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Register a new userLAGER Ly$alpha$ Luminosity Function at $zsim7$, Implications for Reionization
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We present a new measurement of the Ly$alpha$ luminosity function at redshift $z=6.9$, finding moderate evolution from $z=5.7$ that is consistent with a fully or largely ionized $zsim7$ intergalactic medium. Our result is based on four fields of the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. Our survey volume of $6.1times10^{6}$ Mpc$^{3}$ is double that of the next largest $zsim 7$ survey. We combine two new LAGER fields (WIDE12 and GAMA15A) with two previously reported LAGER fields (COSMOS and CDFS). In the new fields, we identify $N=95$ new $z=6.9$ Ly$alpha$ emitters (LAEs); characterize our surveys completeness and reliability; and compute Ly$alpha$ luminosity functions. The best-fit Schechter luminosity function parameters for all four LAGER fields are in good general agreement. Two fields (COSMOS and WIDE12) show evidence for a bright-end excess above the Schechter function fit. We find that the Ly$alpha$ luminosity density declines at the same rate as the UV continuum LF from $z=5.7$ to $z=6.9$. This is consistent with an intergalactic medium that was fully ionized as early as redshift $zsim 7$, or with a volume-averaged neutral hydrogen fraction of $x_{HI} < 0.33$ at $1sigma$.
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We present the largest-ever sample of 79 Ly$alpha$ emitters (LAEs) at $zsim$ 7.0 selected in the COSMOS and CDFS fields of the LAGER project (the Lyman Alpha Galaxies in the Epoch of Reionization). Our newly amassed ultradeep narrowband exposure and deeper/wider broadband images have more than doubled the number of LAEs in COSMOS, and we have selected 30 LAEs in the second field CDFS. We detect two large-scale LAE-overdense regions in the COSMOS that are likely protoclusters at the highest redshift to date. We perform injection and recovery simulations to derive the sample incompleteness. We show significant incompleteness comes from blending with foreground sources, which however has not been corrected in LAE luminosity functions in {the} literature. The bright end bump in the Ly$alpha$ luminosity function in COSMOS is confirmed with 6 (2 newly selected) luminous LAEs (L$_{Lyalpha}$ $>$ 10$^{43.3}$ erg s$^{-1}$). Interestingly, the bump is absent in CDFS, in which only one luminous LAE is detected. Meanwhile, the faint end luminosity functions from the two fields well agree with each other. The 6 luminous LAEs in COSMOS coincide with 2 LAE-overdense regions, while such regions are not seen in CDFS. The bright-end luminosity function bump could be attributed to ionized bubbles in a patchy reionization. It appears associated with cosmic overdensities, thus supports an inside-out reionization topology at $z$ $sim$ 7.0, i.e., the high density peaks were ionized earlier compared to the voids. An average neutral hydrogen fraction of $x_{HI}$ $sim$ 0.2 -- 0.4 is derived at $zsim$ 7.0 based on the cosmic evolution of the Ly$alpha$ luminosity function.
The time frame in which hydrogen reionization occurred is highly uncertain, but can be constrained by observations of Lyman-alpha (Ly$alpha$) emission from distant sources. Neutral hydrogen in the intergalactic medium (IGM) attenuates Ly$alpha$~photons emitted by galaxies. As reionization progressed the IGM opacity decreased, increasing Ly$alpha$~visibility. The galaxy Ly$alpha$~luminosity function (LF) is thus a useful tool to constrain the timeline of reionization. In this work, we model the Ly$alpha$~LF as a function of redshift, $z=5-10$, and average IGM neutral hydrogen fraction, $overline{x}_textsc{hi}$. We combine the Ly$alpha$~luminosity probability distribution obtained from inhomogeneous reionization simulations with a model for the UV LF to model the Ly$alpha$~LF. As the neutral fraction increases, the average number density of Ly$alpha$~emitting galaxies decreases, and are less luminous, though for $overline{x}_textsc{hi} lesssim 0.4$ there is only a small decrease of the Ly$alpha$~LF. We use our model to infer the IGM neutral fraction at $z=6.6, 7.0, 7.3$ from observed Ly$alpha$~LFs. We conclude that there is a significant increase in the neutral fraction with increasing redshift: $overline{x}_textsc{hi}(z=6.6)=0.08^{+ 0.08}_{- 0.05}, , overline{x}_textsc{hi}(z=7.0)=0.28 pm 0.05$ and $overline{x}_textsc{hi}(z=7.3)=0.83^{+ 0.06}_{- 0.07}$. We predict trends in the Ly$alpha$~luminosity density and Schechter parameters as a function of redshift and the neutral fraction. We find that the Ly$alpha$~luminosity density decreases as the universe becomes more neutral. Furthermore, as the neutral fraction increases, the faint-end slope of the Ly$alpha$~LF steepens, and the characteristic Ly$alpha$~luminosity shifts to lower values, concluding that the evolving shape of the Ly$alpha$~LF -- not just its integral -- is an important tool to study reionization.
We detect 20 $z=7.0$ Ly$alpha$ emitter (LAE) candidates to $L({rm Ly}alpha) geq 2 times 10^{42}$ erg s$^{-1}$ or $0.3$ $L^*_{z=7}$ and in $6.1times 10^5$ Mpc$^3$ volume in the Subaru Deep Field and the Subaru/XMM-Newton Deep Survey field by 82 and 37 hours of Subaru Suprime-Cam narrowband NB973 and reddest optical $y$-band imaging. We compare their Ly$alpha$ and UV luminosity functions (LFs) and densities and Ly$alpha$ equivalent widths (EWs) to those of $z=5.7$, 6.6 and 7.3 LAEs from previous Suprime-Cam surveys. The Ly$alpha$ LF (density) rapidly declines by a factor of $times$1.5 (1.9) in $L({rm Ly}alpha)$ at $z=5.7-6.6$ (160 Myr), $times$1.5 (1.6) at $z=6.6-7.0$ (60 Myr) at the faint end and $times$2.0 (3.8) at $z=7.0-7.3$ (40 Myr). Also, in addition to the systematic decrease in EW at $z=5.7-6.6$ previously found, 2/3 of the $z=7.0$ LAEs detected in the UV continuum exhibit lower EWs than the $z=6.6$ ones. Moreover, while the UV LF and density do not evolve at $z=5.7-6.6$, they modestly decline at $z=6.6-7.0$, implying galaxy evolution contributing to the decline of the Ly$alpha$ LF. Comparison of the $z=7.0$ Ly$alpha$ LF to the one predicted by an LAE evolution model further reveals that galaxy evolution alone cannot explain all the decline of Ly$alpha$ LF. If we attribute the discrepancy to Ly$alpha$ attenuation by neutral hydrogen, the intergalactic medium transmission of Ly$alpha$ photons at $z=7.0$ would be $T_{{rm Ly}alpha}^{rm IGM} leq 0.6-0.7$. It is lower (higher) than $T_{{rm Ly}alpha}^{rm IGM}$ at $z=6.6$ (7.3) derived by previous studies, suggesting rapid increase in neutral fraction at $z > 6$.
The Ly-alpha luminosity function (LF) of high-redshift Ly-alpha emitters (LAEs) is one of the few observables of the re-ionization epoch accessible to date with 8-10 m class telescopes. The evolution with redshift allows one to constrain the evolution of LAEs and their role in re-ionizing the Universe at the end of the Dark Ages. We have performed a narrow-band imaging program at 1.06 microns at the CFHT, targeting Ly-alpha emitters at redshift z ~ 7.7 in the CFHT-LS D1 field. From these observations we have derived a photometric sample of 7 LAE candidates at z ~ 7.7. We derive luminosity functions for the full sample of seven objects and for sub-samples of four objects. If the brightest objects in our sample are real, we infer a luminosity function which would be difficult to reconcile with previous work at lower redshift. More definitive conclusions will require spectroscopic confirmation.
The distribution of Ly$alpha$ emission is an presently accessible method for studying the state of the intergalactic medium (IGM) into the reionization era. We carried out deep spectroscopic observations in order to search for Ly$alpha$ emission from galaxies with photometric redshifts $z$ = 5.5 - 8.3 selected from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). Utilizing data from the Keck/DEIMOS spectrograph, we explore a wavelength coverage of Ly$alpha$ emission at $z$ ~ 5 - 7 with four nights of spectroscopic observations for 118 galaxies, detecting five emission lines with ~ 5$sigma$ significance: three in the GOODS-N and two in the GOODS-S field. We constrain the equivalent width (EW) distribution of Ly$alpha$ emission by comparing the number of detected objects with the expected number constructed from detailed simulations of mock emission lines that account for the observational conditions (e.g., exposure time, wavelength coverage, and sky emission) and galaxy photometric redshift probability distribution functions. The Ly$alpha$ EW distribution is well described by an exponential form, $text{dN/dEW}propto text{exp(-EW/}W_0)$, characterized by the $e$-folding scale ($W_0$) of ~ 60 - 100$AA$ at 0.3 < $z$ < 6. By contrast, our measure of the Ly$alpha$ EW distribution at 6.0 < $z$ < 7.0 rejects a Ly$alpha$ EW distribution with $W_0$ > 36.4$AA$ (125.3$AA$) at 1$sigma$ (2$sigma$) significance. This provides additional evidence that the EW distribution of Ly$alpha$ declines at $z$ > 6, suggesting an increasing fraction of neutral hydrogen in the IGM at that epoch.
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