Very blue UV-continuum slopes of low luminosity z~7 galaxies from WFC3/IR: Evidence for extremely low metallicities?

0.23 MB·English

by  R. J. Bouwens

Draftversion January9,2010
PreprinttypesetusingLATEXstyleemulateapjv.11/10/09
VERY BLUE UV-CONTINUUM SLOPES β OF LOW LUMINOSITY Z ∼7 GALAXIES FROM WFC3/IR:
EVIDENCE FOR EXTREMELY LOW METALLICITIES?1
R. J. Bouwens2,3, G. D. Illingworth2, P. A. Oesch4, M. Trenti5, M. Stiavelli6, C. M. Carollo4, M. Franx3, P. G.
van Dokkum7, I. Labb´e8, D. Magee2
Draft version January 9, 2010
ABSTRACT
0 Weusetheultra-deepWFC3/IRdataovertheHUDFandtheEarlyReleaseScienceWFC3/IRdata
1 over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-
0 forming galaxies at z ∼ 7. We determine the UV-continuum slope β in these galaxies, and compare
2 the slopes with galaxies at later times to measure the evolution in β. For luminous L∗ galaxies,we
z=3
measure a mean UV-continuum slope β of −2.0±0.2, which is comparable to the β ∼−2 derived at
n
similar luminosities at z ∼ 5−6. However, for the lower luminosity 0.1L∗ galaxies, we measure a
a z=3
J meanβ of−3.0±0.2. This is substantiallybluer thanis found forsimilarluminosity galaxiesatz∼4,
just 800Myr later,andevenatz∼5-6. Inprinciple, the observedβ of−3.0canbe matchedbya very
9
young, dust-free stellar population, but when nebular emission is included the expected β becomes

] ≥ −2.7. To produce these very blue β’s (i.e., β ∼ −3), extremely low metallicities and mechanisms
O to reduce the red nebular emission seem to be required. For example, a large escape fraction (i.e.,
C fesc & 0.3) could minimize the contribution from this red nebular emission. If this is correct and
the escape fraction in faint z ∼ 7 galaxies is &0.3, it may help to explain how galaxies reionize the
.
h universe.
p Subject headings: galaxies: evolution — galaxies: high-redshift
-
o
tr 1. INTRODUCTION et al. 2009b; Bouwens et al. 2009b; McLure et al. 2009;
s Bunkeretal.2009),and&20z ∼7candidategalaxiesin
a The spectral properties of high-redshift galaxies must
theWFC3/IREarlyReleaseScience(ERS)observations
[ undergo dramatic changes at some point in the past,
as the metallicities in these systems drop to lower val- overthe CDF-South(R.J.Bouwensetal.2009,inprep).
4 Here we take advantage of these early WFC3/IR ob-
ues and these systems become progressively younger.
v servations to study the spectral properties of candidate
In the limit of low metallicities, gas is no longer able
1 z ∼ 7 galaxies. Our principal focus will be on the slope
to cool efficiently, likely resulting in massive extremely
0 of z ∼ 7 galaxy spectra in the UV-continuum – since
low-metallicity (or Population III) stars whose hot at-
0 this slope is the primary observable we can derive from
mospheres are expected to result in a very hard UV-
0 the available broadband imaging data with WFC3. The
continuum spectrum and strong HeII 1640 emission.
0. However, the strong UV flux coming from hot stars is UV-continuum slope β (fλ ∝ λβ: e.g., Meurer et al.
1 expected to be largely offset by the redder nebular con- 1999)provides us with a powerful constrainton the age,
9 tinuum light produced by the ionized gas surrounding metallicity, and dust content of high-redshift galaxies;it
0 these massive stars (e.g., Schaerer 2002). has already been the subject of much study at z ∼3−6
: (Lehnert& Bremer2003;Stanwayetal.2005;Yanetal.
v The newly installed WFC3/IR camera on the Hubble
2005;Bouwensetal.2006;Hathietal.2008;seeBouwens
i Space Telescope permits us to observefaint z &7 galax-
X ies&40×moreefficientlythanbefore,providinguswith etal. 2009aforasystematicstudyatz ∼2−6)andeven
at z ∼7 (Gonzalez et al. 2009) using NICMOS data.
r our most detailed look yet at the UV light and spectral
a properties of z & 7 galaxies. Already &25 likely z∼7- Throughout this work, we quote results in terms of
the luminosity L∗ Steidel et al. (1999) derived at z ∼
8 galaxies have been identified in the early WFC3/IR z=3
3: M = −21.07. We refer to the HST F606W,
data over the Hubble Ultra Deep Field (HUDF: Oesch 1700,AB
F775W,F850LP,F105W, F125W, and F160W bands as
1Based on observations made with the NASA/ESA Hubble V606, i775, z850, Y105, J125, and H160, respectively, for
Space Telescope, which is operated by the Association of Univer- simplicity. Where necessary, we assume Ω0 =0.3, ΩΛ =
sitiesforResearchinAstronomy,Inc.,underNASAcontractNAS 0.7, H = 70km/s/Mpc. All magnitudes are in the AB
0
5-26555. Theseobservationsareassociatedwithprograms#11563, system (Oke & Gunn 1983).
9797.
CA2U95C0O64/Lick Observatory, University of California, Santa Cruz, 2. OBSERVATIONALDATA
3Leiden Observatory, Leiden University, NL-2300 RA Leiden, Our primary dataset is the ultra deep WFC3/IR data
Netherlands
4InstituteforAstronomy,ETHZurich,8092Zurich,Switzerland overtheHUDFandthewide-areaWFC3/IRERSobser-
5UniversityofColorado,CenterforAstrophysicsandSpaceAs- vations over the CDF-South (PI O’Connell: GO 11359).
tronomy,389-UCB,Boulder,CO80309,USA The HUDF data permit us to identify lower luminosity
6Space Telescope Science Institute, Baltimore, MD 21218, z ∼7 galaxies and quantify their properties (e.g., Oesch
UnitedStates
7Department of Astronomy, Yale University, New Haven, CT et al. 2009b), while the ERS data permit us to do the
06520 same for more luminous z ∼7 galaxies.
8CarnegieObservatories,Pasadena, CA91101, HubbleFellow For our HUDF dropout selections, we make use of the
2
v1.0 reductions of the HUDF ACS data (Beckwith et al.
2006) rebinned on a 0.06′′-pixel scale, and our own re-
duction of the HUDF09 WFC3/IR data over the HUDF
(Bouwens et al. 2009b; Oesch et al. 2009b). The op-
tical ACS imaging over the HUDF reach to 29.4, 29.8,
29.7, and 29.0 AB mag (5σ: 0.35′′-diameter apertures)
in the B , V , i , z bands, respectively, while
435 606 775 850
the near-IRWFC3/IRdatareachto 28.8,28.8,and28.8
(5σ: 0.35′′ apertures) in the Y , J , and H bands,
105 125 160
respectively. The PSF FWHMs are ∼0.10′′ for the ACS
data and ∼0.16′′ for the WFC3/IR data.
For our dropout selections over the WFC3/IR ERS
fields,wemakeuseofourownreductionsoftheavailable
ACS/WFC data over the GOODS fields (Bouwens et al.
2006, 2007). Our reductions reach to 28.0, 28.2, 27.5,
and 27.4 in the B , V , i , z , respectively (5σ:
435 606 775 850
0.35′′ apertures). This is similar to the GOODS v2.0 re-
ductions,butreach∼0.1-0.3magdeeperinthez -band
850
due to the inclusion of the SNe follow-up data (Riess et
al. 2007). Our reductions of the WFC3/IR ERS data
was performed using the same procedures as we used on Fig.1.—UV-continuumslopeβversusabsoluteUV magnitude.
the HUDF09 WFC3/IR data. These data reach to 27.7 Theredcirclesshowtheβdeterminationsandabsolutemagnitudes
and 27.4 in the J125 and H160 bands, respectively (5σ: (derivedfromtheJ125−H160 colorsand 21(J125+H160)AB mag-
0.35′′ apertures). nitudes,respectively)forindividualz∼7galaxiesinourHUDF09
and ERS selections. The large red squares (with 1σ error bars)
representthemeanvaluesin1-magbins(dotted lines). Theblack
3. RESULTS points correspond to the β determinations at z ∼ 4 (Bouwens et
3.1. Catalog Creation al. 2009a). The β’s for lower luminosity galaxies at z ∼ 7 (red
circles)aremuchbluer(by∆β of∼1)thanthosederivedatz∼4
Our procedure for source detection and photometry is (a4σ difference). (inset)Therelativevolumesavailableforselect-
similartothatusedinpreviousstudiesbyourteam(e.g., ing galaxies with various β’s using our HUDF09 z-dropout crite-
rion(see §3.4)). These volumes do not depend significantly on β,
Bouwensetal.2007,2009a)andreliesupontheSExtrac-
demonstrating that the blue β’s observed for faint z ∼ 7 galaxies
torsoftware(Bertin&Arnouts1996)runindoubleimage isnotaselectioneffect.
mode. Source detection is performed off the square root
ofχ2 image(Szalayetal.1999: similarto acoaddedim- band color J125−H160 as
age) constructed from all images redward of the Lyman
β =4.29(J −H )−2.00 (1)
125 160
break. Colors are measured in small scalable apertures
with a Kron (1980) parameter of 1.2 (typically ∼0.4′′- The J and H bands here probe rest-frame
125 160
diameter apertures). Fluxes measured in these small ∼1550˚Aand ∼1940˚A, respectively, for the typical z ∼ 7
scalable apertures are then corrected (typically by ∼0.4 z-dropout candidate in our sample, and are not affected
mag) to total magnitudes using the additional flux in a by Lyα emission or IGM absorption (in contrast to the
larger scalable aperture (Kron parameter of 2.5). Y -band). The above equation is derived assuming a
105
flat spectrum source with no absorption lines (found to
3.2. z ∼7 z-dropout selection work very well by Meurer et al. 1999).
In Figure 1, we show the β determinations for z ∼ 7
For our lower-luminosity z ∼ 7 z-dropout selection,
z-dropout candidates versus luminosity. Both our ultra-
we use a criterion very similar that used by Oesch et al.
deep HUDF09 and wide-area ERS selections are in-
(2009b) over the HUDF09 WFC3/IR field:
cluded, as are the z ∼ 4 selections from Bouwens et al.
(z −Y >0.8)∧(Y −J <0.8) (2009a). The trend of β with luminosity is illustrated
850 105 105 125
with the large squares. The difference between z ∼ 7
Over the ERS fields, no deep WFC3 Y -band coverage
105 and z ∼4 is striking.
is available, so we make use of the following criterion
Use of the photometry from Oesch et al. (2009b),
(R.J. Bouwens 2009, in prep):
McLure et al. (2009), or Bunker et al. (2009) for our
(z −J >0.8)∧(J −H <0.3) z ∼ 7 sample yield very consistent colors and similarly
850 125 125 160
blue (or even bluer) β’s. In addition, use of a consistent
Sourcesarerequiredtobeundetected(<2σ)inallbands 0.7′′-diameter aperture for the color measurements yield
bluewardofthebreak(andalsoundetected[<1.5σ]inno equallyblueβ’s(albeitwithlargerrandomuncertainties,
>1 bands) to ensure that our selections are largely free due to the use of larger apertures).
oflowerredshiftcontaminants. Sourcesmustbedetected
at≥5.5σintheJ bandtoensurethatz ∼7candidates 3.4. Consideration of Possible Selection Biases
125
in our sample correspond to real sources.
Thedistributionofβ’sderivedforsourcesinourz ∼7
sample can be affected by the selection process. This
3.3. UV-continuum slope Measurements
effect is seen in lower redshift samples, where galaxies
TheUV-continuumslopesβ weestimateforsourcesin with bluer β’s have larger selection volumes than galax-
our z ∼7 z-dropoutsample are derived from the broad- ieswithredderβ’s. Todeterminetheimportanceofsuch
3
We can take advantage of the very deep, high-quality
WFC3/IR data to obtain self-consistent determinations
of these slopes at z∼5-6. The z ∼ 5 V and z ∼ 6 i-
dropouts over the HUDF09 and GOODS ERS data are
selected in the same way as in Bouwens et al. (2007:
see also Giavalisco et al. 2004; Beckwith et al. 2006),
except that at z ∼ 6 we also require galaxies to satisfy
thecriterion(z −J <0.6). Atz ∼5andz ∼6,the
850 125
β’s are estimated based upon the z −(Y +J )/2
850 105 125
and Y −(2J +H )/3 colors, respectively, which
105 125 160
area goodmatchin rest-framewavelengthto the J −
125
H colors used to estimate β at z ∼7. The conversion
160
formulas we use are
β =4.07(z −(Y +J )/2)−2.00 (z ∼5)(2)
850 105 125
β =3.78(Y −(2J +H )/3)−2.00 (z ∼6)(3)
105 125 160
In Figure 2, we plot these β determinations as a func-
tionof redshift, for both luminous L∗ galaxiesand lower
luminosity galaxies.
3.6. z ∼8 Comparison Sample
Faint z ∼ 8 Y -dropout candidates in the HUDF09
105
WFC3/IR data (Bouwens et al. 2009b) also have very
blueJ −H colors,againsuggestingβ’sof∼−3. We
125 160
decidednottoconsiderthissampleherebecausetheJ -
125
bandfluxcanbeaffectedbyLymanseriesabsorptionand
Lyα emission at z >8.1, and hence the results would be
less robust.
Fig.2.— Mean UV-continuum slope measured for galaxies at
z ∼ 7 (large red circles) versus galaxies of similar luminosities at 4. DISCUSSION
lower redshifts. The top panel shows the evolution in this slope
forgalaxieswithaUV luminosityofL∗z=3(−21ABmag)andthe Before discussing the extraordinarily blue UV-
bottompanelshowsthisevolutionforUV luminositiesof0.1L∗z=3 continuum slopes β ∼ −3 found for lower luminosity
(−18.5 AB mag). The determinations of Bouwens et al. (2009a)
galaxies at z ∼ 7, we first consider the relatively lumi-
baseduponACS+NICMOSdataareshownwiththeopenredcir-
cles. The short error bars are the random errors while the longer nous L∗z=3 galaxy candidates. These sources have mea-
errorbarsalsoincludepossiblesystematicerrors(e.g.,seeBouwens sured β’s of −2, which is similar to that observed for
et al. 2009a). We also show β determinations by Stanway et al. luminous galaxies at z ∼ 3−6 (Figure 2). Such slopes
(2005: cyan square), Ouchi et al. (2004: blue pentagon), Adel-
berger & Steidel (2000: black star), andHathi etal.(2008: green canbe fitby a moderatelyyoung,subsolar(0.2Z⊙)stel-
open squares). Note the dramatic change in β in the ∼800 Myr lar population, with a maximum E(B − V) of ∼0.05
fromz∼7toz∼4forlowerluminositygalaxies. (Calzetti et al. 2000 extinction law: see also Bouwens
et al. 2009a).
effects, we constructed models with various β distribu- The lower luminosity (0.1L∗ ) galaxy candidates at
z=3
tions, added galaxies with these distributions to the ob- z ∼ 7, by contrast, have observed β’s of −3.0 ± 0.2.
servational data, and then attempted to reselect these This is much bluer than for luminous z ∼7 galaxies (by
galaxies using the selection criteria in §3.2 to estimate ∆β ∼1) and also much bluer than is found at z ∼5−6
the effectiveselectionvolumeversusβ. We modelledthe (Figures1-2). Thismakesthesegalaxiesofgreatinterest
pixel-by-pixel profiles of the sources using similar lumi- sincetheyarelikelytobeevenyounger,moremetalpoor,
nosity z ∼4 B-dropouts from the Bouwens et al. (2007) and dust-free than any galaxies known.
HUDF sample, but scaled in size (physical) as (1+z)−1 This is not a selection effect (§3.4), and cannot be at-
to match the observed size-redshift relationship (Oesch tributed to Lyα emission contributing to the broadband
et al. 2009c; Ferguson et al. 2004; Bouwens et al. 2004). fluxes. Lyα does not move into the J -band (used to
125
The selection volumes derived from these simulations estimate β) until z & 8.1, and . 10% of our z-dropout
are shown in the inset to Figure 1. Encouragingly,these sample extends to z >8.
volumesshowonly a modestdependence uponthe input An AGN contribution seem similarly unlikely, given
β forthe range−4<β <−1,andsoselectionbiasesare the rarety of AGN signatures in faint Lyman-Break and
minimal. Lyα-emitter samples (e.g., Nandra et al. 2002; Ouchi et
al. 2008).
3.5. Low-redshift Comparison Samples What then is the explanation for the very blue β’s?
Tointerpettheβ’swederivefromourz ∼7z-dropout We explore several possibilities:
selections, we compare them with the β’s found for sim-
4.1. Standard stellar population models
ilar luminosity galaxies at z ∼ 4 − 6. Bouwens et al.
(2009a)providedeterminationsoftheseslopesasafunc- A first question is whether it is possible to obtain a β
tion of luminosity at z ∼2−6. The β measurements at of ∼ −3 using standard stellar population models (e.g.,
z ∼4 are also shown in Figure 1 for comparison. Leitherer et al. 1999; Bruzual & Charlot 2003). The
4
answer is that it is possible, but only for very young
(<5Myr)star-formingsystems (see Figure3). However,
to do so ignores the nebular continuum emission from
the ionized gas around the young stars. Including this
nebular continuum emission can redden the observed β
by as much as ∆β ∼0.5.
Figure 3 also shows the β’s predicted for several low
metallicity(0.02Z⊙and0.2Z⊙)starbursts,asafunction
of age for the Schaerer (2002) stellar population models
(which – like Starburst99 Leitherer et al. 1999 – include
a nebular contribution). In the best cases, the models
predict β’s as blue as −2.7, which is redder than what
we observe in our faint samples.9 This would suggest
that lower metallicities are needed since the standard
Leithereretal.(1999)orBruzual&Charlot(2003)stellar
populationmodels do not produce blue enoughcolorsto
matchthosefoundinourlowerluminosityz ∼7sample.
Of course, the significance of this result is only modest
(< 1.3σ), but the similarly blue colors observed for z∼8
selections(Section3.6)suggestthatwemaywanttotake
this finding at face value.
Fig. 3.—UV-continuumslopeβwewouldexpectforz∼7galax-
ies as a function of age forconstant star formation(CSF) models
and an instantaneous burst (Schaerer 2002). The gray band de- 4.2. Extremely low metallicities (≤10−3 Z⊙)
notes the observed mean β and its uncertainty. The slopes β de-
In Figure 4 we present the β’s predicted by the
rivedfromthestellarlight(dottedlines)andthestellar+nebular
light (solid lines) are shown. While it is in principle possible to Schaerer (2003) stellar population models (which con-
obtainβ’sof−3withstandardlowmetallicity(≥0.02Z⊙)models, veniently provide predictions at extremely low metallici-
includingthenebularemissionassociatedwithhotstarsmakethe ties)asafunctionofageforinstantaneousburstsassum-
predictedβ’s≥−2.7andhencetoored. ingametallicityof0.5×10−3Z⊙,0.5×10−5Z⊙,andzero
(population III).10 From the figure, it is clear that the
nebular component contributes significantly to the total
light output from .10−3 Z⊙ stellar populations.
Whileinitiallysomewhatredduetothenebularcontri-
bution, the predicted β’s for these ultra-low metallicity
modelsbecomemuchblueratages>10Myr,eventually
reaching β’s of −3. Such metallicities and ages are not
necessarilyunreasonablefor lowerluminosity galaxiesat
z ∼7,andthereforeatleastonepossibleexplanationfor
theveryblueβ’sinourselectionsisthatthemetallicities
for galaxies in our sample may be .10−3Z⊙.
The above explanation may explain some of the very
bluegalaxiesinourselection,butgiventhatβ ∼−3only
for a limited period (10-30 Myr after a burst), it seems
unlikely to be the general explanation (unless updated
models revise the theoretical SEDs).
4.3. Top-heavy IMF
Oneseeminglyattractiveexplanationfortheveryblue
β’s observed is through a top-heavy IMF, since galaxy
stellar populations would be weighted towards massive,
blue stars. The difficulty with this explanation is that
these same massive stars are extraordinarily efficient at
Fig. 4.—UV-continuumslopeβ Schaerer(2003)calculatedasa ionizing the gas around them – resulting in substantial
functionofageforinstantaneous burstmodelsfordifferentmetal-
licities. Both the slopes β derived from the stellar light (dotted nebular emission and leaving the galaxywith a net β no
lines)and thestellar + nebular light(solidlines)are shown. The bluer(andlikelyredder)thantheyoung(<1Myr)bursts
purestellarlight(dottedlines)hasveryblueUV-continuumslopes shown in Figures 3-4 (see also discussion in Leitherer &
β’s (β . −3) for all the low metallicity cases considered here.
Heckman 1995).
ChangingtheIMFdoes notappear tochange theconclusionhere
in any significant way. Of course, these same hot low metallicity
starsalsoionizethegasaroundthem,thusproducingasubstantial 9 To match the wavelength baseline for β measured here, we
amount of redder nebular continuum light. This makes the total added −0.1 to the β’s (1300-1800˚Abaseline) tabulated in the
SED of a galaxy much redder in general, and in the calculations Schaerer(2002, 2003).
bySchaerer (2003)shownhere,β never becomes bluerthan−3.0. 10 The use of instantaneous burst models allows us to explore
themostextremecases. Othermodels(τ,inverseτ,CSF)produce
comparableβ’s.
5
4.5. Changes in Escape Fraction?
One possibility that we have explored to reduce the
nebular emission contribution is if the ionizing radiation
leaksdirectlyintotheIGM(e.g.,duetotheeffectofSNe
on the galaxies’ ISM, possibly from a top-heavy IMF:
Trenti & Shull 2009). We consider such a possibility
schematicallyinFigure5,showingthetime-averagedβ’s
expectedforstellarpopulationsofvariousmetallicitiesas
a function ofthe escape fractionf ofionizing photons
esc
intotheIGM.Forf ofunity,wesimplyrecovertheβ’s
esc
fromthepurestellarSEDsandforf ofzero,werecover
esc
the stellar + nebular SEDs. For fractional f , we in-
esc
terpolate between the two extremes. The time-averaged
β’s used for Figure 5 is based upon those presented in
Figure 4.
Comparingthepredictedβ’swiththatobserved(grey-
shaded region: Figure 5), we see that f &0.3 would
esc
permit us to easily match the observations. Such an es-
cape fraction is signicantly higher than the ∼10% fre-
quently assumed in calculations assessing the sufficiency
of galaxies to reionize the universe. Since most of the
luminosity density at z > 7 comes from low luminosity
Fig. 5.— Mean UV-continuum slopes β’s predicted for high-
galaxies,theestimatednumberofionizingphotonsinthe
redshiftgalaxysamplesversustheescapefractionfesc. Thepredic-
z > 7 universe could increase by factors of &3. Such a
tionsaremadeusingtheSchaerer(2003)stellarpopulationmodels
(includingnebularemission)formetallicitiesof<10−3Z⊙. Galax- large change could provide the needed photons to reion-
iesareassumedtobeobservedatsomerandompointduringtheir ize the universe, providing a resolution to the current
starformationhistoriesandto experience instantaneous bursts of debate (e.g., Bouwens et al. 2008; Oesch et al. 2009a,
starformationevery10-40Myr(thedashed,solid,anddottedlines
2009b;McLureetal. 2009;Bunkeretal. 2009;Gonzalez
givetheaverageβ forthefirst10,20,and40Myr,respectively,of
theinstantaneousburst). Forfesc=1,alloftheionizingradiation et al. 2009; Ouchi et al. 2009; Pawlik et al. 2009).
fromagalaxyescapesintotheIGMandhencedoesnotcontribute
to ionizing the gas within a galaxy (and hence the contribution 5. SUMMARY
from nebular continuum emission to the total light is minimal).
On the other hand, for fesc ∼ 0 (preferred in some simulations: The strikingly blue UV-continum slopes β’s seen at
e.g.,Gnedinetal.2008), theionizingradiationfromthehotstars lower luminosities in candidate z ∼ 7 galaxies (also ap-
does not make it out of galaxies – resulting in substantial nebu- parentintheBouwensetal.2009bz ∼8sample)indicate
lar continuum emission (see Figure 4) and hence a much redder
thatwearenowbeginningtoexplorearegimewherethe
β. Perhaps the veryblue β’sobserved (shaded gray region) could
indicatethat theescapefractionislargeratfesc &0.3thanwhat natureofgalaxiesandtheirstellarpopulationsareunder-
hascommonlybeenconsidered? goingadramaticchange. Theseresultsraisemanyasyet
4.4. Minimizing nebular emission unansweredquestions,butcouldbeheraldingthetransi-
tion from the first stars and youngestobjects within the
A significant obstacle to matching the very blue β’s first400Myr atz &10to the galaxiesthat dominate the
observedistherednebularemissionassociatedwithhot, universe for the next 2 Gyr and provide clues as to the
ionizing stars. The nebular contribution could be re- source of photons that reionize the universe.
duced in a number of ways, by changes to the ioniza-
tionparameter,metallicity, geometry,etc. Assessing the
impact of such changes would benefit from further de- We thank Daniel Schaerer for helpful conversations.
tailed modelling. However, we should emphasize that We are grateful to all those at NASA, STScI and
regardless of changes to the nebular contribution very throughoutthecommunitywhohaveworkedsodiligently
lowmetallicitymodels(orveryyoungages)appeartobe tomakeHubbletheremarkableobservatorythatitisto-
needed to match the very blue β’s observed. day. WeacknowledgethesupportofNASAgrantNAG5-
REFERENCES
Adelberger,K.L.&Steidel,C.C.2000,ApJ,544,218 Bunker,A.,etal.2009,MNRAS,inpress,arXiv:0909.2255
Beckwith,S.V.W.,etal.2006,AJ,132,1729 Calzetti,D.,Armus,L.,Bohlin,R.C.,Kinney,A.L.,Koornneef,
Bertin,E.andArnouts,S.1996,A&AS,117,39 J.,&Storchi-Bergmann,T.2000,ApJ,533,682
Bouwens,R.J.,Illingworth,G.D.,Blakeslee,J.P.,Broadhurst, Ferguson,H.C.etal.2004,ApJ,600,L107
T.J.,&Franx,M.2004,ApJ,611,L1 Giavalisco,M.,etal.2004a, ApJ,600,L93
Bouwens,R.J.,Illingworth,G.D.,Blakeslee,J.P.,&Franx,M. Gnedin,N.Y.,Kravtsov,A.V.,&Chen,H.-W.2008,ApJ,672,
2006, ApJ,653,53 765
Bouwens,R.J.,Illingworth,G.D.,Franx,M.,&Ford,H.2007, Gonzalez, V.,Labbe,I.,Bouwens, R.,Illingworth,G.,Franx,M.,
ApJ,670,928 Kriek,M.,Brammer,G.2009,ApJ,submitted,arXiv:0909.3517
Bouwens,R.J.,Illingworth,G.D.,Franx,M.,&Ford,H.2008, Hathi,N.P.,Malhotra,S.,&Rhoads,J.E.2008,ApJ,673,686
ApJ,686,230 Kron,R.G.1980,ApJS,43,305
Bouwens,R.J.,etal.2009a, ApJ,705,936 Lehnert,M.D.&Bremer,M.2003, ApJ,593,630
Bouwens,R.J.,etal.2009b,ApJ,inpress,arXiv:0909.1803 Leitherer,C.,&Heckman, T.M.1995, ApJS,96,9
Bruzual,G.,&Charlot,S.2003, MNRAS,344,1000 Leitherer,C.,etal.1999,ApJS,123,3
6
McLure,R.,etal.2009,MNRAS,inpress,arXiv:0909.2437 Riess,A.G.,etal.2007,ApJ,659,98
Meurer,G.R.,Heckman,T.M.,&Calzetti,D.1999,ApJ,521,64 Schaerer,D.2002,A&A,382,28
Nandra,K.,Mushotzky, R.F.,Arnaud,K.,Steidel,C.C., Schaerer,D.2003,A&A,397,527
Adelberger,K.L.,Gardner,J.P.,Teplitz,H.I.,&Windhorst, Stanway, E.R.,McMahon,R.G.,&Bunker,A.J.2005,
R.A.2002, ApJ,576,625 MNRAS,359,1184
Oesch,P.A.,etal.2009a,ApJ,690,1350 Steidel,C.C.,Adelberger,K.L.,Giavalisco,M.,Dickinson,M.
Oesch,P.A.,etal.2009b, ApJ,inpress,arXiv:0909.1806 andPettini,M.1999,ApJ,519,1
Oesch,P.A.,etal.2009c, ApJ,inpress,arXiv:0909.5183 Szalay,A.S.,Connolly,A.J.,&Szokoly,G.P.1999,AJ,117,68
Oke,J.B.,&Gunn,J.E.1983,ApJ,266,713 Trenti,M.,&Shull,M.2009, ApJ,submitted,arXiv:0905.4505
Ouchi,M.,etal.2004,ApJ,611,660 Yan,H.,etal.2005,ApJ,634,109
Ouchi,M.,etal.2008,ApJS,176,301
Ouchi,M.,etal.2009,ApJ,706,1136
Pawlik,A.H.,Schaye, J.,&vanScherpenzeel, E.2009, MNRAS,
394,1812