The Abundance of Star-Forming Galaxies in the Redshift Range 8.5

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Field et Deep Bouwens Ultra promising 3 UDF09). Hubble located hereafter initially the Illingworth, PI: in Cam- campaign Field 11563, (WFC3/IR) the Wide 3 near-infrared from the data era with used 2009 HST (2010) in al. the completed et in Yan out’ and (2011) ‘drop progressively which Y sources to ing r mligaLmnbekna h e deo the presented of al. edge et faint red Yan 20 the contrast, of marked near list In col- break a F125W) Lyman filter. - a F105W (F105W with implying F125W ors in detected robustly ta.as on ore nteitra 8 interval the in sources 3 found also al. et nr-xmnn h opee aae,peetdol a redshift only photometric a presented of at dataset, UDFj-39546284 candidate, completed single the re-examining on e G 26 29,P:Psmn a icvrdsev- discovered has distance Postman) sur- diameter PI: HST CLASH 12791, The angular - 2004). 12065 al. their et (GO nature on Kneib vey 2001; multiply-imaged al. limit their et lower (Ellis cases, factors a favorable by offers In magnified be can 2011). sources imaging Such deep of over- with fields. associated can Gravita- blank difficulties galaxies of the common. of of clusters in some foreground are come by candidates lensing al. tional et Bouwens and redshifts had 8 4 > z bn 15 and F105W -band . lxne Rogers B Alexander , × 5 z .Sc otniyhsipratimplications important has continuity Such 8. − 1.,ntdanfo h rgnltre Bouwens three. original the from drawn not =10.3, -0esrn oerlal htmty(ihr tal. et (Richard photometry reliable more ensuring 5-30 0glxe ihJWST. with galaxies 10 0sml eosrtsalmnst density luminosity a demonstrates sample 10 rigglxe eodaredshift a beyond galaxies orming dfo epmlibn near-infrared multi-band deep a from ed ecniaeUF-9424(previously UDFj-39546284 candidate he z 85rpeet aua rnircorrespond- frontier natural a represents =8.5 c itn aais obndwith Combined galaxies. distant uch 3 > z oue ihWC/Ri hsfield this in WFC3/IR with posures ohaiOno Yoshiaki , h ubeUtaDe il.This Field. Deep Ultra Hubble the dtsi ral mrvd Using improved. greatly is idates J 8 bn rpu addtsagigall arguing candidates dropout -band . J .Hwvr oeo h a tal. et Yan the of none However, 5. bn 15 les own tal. et Bouwens filters. F125W -band 6 ihl Cirasuolo Michele , . O1:NWRESULTS NEW 12: TO 8.5 E 2 nfre h measured the ansformed maCurtis-Lake Emma , J bn rpusat dropouts -band 4 ate Schenker A Matthew , z lxe:stellar alaxies: 1. ri an is or =11.9 > z z 2,8 ≃ . 5 8 2 8.5 . Evan , z galaxies 5 < z < z ≃ ≃ 0but, 10 but 8 1 α , 9, . 2 eral such z > 8.5 candidates, three at z ≃9-10(Zheng et al. 2. STAR FORMING GALAXIES WITH Z >8.5 2012; Bouwens et al. 2012b) and a multiply-imaged source at z=10.7(Coe et al. 2012). To select z >8.5 candidates, we examined the stacked com- bination of the 80 orbit F160W (UDF12 plus UDF09), 30 A key issue is the uncertainty in converting the various de- orbit F140W (UDF12) and 34 orbit F125W (UDF09) im- tections into estimates of the abundance of galaxies beyond ages and located all sources to a 5σ limit within filter- z ≃8. Bouwens et al. (2011, see also Oesch et al. (2012)) matched apertures of 0.4 − 0.5 arc sec corresponding to claimed that their detection of a single z ≃10.3 candidate mAB ≃29.9-30.1. Making effective use of our new ultra- in the UDF09 campaign implies a shortfall of a factor ≃3-6 deep 93 orbit (71 from UDF12, 22 from UDF09) F105W compared to that expected from the declining star forma- image and the deep ACS photometry, we utilized the SED tion rate density over 6 8.5 candidates were found. An independent search the CLASH detections are consistent with a continuous de- using the same master sample selecting those which drop cline to z ≃10.7. One limitation of the lensing strategy as a out in F105W (2σ rejection corresponding to mAB > 31.0) means of conducting a census (rather than providing indi- and no detection (2σ) in a combined ACS BV iz stack de- vidual magnified sources for scrutiny) is the uncertainty as- livered the same z >8.5 candidates. All sources but one sociated with estimating the survey volume which depends (see below) are detected in more than one filter and all sensitively on the variation of magnification with position are detected with an appropriately-reduced signal/noise in across the cluster field (c.f., Santos et al. 2004; Stark et al. time-split subsets over the collective UDF09 and UDF12 2007b). campaigns. Figure 1 shows HST broad-band images for There are several drawbacks with the earlier UDF09 cam- these 7 sources. Their SED fits and redshift probability paign with respect to conducting a census of z >8.5 galax- distributions p(z) are given in Figure 2. Identifications, ies. Limiting factors in considering the robustness of the source photometry and optimum redshifts are summarized faint candidates include the poor signal to noise in subsets in Table 1. of the F160W data, the reliance on only a single detection The great advantage of the SED fitting approach is that filter and the limited depth of the critical F105W imaging it allows us to quantify the possibility of alternative low- data whose null detection is central to locating z > 8.5 redshift solutions. Four of our 7 objects (UDF12-3921- candidates. 6322, UDF12-4265-7049, UDF12-4344-6547 & UDF12- This article heralds a series that presents results from 3947-8076) have low probabilities of being at z < 4 (1−4%). a deeper UDF campaign with WFC3/IR completed in UDF12-4106-7304 has a ≃ 10% probability for z < 4 September 2012 (GO 12498, PI; Ellis, hereafter UDF12) and lies close to the diffraction pattern of an adjacent which remedies the above deficiencies by (i) substantially source which may affect the F140W photometry (Figure increasing the depth of the F105W image (by ×4 in expo- 1). UDF12-3895-7114 is the least secure with a 28% proba- sure time) essential for robust rejection of z <8.5 sources, bility of lying at z < 4. We discuss UDF-3954-6284 below. (ii) increasing the depth of the detection filter F160W (a Our deeper F105W data and the new F140W image also en- 50% increase in exposure time) and (iii) adding a deep im- ables us to clarify the nature of z > 8.5 sources claimed in age in the F140W filter matching the depth now attained the earlier UDF09 analyses (see Table 1). In McLure et al. in F160W. This filter partially straddles the F125W and (2011)’s UDF09 analysis, no robust J-band dropout source F160W passbands offering valuable information on all z > 7 was claimed (see also Bunker et al. 2010). However a so- sources, the opportunity for an independent detection for lution with z=8.49 was found for HUDF 2003 which was 8.5 8.5 additions of all earlier UDF data, are available to the com- 10 munity on the team web page9. All magnitudes are in the This source was examined spectroscopically using the VLT SIN- FONI integral field spectrograph by Lehnert et al. (2010) who re- AB system (Oke 1974). ported a detection of Lyα emission at z=8.6 but this claim is refuted by Bunker et al (in preparation) following a separate spectroscopic 9 http://udf12.arizona.edu/ exposure with the higher resolution spectrograph X-shooter. The Abundance of Star-Forming Galaxies in the Redshift Range 8.5 to 12 3 candidates; most appear as tails of bright objects and can- the F160W emission and satisfy our upper limits in Fig- not be reliably photometered by SeXtractor. All of the ‘Y ure 3b, only an infrared spectrum would completely elimi- dropouts’ claimed by Lorenzoni et al. (2011) have robust nate the possibility of some exotic foreground emission line F105W detections in our deeper data and lie below z=8.5. source. As the source has HAB=29.3, this would be a very In summary, only one object claimed to be at z > 8.5 challenging observation. For the remainder of the paper we from the earlier UDF09 analysis remains and that is the will interpret this source with caution. final J-band dropout presented by Bouwens et al. (2011) 3. at z=10.3, UDFj-39546284 (≡UDF12-3954-6384 in Table THE ABUNDANCE OF GALAXIES WITH 1). However, its non-detection in the UDF12 F140W data 8.5 8.5 candidates(Coe et al. 2012) im- case for all but one of our UDF12 candidates (Table 1). plying star formation rate densities higher than claimed by A major surprise is the non-detection in F140W of UDFj- Bouwens et al. (2011) at z ∼ 10. However, the uncertain 30546284 implying a redshift of z=11.90 (Figures 1 and 2). search volumes inherent in the lensing method are a major Single band detections are naturally less convincing, concern. although UDFj-30546284 is confirmed in F160W sub- In Figure 4 we present the implications of the significant exposures through UDF09 and UDF12, leaving no doubt it increase in the number of 8.5 1.5 (2σ) color precludes any reasonable direct determination of the luminosity function at z > 8.5 foreground continuum source (Figure 2) but a possible ex- is not yet possible, to estimate the UV luminosity densi- planation might be the presence of a very strong emission ties for our four detections at 8.5 . z < 9.5 we calculate line. Recent WFC3/IR imaging and grism spectroscopy of the required redshift evolution in the characteristic lumi- z ≃2 galaxies has revealed a population of extreme emis- nosity dM⋆/dz such that a survey of our depth and se- sion line galaxies (EELGs). van der Wel et al. (2011) have lection efficiency would recover the number of sources we identified an abundant population of EELGs at z ≃1.7 in find. This calculation is performed assuming simple lumi- the CANDELS survey using purely photometric selection nosity evolution from z ∼ 8, keeping the luminosity func- techniques. Spectroscopy of a subset has verified the pres- tion normalization φ⋆ and faint-end slope α fixed at the ence of sources with rest-frame [O III] equivalent widths up z ∼ 8 values measured by Bradley et al. (2012). To repro- to ≃ 1000 A˚ . Independently, Atek et al. (2011) located a duce our sample with mean redshift hzi ≈ 8.9, we find similar population in the WISP survey over 0.35 40% (Robertson 2010b). Within for starbursts with ages of 1 and 10 Myr demonstrating 10.5 . z . 11.5, we find no candidates. Nonetheless we that it is not possible to account for the significant excess can use the same methodology to provide an upper limit of flux in F160W from either intense [O II] 3727 A˚ at z ≃3.4 or ρ (z ∼ 10.8) < 9.57 × 1024 ergs s−1 Hz−1Mpc−3 (Figure [O III] 5007 A˚ at z ≃2.4. Figure 3b illustrates, for the case UV 4, purple upper limit). of intense [O III] emission that the expected stellar plus nebular emission spectrum of a 10 Myr starburst would vi- Considering the putative z ∼ 12 source, both its mor- olate the photometric flux limits provided by the various phology (Figure 1) and its luminosity cause us to be cau- ACS and WFC3/IR broad band non-detections. If all of tious, particularly given the paucity of other detections the emission in F160W arises from [O III] above a blue β=- beyond z ≃10.5. Nonetheless, since the emission line hy- 2 stellar continuum, the rest-frame equivalent width would pothesis is equally difficult to accept (Section 2.1), we es- have to be >4500 A˚ , beyond that of any known object. timated the luminosity density using only the source lumi- A further difficulty is the absence of the expected Lyman nosity (MUV = −19.6AB accounting for IGM absorption in −1 −1 α emission following a recent Keck optical spectrum (Fig- F160W, or log10 LUV = 28.48 log10 ergs s Hz ) and the ure 3b). However, although we can find no physically self- UDF survey volume V (11.5 . z . 12.5)=6.37×103 Mpc3. consistent starburst model that can simultaneously explain The resulting luminosity density ρUV(z ∼ 11.8) > 4.7 × 4

1024 ergs s−1 Hz−1Mpc−3 is thus a lower limit (Figure 4, red faint end slope (Schenker et al. 2012; McLure et al. 2012, point), and conservatively does not include multiplicative see Robertson et al. 2012) this work has placed the first effects of selection efficiency or involve extrapolations from constraint on the SFR density only 360 million years after the z ∼ 8 luminosity function. An additional possibility the Big Bang. Evidence for actively star-forming galaxies is that the F160W is contaminated by Lyα emission. The significantly beyond the instantaneous reionization redshift additional z=12 point (yellow) illustrates how this limit zreion ≈ 10.6 ± 1.2 implied by observations of the cosmic would be affected for a rest-frame equivalent width of 260 microwave background (Komatsu et al. 2011) motivates fu- A˚ of which half is absorbed by neutral hydrogen. ture observations with James Webb Space Telescope. Our In summary, the new galaxy sample provided by UDF12 estimate of the z ∼ 10 star formation rate densities are con- has enabled us to present the first meaningful estimate of sistent with previous analyses aimed at explaining the mea- ρUV (z) beyond z ≃ 8.5. The six galaxies with 8.5

4. DISCUSSION US authors acknowledge financial support from the The UDF12 data has demonstrated the continued effec- Space Telescope Science Institute under award HST-GO- tiveness of HST to undertake a census of very high red- 12498.01-A. JSD acknowledges support of the European shift galaxies. Our discovery of the first robust sample Research Council and the Royal Society. RJM acknowl- of galaxies with z > 8.5 and possibly the most distant edges funding from the Leverhulme Trust. We thank Kim- galaxy at z ∼ 12 extends HST’s reach further into the ihiko Nakajima for assistance with the Keck spectroscopy. reionization epoch than previously thought possible (c.f., This work is based on data from the Hubble Space Telescope Bouwens et al. 2011). While the question of whether star- operated by NASA through the Space Telescope Science forming galaxies were solely responsible for reionizing in- Institute via the association of Universities for Research in tergalactic hydrogen is more reliably addressed through Astronomy, Inc. under contract NAS5-26555. precise constraints on the z ∼ 7 − 8 luminosity function

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TABLE 1 z > 8.5 Candidates

ID RA Dec zSED(±1σ) Y105W J125W J140W H160W Notes UDF12 Survey Depth 5-σ AB (aperture diameter arcsec - 70% enclosed point source ﬂux) 30.0 (0.40) 29.5 (0.44) 29.5 (0.47) 29.5 (0.50) UDF12 Galaxiesa +0.3 b UDF12-3954-6284 3:32:39.54 -27:46:28.4 11.9 −0.5 > 31.2 > 30.7 > 30.5 29.3 ± 0.2 UDFj-39546284 B11 +0.4 UDF12-4106-7304 3:32:41.06 -27:47:30.4 9.5 −0.8 > 30.8 > 30.0 29.8 ± 0.3 29.7 ± 0.3 +0.4 UDF12-4265-7049 3:32:42.65 -27:47:04.9 9.5 −0.7 > 31.2 30.4 ± 0.6 29.9 ± 0.4 29.7 ± 0.4 +0.4 UDF12-3921-6322 3:32:39.21 -27:46:32.2 8.8 −0.2 > 31.2 29.9 ± 0.3 29.6 ± 0.3 29.9 ± 0.3 +0.5 UDF12-4344-6547 3:32:43.44 -27:46:54.7 8.8 −0.5 > 31.2 30.0 ± 0.3 30.1 ± 0.4 30.1 ± 0.3 +0.8 UDF12-3895-7114 3:32:38.95 -27:47:11.4 8.6 −0.6 > 30.9 30.4 ± 0.5 30.1 ± 0.3 30.1 ± 0.4 +0.2 b UDF12-3947-8076 3:32:39.47 -27:48:07.6 8.6 −0.2 31.0 ± 0.5 29.5 ± 0.2 29.0 ± 0.1 29.0 ± 0.1 UDFy-39468075 B11

Earlier Candidatesa +0.3 b UDFj-39546284 3:32:39.54 -27:46:28.4 11.9 −0.5 > 31.2 > 30.7 > 30.5 29.3 ± 0.2 B11 z≃10.3 UDFj-38116243 3:32:38.11 -27:46:24.3 − > 31.2 > 30.1 30.3 ± 0.5 30.0 ± 0.3 B UDF09 c #1, B11b #2 +0.4 c UDFj-43696407 3:32:43.69 -27:46:40.7 7.6 −0.6 31.0 ± 0.6 > 30.1 29.9 ± 0.3 29.5 ± 0.2 B UDF09 #2 +0.9 c UDFj-35427336 3:32:35.42 -27:47:33.6 7.9 −0.8 > 30.8 30.3 ± 0.4 30.2 ± 0.4 29.6 ± 0.2 B UDF09 #3 +0.2 b UDFy-38135539 3:32:38.13 -27:45:53.9 8.3 −0.1 30.1 ± 0.2 28.6 ± 0.1 28.5 ± 0.1 28.4 ± 0.1 B11 8.5

Fig. 1.— Hubble Space Telescope WFC3/IR images of the promising z > 8.5 candidates from combined UDF12 and earlier data. Each panel is 2.4 arcsec on a side. (Top two rows) Summed (F125W+F140W+F160W) images for 6 sources with 8.5

Fig. 2.— Spectral energy distributions for 7 promising z >8.5 candidates from combined UDF12 and earlier data. Blue lines represent the adopted high z solution, orange lines the best rejected low z alternative. Upper limits are 1-σ. IRAC limits of m3.6 > 28.5 and m4.5 > 28.0 are based on a deconfusion analysis of the Labbéet al. (2012) data using the UDF12 H160 image and the technique described in McLure et al. (2011). The final two panels show photometric likelihood fits for the sub-samples with 8.5

Fig. 3.— Possible contamination by foreground extreme emission line galaxies. (Left) F105W minus F160W color as a function of redshift for a 1 Myr (red) and 10 Myr (black) metal-poor dust-free stellar population incorporating nebular emission following the precepts of Ono et al (2010). Expectations for a stellar continuum only are shown by the dotted lines. The upper dashed line is the lower limit for UDFj-39546284 derived from the 2σ F105W limit and the >6σ F160W detection. (Right). Simulated spectrum of UDFj-39546284 assuming a 10 Myr starburst at z=2.24 where [O III] emission dominates the F160W signal. Arrows indicate 2σ upper limits from non-detections in the various WFC3/IR and ACS bands. The blue circle indicates the expected Lyman α strength rejected by a Keck spectrum (blue arrow). 8

Fig. 4.— Luminosity and star formation rate (SFR) density versus redshift inferred from UDF12. Reddening corrected luminosity densities are shown from Bouwens et al. (2007, 2011) over the redshift range 5