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Register a new userDetections of Faint Lyman-alpha Emitters at z = 5.7: Galaxy Building Blocks and Engines of Reionization
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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.
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We investigate the stellar populations of Lyman alpha emitters (LAEs) at z=5.7 and 6.6 in a 0.65 deg^2 sky of the Subaru/XMM-Newton Deep Survey (SXDS) Field, using deep images taken with Subaru/Suprime-Cam, UKIRT/WFCAM, and Spitzer/IRAC. We produce stacked multiband images at each redshift from 165 (z=5.7) and 91 (z=6.6) IRAC-undetected objects, to derive typical spectral energy distributions (SEDs) of z~6-7 LAEs for the first time. The stacked LAEs have as blue UV continua as the HST/WFC3 z-dropout galaxies of similar Muv, with a spectral slope beta ~ -3, but at the same time they have red UV-to-optical colors with detection in the 3.6um band. Using SED fitting we find that the stacked LAEs have low stellar masses of ~(3-10)*10^7 Msun, very young ages of ~1-3 Myr, negligible dust extinction, and strong nebular emission from the ionized interstellar medium, although the z=6.6 object is fitted similarly well with high-mass models without nebular emission; inclusion of nebular emission reproduces the red UV-to-optical color while keeping the UV color sufficiently blue. We infer that typical LAEs at z~6-7 are building blocks of galaxies seen at lower redshifts. We find a tentative decrease in the Lyman alpha escape fraction from z=5.7 to 6.6, which may imply an increase in the intergalactic medium neutral fraction. From the minimum contribution of nebular emission required to fit the observed SEDs, we place an upper limit on the escape fraction of ionizing photons to be f_esc^ion~0.6 at z=5.7 and ~0.9 at z=6.6. We also compare the stellar populations of our LAEs with that of stacked HST/WFC3 z-dropout galaxies.
We present NICMOS J110 (rest-frame 1200-2100 A) observations of the three z=5.7 Lyman Alpha emitters discovered in the blind multislit spectroscopic survey by Martin et al. (2008). These images confirm the presence of the two sources which were previously only seen in spectroscopic observations. The third source, which is undetected in our J110 observations has been detected in narrowband imaging of the Cosmic Origins Survey (COSMOS), so our nondetection implies a rest frame equivalent width >146 Angstroms (3 sigma). The two J110-- detected sources have more modest rest frame equivalent widths of 30-40 Angstroms, but all three are typical of high-redshift LAEs. In addition, the J110- detected sources have UV luminosities that are within a factor of two of L*_{UV}, and sizes that appear compact (r_{hl} ~ 0.15) in our NIC2 images -- consistent with a redshift of 5.7. We use these UV-continuum and Lyman Alpha measurements to estimate the i-z colors of these galaxies, and show that at least one, and possibly all three would be missed by the i-dropout LBG selection. These observations help demonstrate the utility of multislit narrowband spectroscopy as a technique for finding faint emission line galaxies.
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.
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.
We present the results of the extensive narrow-band survey of Lyalpha emission-line objects at z=3.1 in the 1.38 deg^2 area surrounding the high density region of star-forming galaxies at z=3.09 in the SSA22 field, as well as in the 1.04 deg^2 area of the three separated general blank fields. In total of 2161 Lyalpha emitters, 1394 in the SSA22 fields and 767 in the general fields, respectively, are detected to the narrow-band AB magnitude limit of 25.73, which corresponds to the line flux of 1.8 x 10^{-17} erg s^{-1} cm^{-2} or luminosity of 1.5 x 10^{42} erg s^{-1} at z=3.1, above the observed equivalent width threshold, 190AA . The average surface number density of the emitters at z=3.1 in the whole general fields above the thresholds is 0.20+-0.01 arcmin^{-2}. The SSA22 high-density region at z=3.09 whose peak local density is 6 times the average is found to be the most prominent outstanding structure in the whole surveyed area and is firmly identified as a robust `protocluster with the enough large sample. We also compared the overdensity of the 100 arcmin^2 and 700 arcmin^2 areas which contain the protocluster with the expected fluctuation of the dark matter as well as those of the model galaxies in cosmological simulations. We found that the peak height values of the overdensity correspond to be 8-10 times and 3-4 times of the expected standard deviations of the counts of Lyalpha emitters at z=3.1 in the corresponding volume, respectively. We conclude that the structure at z=3.09 in the SSA22 field is a very significant and rare density peak up to the scale of 60 Mpc.
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