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Register a new userTexas Spectroscopic Search for Ly$alpha$ Emission at the End of Reionization III. the Ly$alpha$ Equivalent-width Distribution and Ionized Structures at $z > 7$
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Authors
Intae Jung
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Ly$alpha$ emission from galaxies can be utilized to characterize the ionization state in the intergalactic medium (IGM). We report our search for Ly$alpha$ emission at $z>7$ using a comprehensive Keck/MOSFIRE near-infrared spectroscopic dataset, as part of the Texas Spectroscopic Search for Ly$alpha$ Emission at the End of Reionization Survey. We analyze data from 10 nights of MOSFIRE observations which together target 72 high-$z$ candidate galaxies in the GOODS-N field, all with deep exposure times of 4.5-19 hr. Utilizing an improved automated emission-line search, we report 10 Ly$alpha$ emission lines detected ($>$4$sigma$) at $z>7$, significantly increasing the spectroscopically confirmed sample. Our sample includes large equivalent-width (EW) Ly$alpha$ emitters ($>$50r{A}), and additional tentative Ly$alpha$ emission lines detected at 3 - 4$sigma$ from five additional galaxies. We constrain the Ly$alpha$ EW distribution at $zsim7.6$, finding a significant drop from $zlesssim6$, suggesting an increasing fraction of neutral hydrogen (HI) in the IGM in this epoch. We estimate the Ly$alpha$ transmission through the IGM ($=$EW$_{zsimtext{7.6}}$/EW$_{zsimtext{2-6}}$), and infer an IGM HI fraction ($X_{text{HI}}$) of $49^{+19}_{-19}%$ at $zsim7.6$, which is lower in modest tension ($>$1$sigma$) with recent measurements at $z sim$ 7.6. The spatial distribution of the detected Ly$alpha$ emitters implies the presence of a potential highly ionized region at $zsim7.55$ which hosts four Ly$alpha$ emitters within a $sim$ 40 cMpc spatial separation. The prominence of this ionized region in our dataset could explain our lower inferred value of $X_{text{HI}}$, though our analysis is also sensitive to the chosen reference Ly$alpha$ EW distribution values and reionization models.
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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.
We report the Subaru Hyper Suprime-Cam (HSC) discovery of two Ly$alpha$ blobs (LABs), dubbed z70-1 and z49-1 at $z=6.965$ and $z=4.888$ respectively, that are Ly$alpha$ emitters with a bright ($log L_{rm Lyalpha}/{rm [erg s^{-1}]}>43.4$) and spatially-extended Ly$alpha$ emission, and present the photometric and spectroscopic properties of a total of seven LABs; the two new LABs and five previously-known LABs at $z=5.7-6.6$. The z70-1 LAB shows the extended Ly$alpha$ emission with a scale length of $1.4pm 0.2$ kpc, about three times larger than the UV continuum emission, making z70-1 the most distant LAB identified to date. All of the 7 LABs, except z49-1, exhibit no AGN signatures such as X-ray emission, {sc Nv}$lambda$1240 emission, or Ly$alpha$ line broadening, while z49-1 has a strong {sc Civ}$lambda$1548 emission line indicating an AGN on the basis of the UV-line ratio diagnostics. We carefully model the point-spread functions of the HSC images, and conduct two-component exponential profile fitting to the extended Ly$alpha$ emission of the LABs. The Ly$alpha$ scale lengths of the core (star-forming region) and the halo components are $r_{rm c}=0.6-1.2$ kpc and $r_{rm h}=2.0-13.8$ kpc, respectively. The average $r_{rm h}$ of the LABs falls on the extrapolation of the $r_{rm h}$-Ly$alpha$ luminosity relation of the Ly$alpha$ halos around VLT/MUSE star-forming galaxies at the similar redshifts, suggesting that typical LABs at $zgtrsim5$ are not special objects, but star-forming galaxies at the bright end.
We present rest-frame optical spectra of 60 faint ($R_{AB}sim 27$; $Lsim0.1 L_*$) Ly$alpha$-selected galaxies (LAEs) at $zapprox2.56$. The average LAE is consistent with the extreme low-metallicity end of the continuum-selected galaxy distribution at $zapprox2-3$. In particular, the LAEs have extremely high [OIII] $lambda$5008/H$beta$ ratios (log([OIII]/H$beta$) $sim$ 0.8) and low [NII] $lambda$6585/H$alpha$ ratios (log([NII]/H$alpha$) $<-1.15$). Using the [OIII] $lambda$4364 auroral line, we find that the star-forming regions in faint LAEs are characterized by high electron temperatures ($T_eapprox1.8times10^4$K), low oxygen abundances (12 + log(O/H) $approx$ 8.04, $Z_{neb}approx0.22Z_odot$), and high excitations with respect to more luminous galaxies. Our faintest LAEs have line ratios consistent with even lower metallicities, including six with 12 + log(O/H) $approx$ 6.9$-$7.4 ($Z_{neb}approx0.02-0.05Z_odot$). We interpret these observations in light of new models of stellar evolution (including binary interactions). We find that strong, hard ionizing continua are required to reproduce our observed line ratios, suggesting that faint galaxies are efficient producers of ionizing photons and important analogs of reionization-era galaxies. Furthermore, we investigate physical trends accompanying Ly$alpha$ emission across the largest current sample of combined Ly$alpha$ and rest-optical galaxy spectroscopy, including 60 faint LAEs and 368 more luminous galaxies at similar redshifts. We find that Ly$alpha$ emission is strongly correlated with nebular excitation and ionization and weakly correlated with dust attenuation, suggesting that metallicity plays a strong role in determining the observed properties of these galaxies by modulating their stellar spectra, nebular excitation, and dust content.
Spectroscopic confirmation of galaxies at z~7 and above has been extremely difficult, owing to a drop in intensity of Ly-alpha emission in comparison with samples at z~6. This crucial finding could potentially signal the ending of cosmic reionization. However it is based on small datasets, often incomplete and heterogeneous in nature. We introduce a flexible Bayesian framework, useful to interpret such evidence. Within this framework, we implement two simple phenomenological models: a smooth one, where the distribution of Ly-alpha is attenuated by a factor es with respect to z~6; a patchy one where a fraction ep is absorbed/non-emitted while the rest is unabsorbed. From a compilation of 39 observed z~7 galaxies we find es=0.69+-0.12 and ep=0.66+-0.16. The models can be used to compute fractions of emitters above any equivalent width W. For W>25AA, we find X^{25}_{z=7}=0.37+-0.11 (0.14+-0.06) for galaxies fainter (brighter) than M_{UV}=-20.25 for the patchy model, consistent with previous work, but with smaller uncertainties by virtue of our full use of the data. At z~8 we combine new deep (5-sigma flux limit 10^{-17}ergs^{-1}cm^{-2}) Keck-NIRSPEC observations of a bright Y-dropout identified by our BoRG Survey, with those of three objects from the literature and find that the inference is inconclusive. We compute predictions for future near-infrared spectroscopic surveys and show that it is challenging but feasible to constrain the distribution of Ly-alpha emitters at z~8 and distinguish between models.
Context. Searching for high-redshift galaxies is a field of intense activity in modern observational cosmology that will continue to grow with future ground-based and sky observatories. Over the last few years, a lot has been learned about the high-z Universe. Aims. Despite extensive Ly-alpha Blobs (LAB) surveys from low to high redshifts, giant LABs over 100 kpc have been found mostly at z~2-4. This redshift range is coincident with the transition epoch of galactic gas-circulation processes from inflows to outflows at z~2.5-3. This suggests that the formation of giant LABs may be related to a combination of gas inflows and outflows. Their extreme youth makes them interesting objects in the study of galaxy formation as they provide insight into some of the youngest known highly star forming galaxies, with only modest time investments using ground-based telescopes. Methods. Systematic narrow-band Ly-alpha nebula surveys are ongoing, but they are limited in their covered redshift range and their comoving volume. This poses a significant problem when searching for such rare sources. To address this problem, we developed a systematic searching tool, ATACAMA (A Tool for seArChing for lArge LyMan Alpha nebulae) designed to find large Ly-alpha nebulae at any redshift within deep multi-wavelength broad-band imaging. Results. We identified a Ly-alpha nebula candidate at zphot~3.3 covering an isophotal area of 29.4sq.arcsec. Its morphology shows a bright core and a faint core which coincides with the morphology of previously known Ly-alpha blobs. A first estimation of the Ly-alpha equivalent width and line flux agree with the values from the study led by several groups.
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