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Register a new userConfirmation of a Steep Luminosity Function for Lyman-alpha Emitters at z = 5.7: A Major Component of Reionization
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We report the first direct and robust measurement of the faint-end slope of the Lyman-alpha emitter (LAE) luminosity function at z = 5.7. Candidate LAEs from a low-spectral-resolution blind search with IMACS on Magellan-Baade were targeted at higher resolution to distinguish high redshift LAEs from foreground galaxies. All but 2 of our 42 single-emission-line systems have flux F $< 2.0 times 10^{-17}$ ergs s$^{-1}$ cm$^{-2}$, making these the faintest emission-lines observed for a z = 5.7 sample with known completeness, an essential property for determining the faint end slope of the LAE luminosity function. We find 13 LAEs as compared to 29 foreground galaxies, in very good agreement with the modeled foreground counts predicted in Dressler et al. (2011a) that had been used to estimate a faint-end slope of $alpha$ = -2.0 for the LAE luminosity function. A 32% LAE fraction, LAE/(LAE+foreground), within the flux interval F = $2-20 times 10^{-18}$ ergs s$^{-1}$ cm$^{-2}$, constrains the faint end slope of the luminosity function to -2.35 < $alpha$ < -1.95 (1-$sigma$). We show how this steep LF should provide, to the limit of our observations, more than 20% of the flux necessary to maintain ionization at z=5.7, with a factor-of-ten extrapolation in flux reaching more than 55%. This is in addition to a comparable contribution from Lyman Break Galaxies M$_{UV} le$ -18. We suggest that this bodes well for a sufficient supply of Lyman continuum photons by similar, low-mass star forming galaxies within the reionization epoch at z $approx$ 7, only 250 Myr earlier.
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We report results of a deep wide-field narrowband survey for redshift z~5.7 Ly alpha emitters carried out with SuprimeCam on Subaru 8.3-m telescope. Deep narrowband imaging of the SSA22 field through a 120 A bandpass filter centered at 8150 A was combined with deep multicolor RIz SuprimeCam broadband imaging, and BVRZ imaging taken with CFHTs CFH12K camera to select high-redshift galaxy candidates. Spectroscopic observations were made using the new wide-field multi-object DEIMOS spectrograph on Keck for 22 of the 26 candidate objects. Eighteen objects were identified as z~5.7 Lyman alpha emitters, and a further nineteenth candidate was identified based on an LRIS spectrum. At the 3.3 A resolution of the DEIMOS spectra the asymmetric profile for Ly alpha emission with its steep blue fall-off can be clearly seen. We use this to describe the distribution of equivalent widths and the continuum color break properties for z~5.7 Ly alpha galaxies compared with foreground objects. The large majority (>75%) of Ly alpha lines have rest frame equivalent widths less than 240 A and can be understood in terms of young star forming galaxies with a Salpeter initial mass function for the stars. With narrowband selection criteria of I-N > 0.7 and N<25.05 (AB mags) we find a surface density of Ly alpha emitters of 0.03 per square arcminute per (deltaz=0.1) to a limiting flux just under 2e-17 erg/cm2/s. The luminosity function of the Ly alpha emitters is similar to that at lower redshifts to the lowest measurable luminosity of 1e43 ergs/s as is the universal star formation rate based on their continuum properties. We note that the objects are highly structured in both spatial and spectral properties on the angular scale of the fields (~60 Mpc), and that multiple fields will have to be averaged to accurately measure their ensemble properties.
Lyman alpha (Lya) emission lines should be attenuated in a neutral intergalactic medium (IGM). Therefore the visibility of Lya emitters at high redshifts can serve as a valuable probe of reionization at about the 50% level. We present an imaging search for z=7.7 Lya emitting galaxies using an ultra-narrowband filter (filter width= 9A) on the NEWFIRM imager at the Kitt Peak National Observatory. We found four candidate Lya emitters in a survey volume of 1.4 x 10^4 Mpc^3, with a line flux brighter than 6x10^-18 erg/cm^2/s (5 sigma in 2 aperture). We also performed a detailed Monte-Carlo simulation incorporating the instrumental effects to estimate the expected number of Lya emitters in our survey, and found that we should expect to detect one Lya emitter, assuming a non-evolving Lya luminosity function (LF) between z=6.5 and z=7.7. Even if one of the present candidates is spectroscopically confirmed as a z~8 Lya emitter, it would indicate that there is no significant evolution of the Lya LF from z=3.1 to z~8. While firm conclusions would need both spectroscopic confirmations and larger surveys to boost the number counts of galaxies, we successfully demonstrate the feasibility of sensitive near-infrared (1.06 um) narrow-band searches using custom filters designed to avoid the OH emission lines that make up most of the sky background.
We report results of a unprecedentedly deep, blind search for Ly-alpha emitters (LAEs) at z = 5.7 using IMACS, the Inamori-Magellan Areal Camera & Spectrograph, with the goal of identifying missing sources of reionization that could also be basic building blocks for todays L* galaxies. We describe how improvements in wide field imaging with the Baade telescope, upgrades to IMACS, and the accumulation of ~20 hours of integration per field in excellent seeing led to the detection of single-emission-line sources as faint as F ~ 2 x 10^{-18} ergs s^{-1} cm^{-2}, a sensitivity 5 times deeper than our first search (Martin et al. 2008). A reasonable correction for foreground interlopers implies a steep rise of approximately an order of magnitude in source density for a factor of four drop in flux, from F = 10^{-17.0} ergs s^{-1} cm^{-2} to F = 10^{-17.6} (2.5) x 10^{-18} ergs s^{-1} cm^{-2}. At this flux the putative LAEs have reached a surface density of ~1 per sq arcminute -- a comoving volume density of 4 x 10^{-3} Mpc^{-3}, several times the density of L* galaxies today. Such a population of faint LAEs would account for a significant fraction of the critical flux density required to complete reionization at this epoch, and would be good candidates for building blocks of stellar mass ~10^{8-9} Msun for the young galaxies of this epoch.
We present the luminosity function (LF) for ultraluminous Ly$alpha$ emitting galaxies (LAEs) at z = 6.6. We define ultraluminous LAEs (ULLAEs) as galaxies with logL(Ly$alpha$) > 43.5 erg s$^{-1}$. We select our main sample using the g, r, i, z, and NB921 observations of a wide-area (30 deg$^2$) Hyper Suprime-Cam survey of the North Ecliptic Pole (NEP) field. We select candidates with g, r, i > 26, NB921 $leq$ 23.5, and NB921 - z $leq$ 1.3. Using the DEIMOS spectrograph on Keck II, we confirm 9 of our 14 candidates as ULLAEs at z = 6.6 and the remaining 5 as an AGN at z = 6.6, two [OIII]$lambda$5007 emitting galaxies at z = 0.84 and z = 0.85, and two non-detections. This emphasizes the need for full spectroscopic follow-up to determine accurate LFs. In constructing the ULLAE LF at z = 6.6, we combine our 9 NEP ULLAEs with two previously discovered and confirmed ULLAEs in the COSMOS field: CR7 and COLA1. We apply rigorous corrections for incompleteness based on simulations. We compare our ULLAE LF at z = 6.6 with LFs at z = 5.7 and z = 6.6 from the literature. Our data reject some previous LF normalizations and power law indices, but they are broadly consistent with others. Indeed, a comparative analysis of the different literature LFs suggests that none is fully consistent with any of the others, making it critical to determine the evolution from z = 5.7 to z = 6.6 using LFs constructed in exactly the same way at both redshifts.
The time frame in which hydrogen reionization occurred is highly uncertain, but can be constrained by observations of Lyman-alpha (Ly$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.
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