens et∼ al. 2007, which≈ corresponds − to magnification of A1689-zD1 is greater along its minor d(log10 N)/dm 0.3). Despite the small expected num- axis than its major axis. This results in a source-to- bers, the present∼ search around Abell 1689 should be image-plane size scaling along the galaxy major axis of more efficient at finding L∗ galaxies at z & 7 than a 2.5, which is slightly smaller than the value one would search without the lensing amplification. However, lens- obtain by assuming that the magnification is the same in ing may not provide as great an advantage in searches all directions (√9.3=3.0). for galaxies fainter than L∗, given recent measurements Though there is a fair amount of dispersion in the of the faint-end slope [where d(log N)/dm 0.3]. 10 ∼ size of individual galaxies in high-redshift dropout sam- 7. PREVIOUS z & 6 LENSED LBG CANDIDATES ples, the mean size of galaxies at a given luminosity STRONGLY LENSED LYMAN BREAK GALAXY CANDIDATE 7 N N E E 1 kpc 1 kpc
Fig. 8.— Source-plane deprojection of the NICMOS J110 (left) and H160 (right) images of A1689-zD1. The morphology is remarkably consistent in both images, showing an extended morphology that spans ∼ 0.4′′ (2.0 kpc) in the source plane. A1689-zD1 appears to be comprised of two compact knots, denoted by the dotted red circles, connected by a lower-luminosity region. Each knot appears to have an ′′ intrinsic half-light radius that is less than 300 pc (see § 5.1). The solid white ellipses represent the NIC3 J110 and H160 PSFs (0.33 and 0.37′′ FWHM, respectively), delensed to the source plane at z = 7.6. High-redshift z & 6 galaxy candidates have been pre- these data. This suggests that their sample of first, sec- viously identified in searches around strong lensing clus- ond, and third category z & 6 candidates is largely spu- ters (Hu et al. 2002; Kneib et al. 2004; Richard et al. rious (Bouwens et al. 2008). Given its secure detection 2006). For example, in the field of the lensing cluster significance (8 σ), broadband colors, and strength of its Abell 2218, Kneib et al. (2004) found a very highly mag- z850 J110 break, A1689-zD1 is the brightest LBG can- nified (µ 25) and triply imaged LBG candidate near didate− that we can confidently place at a redshift z & 7. ∼ the critical curve. They infer a redshift for the candidate 8. of 6.6
−1 and star-formation rates < 7.6 M⊙ yr , properties gen- Brad Holden for helpful discussions. ACS was developed erally similar to z 5 6 LBGs. A1689-zD1, with a under NASA contract NAS 5-32865, and this research redshift of z 7.6∼ and− a formation redshift z & 8.0 has been supported by NASA grants NAG5-7697 and (t . 0.63 Gyr),∼ is the brightest observed, highly reliable HST-GO10150.01-A and by an equipment grant from z > 7.0 galaxy candidate found to date. Sun Microsystems, Inc. The Space Telescope Science In- Given the unique brightness of A1689-zD1, we are ac- stitute is operated by AURA Inc., under NASA contract tively working to confirm its redshift with near-IR spec- NAS5-26555. We are grateful to K. Anderson, J. Mc- troscopy. A spectrum of this bright galaxy would allow Cann, S. Busching, A. Framarini, S. Barkhouser, and for the most detailed study of a z > 7 LBG to date and T. Allen for their invaluable contributions to the ACS provide valuable insights into the properties of galaxies project at JHU. L.I. acknowledges support from a Coni- in the early universe. cyt Fondap grant.
We would like to thank Dan Coe, Andrew Zirm, and
REFERENCES Beckwith, S. V. W., et al. 2006, AJ, 132, 1729 Kneib, J.-P., Ellis, R. S., Santos, M. R., & Richard, J. 2004, ApJ, Ben´ıtez, N. 2000, ApJ, 536, 571 607, 697 Bouwens, R. J., & Illingworth, G. D. 2006, Nature, 443, 189 Kneib, J.-P., Ellis, R. S., Smail, I., Couch, W. J., & Sharples, Bouwens, R. J., Illingworth, G. D., Blakeslee, J. P., Broadhurst, R. M. 1996, ApJ, 471, 643 T. J., & Franx, M. 2004a, ApJ, 611, L1 Kron, R. G. 1980, ApJS, 43, 305 Bouwens, R. J., Illingworth, G. D., Blakeslee, J. P., & Franx,M. Labb´e, I., Bouwens, R., Illingworth, G. D., & Franx, M. 2006, 2006, ApJ, 653, 53 ApJ, 649, L67 Bouwens, R. J., Illingworth, G. D., Franx, M., & Ford, H. 2007, Law, D. R., Steidel, C. C., Erb, D. K., Pettini, M., Reddy, N. A., ApJ, 670, 928 Shapley, A. E., Adelberger, K. L., & Simenc, D. J. 2007, ApJ, —. 2008, ApJ, submitted (arXiv: 0803.0548) 656, 1 Bouwens, R. J., et al. 2004b, ApJ, 616, L79 Lehnert, M. D., & Bremer, M. 2003, ApJ, 593, 630 Broadhurst, T., et al. 2005, ApJ, 621, 53 Limousin, M., et al. 2007, ApJ, 668, 643 Broadhurst, T. J., Taylor, A. N., & Peacock, J. A. 1995, ApJ, Madau, P. 1995, ApJ, 441, 18 438, 49 Meurer, G. R., Heckman, T. M., Leitherer, C., Kinney, A., Bruzual, G., & Charlot, S. 2003, MNRAS, 344, 1000 Robert, C., & Garnett, D. R. 1995, AJ, 110, 2665 Bunker, A. J., Stanway, E. R., Ellis, R. S., & McMahon, R. G. Mobasher, B., et al. 2005, ApJ, 635, 832 2004, MNRAS, 355, 374 Oke, J. B. 1974, ApJS, 27, 21 Calzetti, D., Armus, L., Bohlin, R. C., Kinney, A. L., Koornneef, Overzier, R. A., et al. 2007, preprint (arXiv: 0709.3304) J., & Storchi-Bergmann, T. 2000, ApJ, 533, 682 Page, L., et al. 2007, ApJS, 170, 335 Chabrier, G. 2003, PASP, 115, 763 Pell´o, R., et al. 1999, A&A, 346, 359 Chary, R.-R., Teplitz, H. I., Dickinson, M. E., Koo, D. C., Le Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H.-W. 2002, AJ, Floc’h, E., Marcillac, D., Papovich, C., & Stern, D. 2007, ApJ, 124, 266 665, 257 Richard, J., Pell´o, R., Schaerer, D., Le Borgne, J.-F., & Kneib, Dow-Hygelund, C. C., et al. 2005, ApJ, 630, L137 J.-P. 2006, A&A, 456, 861 Egami, E., et al. 2005, ApJ, 618, L5 Salpeter, E. E. 1955, ApJ, 121, 161 Ellis, R., Santos, M. R., Kneib, J.-P., & Kuijken, K. 2001, ApJ, Smail, I., et al. 2007, ApJ, 654, L33 560, L119 Spergel, D. N., et al. 2007, ApJS, 170, 377 Eyles, L. P., Bunker, A. J., Ellis, R. S., Lacy, M., Stanway, E. R., Stanway, E. R., McMahon, R. G., & Bunker, A. J. 2005, Stark, D. P., & Chiu, K. 2006, MNRAS, 1348 MNRAS, 359, 1184 Fan, X., Carilli, C. L., & Keating, B. 2006, ARA&A, 44, 415 Stark, D. P., Ellis, R. S., Richard, J., Kneib, J.-P., Smith, G. P., Ferguson, H. C., et al. 2004, ApJ, 600, L107 & Santos, M. R. 2007, ApJ, 663, 10 F¨orster Schreiber, N. M., et al. 2004, ApJ, 616, 40 Steidel, C. C., Adelberger, K. L., Giavalisco, M., Dickinson, M., Franx, M., Illingworth, G. D., Kelson, D. D., van Dokkum, P. G., & Pettini, M. 1999, ApJ, 519, 1 & Tran, K.-V. 1997, ApJ, 486, L75+ Stern, D., Yost, S. A., Eckart, M. E., Harrison, F. A., Helfand, Frye, B. L., et al. 2007, ApJ, 665, 921 D. J., Djorgovski, S. G., Malhotra, S., & Rhoads, J. E. 2005, Giavalisco, M., et al. 2004, ApJ, 600, L103 ApJ, 619, 12 Halkola, A., Seitz, S., & Pannella, M. 2006, MNRAS, 372, 1425 Yan, H., et al. 2005, ApJ, 634, 109 Hu, E. M., Cowie, L. L., McMahon, R. G., Capak, P., Iwamuro, Yan, H., & Windhorst, R. A. 2004, ApJ, 600, L1 F., Kneib, J.-P., Maihara, T., & Motohara, K. 2002, ApJ, 568, Zekser, K. C., et al. 2006, ApJ, 640, 639 L75 Iye, M., et al. 2006, Nature, 443, 186 Jee, M. J., et al. 2007, ApJ, 661, 728 Kinney, A. L., Calzetti, D., Bohlin, R. C., McQuade, K., Storchi-Bergmann, T., & Schmitt, H. R. 1996, ApJ, 467, 38