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Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies:- II. A High Fraction of Line Emitters at Redshift Six

As Lyman-alpha photons are scattered by neutral hydrogen, a change with redshift in the Lyman-alpha equivalent width distribution of distant galaxies offers a promising probe of the degree of ionization in the intergalactic medium and hence when cosmic reionization ended. This simple test is complicated by the fact that Lyman-alpha emission can also be affected by the evolving astrophysical details of the host galaxies. In the first paper in this series, we demonstrated both a luminosity and redshift dependent trend in the fraction of Lyman-alpha emitters seen within color-selected Lyman-break galaxies (LBGs) over the range 3<z<6; lower luminosity galaxies and those at higher redshift show an increased likelihood of strong emission. Here we present the results from much deeper 12.5 hour exposures with the Keck DEIMOS spectrograph focused primarily on LBGs at z~6 which enable us to confirm the redshift dependence of line emission more robustly and to higher redshift than was hitherto possible. We find 54+/-11% of faint z~6 LBGs show strong (W_0>25 A) emission, an increase of 1.6x from a similar sample observed at z~4. With a total sample of 74 z~6 LBGs, we determine the luminosity-dependent Lyman-alpha equivalent width distribution. Assuming continuity in these trends to the new population of z~7 sources located with the Hubble WFC3/IR camera, we predict that unless the neutral fraction rises in the intervening 200 Myr, the success rate for spectroscopic confirmation using Lyman-alpha emission should be high.


INTRODUCTION
The reionization of neutral hydrogen in the intergalactic medium (IGM) was a landmark event in cosmic history, rendering the Universe transparent to UV photons and dramatically reducing the star formation efficiency in dwarf galaxies. In spite of its importance, there are few robust constraints on when reionization occurred. Polarization measures of the microwave background radiation (Larson et al. 2010) demonstrate scattering by free electrons in the redshift range 7 < z < 20 but do not describe the evolving neutral fraction, x HI . Absorption line spectra of high-z quasars are largely sensitive to the very late stages (x HI ≃ 10 −3 ) of reionization (Fan et al. 2006), and progress has been slow due to the paucity of sources so far detected beyond z ≃ 6.5.
One of the most promising probes of reionization with current facilities is through the study of Lyα emission from star forming galaxies. Since Lyα photons are resonantly scattered by neutral hydrogen, the abundance of Lyα emitters should decrease as observations probe into the era where there are pockets of neutral gas. Studies of the redshift-dependent luminosity function (LF) of Lyα emitters (LAEs) selected via narrowband filters have revealed a possible decline in abundance between z = 5.7 and z = 7.0 (Kashikawa et al. 2006;Iye et al. 2006;Ota et al. 2008;Ouchi et al. 2010), offering tantalizing evidence that this short time interval (≃200 Myr) may correspond to one during which there is some evolution in the neutral fraction. But since a number of astrophysical factors can also affect the presence of Lyα emission, it may be dangerous to directly link evolution in the Lyα LF to reionization (e.g., Dayal et al. 2010). These factors include time-dependent changes in the host galaxy number density, dust obscuration and interstellar gas content and kinematic properties. By enlarging the LAE samples, it may be possible to bypass some of these complications by testing for the expected change in their spatial clustering and line profiles as the neutral era is entered .
A complementary approach introduced in Stark et al. (2010) (hereafter Paper I) is to spectroscopically measure the fraction of strong Lyα emitters within the colorselected Lyman Break Galaxy (LBG) population. By tracing the redshift-dependent fraction, the host galaxy number density is not a factor. Evolution in dust obscuration can be independently tracked using the continuum colors and ISM kinematics through deep spectroscopy (Steidel et al. 2010;Bouwens et al. 2009Bouwens et al. , 2010aVanzella et al. 2009). Although demanding observationally, high throughput spectrographs such as FORS2 on the ESO Very Large Telescope and DEIMOS on the Keck II telescope have enabled progress in recent years (Paper I, Vanzella et al. 2009). With the additional information on the host galaxies possible for the LBG population, we can hope to more reliably link any redshift-dependence in the Lyα fraction to ionization changes in the IGM. Most importantly of all, the proposed approach can now be readily extended to z ≃ 7-8 and beyond given the availability of LBG samples at these early epochs following the advent of the WFC3/IR camera onboard Hubble Space Telescope (HST, e.g. Bouwens et al. 2010a).
In Paper I we introduced a large Keck spectroscopic survey of z > 3 LBGs and demonstrated the practical details of the above method through analyses of the Lyα fraction (X Lyα ) in B-band (z ≃ 4) and V-band (z ≃ 5) dropouts to which we added a sample of i ′band (z ≃ 6) dropouts drawn from other programs (e.g., Vanzella et al. 2009. Correcting for minor magnitude and redshift-dependent biases in completeness and contamination, we determined the luminosity and redshift dependence of X Lyα over 3 < z < 6. Since the IGM is known to be highly ionized over this interval, this dataset enabled us to explore the importance of factors other than the IGM neutral fraction. We found that galaxies with lower rest-frame UV continuum luminosities exhibit Lyα emission more frequently than luminous systems. Correlations between line strength and UV continuum slope suggest reduced dust obscuration is the primary cause. The data also suggest an increase in X Lyα with redshift (dx Lyα /dz ≃ 0.05 ± 0.03), as originally claimed based on the relative evolution of the UV luminosity function of LAEs and LBGs over this redshift range (Ouchi et al. 2008). However, since the size of our archival z ≃ 6 samples were considerably smaller than that of our lower redshift database, the constraints on the redshift evolution were primarily derived from data spanning only the 300 Myr between z ≃ 4 and z ≃ 5.
Ideally one would construct the full equivalent width (EW) distribution function of Lyα emission for a range of UV luminosities at the highest redshift where the IGM is known to be highly ionized, i.e. z ≃ 6. This could then form the basis for comparisons with spectroscopic data at z > 7 where reionization may be incomplete. With this motivation, we have thus extended the sample introduced in Paper I, through ultra-deep spectroscopy of a sample of i ′ -band dropouts in the GOODS North field. The increased sample size and deep spectroscopic exposures provide statistically-significant constraints on the EW distribution of feeble sources, ensuring an adequate basis for comparisons with higher redshift spectroscopic samples.

OBSERVATIONS
Our dataset is primarily comprised of spectra obtained using the DEep Imaging Multi-Object Spectrograph (DEIMOS) at the Nasmyth focus of the 10 m Keck II telescope (Faber et al. 2003). We direct the reader to Paper I for a full description of our survey strategy. In Paper I we presented analysis of 513 DEIMOS spectra, including 268 unique B-drops and 95 unique V-drops. To this we added publicly available spectra from the VLT/FORS2 survey of z ≃ 4, 5, and 6 LBGs (Vanzella et al. 2009) and 2 unique z ≃ 6 LBGs from the Keck survey of Bunker et al (2010, in prep). The total sample drawn from Paper I is thus 351 B-drops, 151 V-drops, and 44 i ′ -drops.
The major step forward here is the inclusion of new z ≃ 6 spectra following ultra-deep Keck exposures of faint i ′ -band dropouts in GOODS-North. The archival data in Paper I was mostly based on the equivalent of 3-4 hour exposures with a 10 meter aperture. The new sample consists of 23 i ′ -band dropouts with 12.5 hour exposures (and an additional 7 with 3.67 hours of integration) enabling constraints to be placed further down the EW distribution at z ≃ 6 (allowing a uniform sampling over the full redshift range) and increasing the total z ≃ 6 LBG sample by nearly ≃ 70% to 74 across both GOODS fields.
The new data were taken during April 2010. Over 11-12 April, we obtained 12.5 hours of on-source integration in good seeing (<0. ′′ 8) for one mask containing 23 i ′ -band dropouts. Over 13-14 April, we obtained 3.67 hours of integration on a separate mask containing 7 i ′band dropouts. For both masks, we used the 830 line mm −1 grating, typically providing spectral coverage between 7000Å and 10400Å. Slit lengths were generally ≃ 7 ′′ , and slit widths were 1 ′′ . Skylines are measured to have a Gaussian σ of 1.1Å. Reduction was performed using the spec2d IDL pipeline developed for the DEEP2 survey (Davis et al. 2003) 7 . Wavelength calibration was performed using Ne+Xe+Cd+Hg+Zn reference arc lamps. As in Paper I, we flux calibrate our data using the spectra of alignment stars included on the slitmask (observed in 2 ′′ by 2 ′′ boxes). We compared this calibration to that obtained using spectroscopic standard stars and found it to be consistent to within ±20% (with no significant systematic offset) for the alignment stars. Using the flux calibration, we computed our survey sen- . Overplotted in red is the Lyα EW distribution for LBGs at 4 < z < 5 derived from the sample in Paper I (dotted lines provide 1σ uncertainties). Right: Evolution in the overall fraction of Lyα emitters (X Lyα ) in the LBG population over 4 < z < 6. Luminous LBGs are considered in the bottom panel, and less luminous systems in the top panel. In each panel, we derive the Lyα fraction of LBGs with Lyα EWs larger than 25Å (circles) and 55Å (squares). Assuming a linear relationship between X Lyα and z, we extrapolate these trends to z ≃ 7 (triangles with dashed-line error bars).
sitivity as a function of wavelength. The 5σ limiting line flux is 3.1±0.5×10 −18 erg cm −2 s −1 (assuming a range of Lyα line widths typical of our LBG samples), implying that we should be able to detect Lyα with rest-frame equivalent widths of greater than 20 ± 3Å for i ′ -drops with z 850 ≃ 27.

ANALYSIS
We searched for Lyα emission at the spatial position of the targeted LBGs in the Keck spectra. Line fluxes and EWs were calculated following the procedures discussed in Paper I. We account for the effects of line contamination and Lyα forest absorption (estimated using relations presented in Meiksin 2006) on the observed z 850band fluxes. Of the 23 i ′ -band dropouts for which we obtained ultra-deep spectra, 11 show Lyα emission, while 2 of the 7 i ′ -drops for which we obtained 3.67 hour integrations show Lyα (Figure 1). These results imply a large fraction of i ′ -drops have prominent Lyα emission. The rest-frame EWs for the i ′ -drops range between 9.4 A and 350Å. The vast majority of the emission lines are detected with high significance. Even so, we take a conservative cut, limiting our analysis to those sources with rest-frame EWs greater than 25Å and S/N > 7. This excises the Lyα detection in the middle right bottom panel of Figure 1. As a result, even among the faintest sources, the emission lines used in our analysis are very confidently detected (< S/N >=18), removing concern regarding spurious features.
As in Paper I, we determine the completeness of our Lyα detections as a function of wavelength by adding and recovering fake emission lines at random positions across the 2-D spectra. We compute the Lyα recovery rate as a function of absolute magnitude and wavelength for all masks observed (including those in Paper I) and make appropriate corrections. This test demonstrates that in our deep 12.5 hour mask we are >90% complete to lines with W Lyα,0 > 50Å even for the faintest i ′ -drops on our mask (z 850 ≃ 27). For lines with W Lyα,0 ≃ 20Å, the completeness implied by our simulations is ≃ 75 − 80% for sources in the faintest magnitude bin covered by our z ≃ 6 spectra. The completeness is of course lower on the DEIMOS mask observed for only 3.67 hours, reaching below ≃ 50% for faint sources with W Lyα,0 ≃ 20Å, and we therefore do not include these sources when computing the fraction of LBGs with low EW Lyα emission.
An additional concern is that the color-cut and z-band selection of i ′ -band dropouts are affected by Lyα emission and Lyα forest absorption. We investigate the extent to which these effects transform the observed EW distribution using Monte Carlo simulations. We create a large sample (>10 6 ) of artificial galaxies with intrinsic absolute magnitudes (normalized at 1500Å) spanning −21.5 < M UV < −18.5 and redshifts spanning 5.6 < z < 6.5. The intrinsic luminosity distribution of the fake galaxies matches the observed i ′ -drop luminosity function (e.g., Bouwens et al. 2006). For the spectral shape, we use synthetic templates (Charlot & Bruzual 2010, in preparation) with parameters fixed to those which provide reasonable fits for similarly bright i ′ -dropouts (e.g. Stark et al. 2009). Changing these parameters to other reasonable values does not affect our results. We attach Lyα luminosities to each of the galaxies according to an assumed Lyα EW distribution (which we describe below) and we also account for Lyα forest absorption using the relations presented in Meiksin (2006). Finally we derive i ′ 775 and z 850 -band magnitudes from the model SEDs and construct an artificial sample of i ′ -drops which satisfy the color criteria and z-band magnitude limit.
We find that the output EW distribution matches the input EW distribution of galaxies at the mean redshift of the i ′ -drop population. For example, if we adopt an input EW distribution with the form p(W Lyα,0 = exp[-W Lyα,0 /W 0 ] and set W 0 =20.0Å, we find that the output EW distribution is nearly identical to the input distribution (W 0 =20.1Å). It should be noted that Poisson noise (which tends to scatter faint sources toward slightly brighter magnitudes) will alter the EW distribution if the intrinsic EW distribution is luminosity-dependent, as suggested by Paper I. But this effect should occur at each redshift and hence should not affect the measured redshift evolution in the EW distribution.

RESULTS
We now derive the EW distribution and Lyα fraction (X Lyα ) for z ≃ 6 galaxies and compare with the lower redshift samples of Paper I. We group our i ′ -drop sample into two bins of rest-UV absolute magnitude, taking care to apply minor corrections to the observed broadband magnitudes to compensate for the effects of Lyα emission and IGM Lyα forest absorption. For galaxies without Lyα emission, we correct for Lyα forest absorption statistically using the redshift distribution predicted from the Monte Carlo simulations in §3.
In Figure 2, we present our observed Lyα EW distribution, with emission lines grouped in 30Å bins. For the luminous sub-sample, the distribution, p(W Lyα,0 ), rises toward lower EW widths, reaching p=12 ± 6.8% in the lowest EW bin considered. When compared to the EW distribution of luminous sources at 4 < z < 5 (from Paper I), we find that while Lyα is marginally more common in each EW bin at z ≃ 6, the uncertainties are too large to distinguish the two distributions. In contrast, in the lower luminosity bin, the EW distribution shows stronger positive evolution from 4 < z < 5, with Lyα considerably more prevalent among z ≃ 6 LBGs.
We next compute the LBG Lyα fraction by integrating the EW distribution above 25Å and 55Å to yield the fractions X 25 Lyα and X 55 Lyα . We group galaxies in the same two luminosity bins as in the analysis above. In the faint subset, we find 54 ± 11% have W Lyα,0 > 25Å and 27 ± 8.0% have W Lyα,0 > 55Å. Luminous galaxies ex- hibit Lyα emission less frequently, with 20 ± 8.1% and 7.4±5.0% observed with Lyα emission in excess of 25 and 55Å. Combining our results with those from Paper I, we find that the fraction of Lyα emitters among the LBG population increases with redshift for lower luminosity galaxies. Assuming a linear relationship between X Lyα and redshift, we find dX 25 Lyα /dz = 0.11 ± 0.04. In contrast, less redshift evolution is seen in the larger EW bin (dX 55 Lyα /dz = 0.018 ± 0.036), consistent with the findings from Paper I. Similar (albeit noisier) trends are seen in the more luminous sub-sample, with the lowest EW bin showing the strongest indications of positive evolution with redshift.
This improved determination of the EW distribution and Lyα fraction for LBGs at z ≃ 6 is a necessary step toward providing the essential baseline for predicting the outcome of spectroscopic campaigns beyond z ≃ 7 and interpreting any downturn in the Lyα fraction that may be associated with reionization (see §5).

Z > 7 LBGS
Our new results, taken together with those in Paper I, now suggest that ≃ 54% of moderately faint (−20.25 < M UV < −18.75) z ≃ 6 LBGs exhibit very strong Lyα emission. In Paper I, we argued that both the redshift and luminosity dependence of the Lyα fraction was likely due in large part to variations in dust obscuration as evidenced by the correlation between Lyα EW and the rest-frame UV slope, β. Recent analyses of the colors of the z ≃ 7 LBGs indicate that these systems are yet bluer than those at z ≃ 6 (Bouwens et al. 2010a), implying even less or no dust obscuration. Hence it seems likely that the redshift trend in the Lyα fraction presented in Figure 2 should continue to z ≃ 7 and that Lyα should be readily detectable in sufficiently deep spectroscopic campaigns.
Given the short cosmic time spanning 6 < z < 7 (≃ 170 Myr), it seems plausible to use the EW distribution and Lyα fractions presented in Figure 2 to predict the expected Lyα visibility for sources at z ≃ 7, assuming Lyα flux is not significantly attenuated by neutral hydrogen in the IGM. Motivated by the blue z ≃ 7 UV slopes discussed above, we extrapolate the evolution in X Lyα to z ≃ 7 (Figure 2). For low luminosity sources, this results in a small increase in the Lyα fraction (∆X 25 Lyα = 0.14) which we divide into the three EW bins using weights set by p(W Lyα,0 ). We follow the same procedure for the luminous sources. The results, presented in Figure 3, suggest a survey of ≃ 20-30 galaxies drawn from the nowavailable WFC3/IR target list (e.g., McLure et al. 2010;Bouwens et al. 2010b;Bunker et al. 2010) would yield interesting results. While uncertainty in the observed Lyα trends and their extrapolation to z ≃ 7 obviously affects our prediction, it seems clear that Lyα should be common in z ≃ 7 samples if the IGM is highly ionized. The failure to detect emission in such a sample might therefore be a strong indicator of a rising neutral fraction beyond z ≃ 6 as claimed originally by Kashikawa et al. (2006) from the luminosity function of LAEs.
How practical is a search for line emission to the EW limit of 20Å discussed above? In terms of an integrated line flux F Lyα , the EW limit corresponds to F Lyα ≃ 3(7)×10 −18 erg cm −2 s −1 for z ≃ 7 galaxies with M UV =-20(-21) (corresponding to galaxies with apparent AB magnitudes of J ≃ 27 and 26, respectively). Such line flux limits are feasible with spectrographs on 8-10 meter telescopes such as the Near InfraRed SPECtrograph (NIRSPEC) on Keck II (McLean et al. 1998). Earlier work with NIRSPEC has reached such a limit at 5-σ significance between the atmospheric sky lines in the Y and J-band in ≃ 4 hours (Stark et al. 2007;Richard et al. 2008). As more multi-object infrared spectrographs become available, it will be feasible to observe many z > 7 sources simultaneously, allowing ultra-deep exposures of galaxies at least as faint as M UV ≃ -19.

CONCLUSIONS
We present new ultra-deep spectroscopic observations of 30 i ′ -band dropouts in GOODS-N using DEIMOS on the Keck II telescope. By adding these spectra to the large database of DEIMOS and FORS2 spectra of B, V, and i ′ -band dropouts discussed in Paper I, we demonstrate more robustly the rise with redshift over 4 < z < 6 in the fraction of low luminosity LBGs that show prominent Lyα emission. We also derive a much-improved EW distribution of Lyα at z ≃ 6, the highest redshift where the intergalactic medium is known to be highly ionized. Motivated by the continued blueward evolution of the continuum UV slopes to z ≃ 7, we extrapolate the Lyα fraction trends presented in this paper to redshift 7, and predict the likely success rate of recovering Lyα emission in the new population of z > 7 sources located with HST. Our results suggest line emission should be readily detected given adequate observing time and that quantitative results would therefore test for the presence of neutral gas associated with the end of cosmic reionization near z ≃ 7.
ACKNOWLEDGMENTS DPS acknowledges financial support from a postdoctoral fellowship from the Science Technology and Research Council and a Schlumberger Interdisciplinary Research Fellowship at Darwin College, University of Cambridge. MO is grateful to financial support from Carnegie Observatories via the Carnegie fellowship.