High-redshift Lyα Emitters Research Articles

Resolving Clumpy versus Extended Lyα in Strongly Lensed, High-redshift Lyα Emitters

We present six strongly gravitationally lensed Lyα emitters (LAEs) at z ∼ 4–5 with Hubble Space Telescope (HST) narrowband imaging isolating Lyα. Through complex radiative transfer Lyα encodes information about the spatial distribution and kinematics of the neutral hydrogen upon which it scatters. We investigate the galaxy properties and Lyα morphologies of our sample. Many previous studies of high-redshift LAEs have been limited in Lyα spatial resolution. In this work we take advantage of high-resolution Lyα imaging boosted by lensing magnification, allowing us to probe subgalactic scales that are otherwise inaccessible at these redshifts. We use broadband imaging from HST (rest-frame UV) and Spitzer (rest-frame optical) in spectral energy distribution fitting, providing estimates of the stellar masses (∼108–109 M ⊙), stellar population ages (t 50 < 40 Myr), and amounts of dust (A V ∼ 0.1–0.6, statistically consistent with zero). We employ nonparametric star formation histories to probe the young stellar populations which create the Lyα. We also examine the offsets between the Lyα and stellar continuum, finding small upper limits of offsets (<0.″1) consistent with studies of low-redshift LAEs, indicating our galaxies are not interacting or merging. Finally, we find a bimodality in our sample’s Lyα morphologies: clumpy and extended. We find a suggestive trend: our LAEs with clumpy Lyα are generally younger than the LAEs with extended Lyα, suggesting a possible correlation with age.

On the evolution of the size of Lyman alpha haloes across cosmic time: no change in the circumgalactic gas distribution when probed by line emission

ABSTRACT Lyman α (Lyα) is now routinely used as a tool for studying high-redshift galaxies, and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Lyα emitters indicate that significant extended Lyα halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-z samples while optimizing the observations for detection of low surface brightness Lyα emission. The Lyα escape fractions range between 0.7 and 37 per cent, and we detect extended Lyα emission around six out of seven targets. We find Lyα halo to UV scale length ratios around 6:1, which is marginally lower than high-redshift observations, and halo flux fractions between 60 and 85 per cent – consistent with high-redshift observations – when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Lyα halo fractions. We find that deeper Hα data would be required to firmly determine the origin of Lyα halo emission; however, there are indications that Hα is more extended than the central FUV profile, potentially indicating conditions favourable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation.

The Compact UV Size of Green Pea Galaxies As Local Analogs of High-redshift Lyα-Emitters

Abstract We study the dependence of Lyα escape from galaxies on UV continuum size and luminosity using a sample of 40 Green Pea (GP) galaxies, which are the best local analogs of high-redshift Lyα emitters (LAEs). We use the Cosmic Origins Spectrograph near-ultraviolet images from the Hubble Space Telescope to measure the UV size and luminosity with 0.″047 spatial resolution. Like most galaxies the GPs show a log-normal size distribution. They also show a positive correlation between size and UV-continuum luminosity. The slope of the size-continuum luminosity relation for GPs is consistent with those of continuum-selected star-forming galaxies (SFGs) at low and high redshifts. A distinctive feature of GPs is a very compact typical radius of 0.33 kpc with a population spread (1σ) of 0.19 kpc. The peak of the size distribution and the intercept of the size–luminosity relation of GPs are noticeably smaller than those of continuum-selected SFGs at similar redshifts. There are statistically significant anticorrelations found between the circularized half-light radius (r cir,50), the Lyα equivalent width (EW(Lyα)), and the Lyα escape faction ( ), suggesting that small UV-continuum radii are crucial for Lyα emission. GPs and high-redshift LAEs have similar sizes, once spatial resolution effects are properly considered. Our results show that a compact small size is crucial for escape of Lyα photons, and that Lyα emitters show constant characteristic size independent of their redshift.

Ultrafaint [C ii] Emission in a Redshift = 2 Gravitationally Lensed Metal-poor Dwarf Galaxy

Abstract Extreme emission-line galaxies (EELGs) at redshift z = 1−2 provide a unique view of metal-poor, starburst sources that are the likely drivers of the cosmic reionization at z ≥ 6. However, the molecular gas reservoirs of EELGs—the fuel for their intense star formation—remain beyond the reach of current facilities. We present ALMA [C ii] and PdBI CO(2–1) observations of the z = 1.8, strongly lensed EELG SL2S 0217, a bright Lyα emitter with a metallicity 0.05 Z ⊙. We obtain a tentative (∼3σ–4σ) detection of the [C ii] line and set an upper limit on the [C ii]/SFR (star-forming rate) ratio of ≤1 × 106 L ⊙/(M ⊙ yr−1), based on the synthesized images and visibility-plane analysis. The CO(2–1) emission is not detected. Photoionization modeling indicates that up to 80% of the [C ii] emission originates from neutral or molecular gas, although we cannot rule out that the gas is fully ionized. The very faint [C ii] emission is in line with both nearby metal-poor dwarfs and high-redshift Lyα emitters, and predictions from hydrodynamical simulations. However, the [C ii] line is 30× fainter than predicted by the De Looze et al. [C ii]–SFR relation for local dwarfs, illustrating the danger of extrapolating locally calibrated relations to high-redshift, metal-poor galaxies.

Void Probability Function of Simulated Surveys of High-redshift Lyα Emitters

Abstract We calculate the void probability function (VPF) in simulations of Lyα emitters (LAEs) across a wide redshift range (z = 3.1, 4.5, 5.7, 6.6). The VPF measures the zero-point correlation function (i.e., places devoid of galaxies) and naturally connects to higher-order correlation functions while being computationally simple to calculate. We explore the Poissonian and systematic errors on the VPF, specify its accuracy as a function of average source density and the volume probed, and provide the appropriate size scales to measure the VPF. At small radii, the accuracy of the VPF is limited by galaxy density, while at large radii, the VPF is limited by the number of independent volumes probed. We also offer guidelines for understanding and quantifying the error in the VPF. We approximate the error in the VPF by using independent subvolumes of the catalogs, after finding that jackknife statistics underestimate the uncertainty. We use the VPF to probe the strength of higher-order correlation functions by measuring and examining the hierarchical scaling between the correlation functions using the count-in-cells method. The negative binomial model (NBM) has been shown to best describe the scaling between the two-point correlation function and the VPF for low-redshift galaxy observations. We further test the fit of the NBM by directly deriving the volume-averaged two-point correlation function from the VPF and vice versa. We find the NBM best describes the simulated LAEs, with a 1σ deviation from the model in the z = 6.6 catalog. This suggests that LAEs show higher-order clustering terms similar to those of normal low-redshift galaxies.

On the Star Formation Efficiency in High-redshift Lyα Emitters

We present semi-analytical models of high-redshift Lyα emitters (LAEs) to constrain the star formation efficiency in those galaxies. Our supernova feedback induced star formation model along with Sheth-Tormman halo mass function correctly reproduces the shape, amplitude and the redshift evolution of UV and Lyα luminosity functions of LAEs in the redshift range z = 2 to 7.3. We show that the fraction of Lyα emitting galaxies increases with increasing redshifts reaching to unity just after the reionisation. However, we show that star formation efficiency in those LAEs does not show any redshift evolution within the uncertainty in available observations. This would have significant repercussion on the reionisation of the intergalactic medium.

Observing the high redshift Universe with Euclid

AbstractEuclid enables the exploration of large sky areas with diffraction limited resolution in the optical and near-infrared, and is sensitive enough to detect targets at cosmological distances. This combination of capabilities gives Euclid a clear advantage over telescope facilities with larger apertures, both on ground and in space. The decision to mount in the NISP instrument one extra grism for the wavelength range 0.92-1.3 μm with a spectral resolution of R ≈260 makes possible a rest-frame UV survey of the early Universe in the redshift range 6.5 &lt; z &lt; 9.7. Euclid’s standard imaging with VIS in the 0.55-0.9 μm band and with NISP in the Y, J, H bands provide complementary photometry for further target identification and characterization. Euclid is a suitable facility to discover and map the spatial distribution of rare high-redshift targets and to collect statistically relevant samples, in particular of high redshift Lyα emitters and QSOs, which can be used as signposts of the cosmic structures. The Euclid surveys are also a starting point for deeper follow up observations of the individual high-z objects. We present the Euclid mission and discuss the detectability of high-z objects to probe the epoch of ionization.

THE BOSS EMISSION-LINE LENS SURVEY. III. STRONG LENSING OF Lyα EMITTERS BY INDIVIDUAL GALAXIES

ABSTRACT We introduce the Baryon Oscillation Spectroscopic Survey (BOSS) Emission-Line Lens Survey GALaxy-Lyα EmitteR sYstems (BELLS GALLERY) Survey, which is a Hubble Space Telescope program to image a sample of galaxy-scale strong gravitational lens candidate systems with high-redshift Lyα emitters (LAEs) as the background sources. The goal of the BELLS GALLERY Survey is to illuminate dark substructures in galaxy-scale halos by exploiting the small-scale clumpiness of rest-frame far-UV emission in lensed LAEs, and to thereby constrain the slope and normalization of the substructure-mass function. In this paper, we describe in detail the spectroscopic strong-lens selection technique, which is based on methods adopted in the previous Sloan Lens ACS (SLACS) Survey, BELLS, and SLACS for the Masses Survey. We present the BELLS GALLERY sample of the 21 highest-quality galaxy–LAE candidates selected from galaxy spectra in the BOSS of the Sloan Digital Sky Survey III. These systems consist of massive galaxies at redshifts of approximately 0.5 strongly lensing LAEs at redshifts from 2–3. The compact nature of LAEs makes them an ideal probe of dark substructures, with a substructure-mass sensitivity that is unprecedented in other optical strong-lens samples. The magnification effect from lensing will also reveal the structure of LAEs below 100 pc scales, providing a detailed look at the sites of the most concentrated unobscured star formation in the universe. The source code used for candidate selection is available for download as a part of this release.

GREEN PEA GALAXIES REVEAL SECRETS OF Lyα ESCAPE

ABSTRACT We analyze archival Lyα spectra of 12 “Green Pea” galaxies observed with the Hubble Space Telescope, model their Lyα profiles with radiative transfer models, and explore the dependence of the Lyα escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [O iii] λ5007 equivalent widths (EWs) of hundreds of Å. All 12 Green Pea galaxies in our sample show Lyα lines in emission, with an Lyα EW distribution similar to high-redshift Lyα emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Lyα escape fractions and find correlations between Lyα escape fraction and kinematic features of Lyα profiles. The escape fraction of Lyα in these galaxies ranges from 1.4% to 67%. We also find that the Lyα escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Lyα profiles with single H i shell radiative transfer models and find that the Lyα escape fraction anticorrelates with the derived H i column densities. Single-shell models fit most Lyα profiles well, but not the ones with the highest escape fractions of Lyα. Our results suggest that low H i column density and low metallicity are essential for Lyα escape and make a galaxy an Lyα emitter.

Predicting the merger fraction of Lyα emitters from redshift z ∼ 3 to 7

Rapid mass assembly, likely from mergers or smooth accretion, has been predicted to play a vital role in star formation in high-redshift Lyα emitters. Here, we predict the major merger, minor merger and smooth accreting Lyα emitter fraction from z ≈ 3 to 7 using a large dark matter simulation and a simple physical model that is successful in reproducing many observations over this large redshift range. The central tenet of this model, different from many of the earlier models, is that the star formation in Lyα emitters is proportional to the mass accretion rate rather than the total halo mass. We find that at z ≈ 3 nearly 35 per cent of the Lyα emitters accrete their mass through major (3:1) mergers, and this fraction increases to about 50 per cent at z ≈ 7. This implies that the star formation in a large fraction of high-redshift Lyα emitters is driven by mergers. Comparing our predictions with observations, we find that the model-predicted fractions are relatively larger. While there is discrepancy between the model predictions and observed merger fractions, some of this difference (˜15 per cent) can be attributed to the mass-ratio used to define a merger in the simulation. We predict that future, deeper observations which use a 3:1 definition of major mergers will find ≥30 per cent major merger fraction of Lyα emitters at redshifts ≥3.

Studying reionization with Lyα emitters

We show that observations of high-redshift Lyα emitters (LAEs) have the potential to provide definitive evidence for reionization in the near future. Using 200-Mpc radiative transfer simulations, we calculate the effect that patchy reionization has on the line profile, on the luminosity function, and, most interestingly, on the clustering of emitters for several realistic models of reionization. Reionization increases the measured clustering of emitters, and we show that this enhancement would be essentially impossible to attribute to anything other than reionization. Our results motivate looking for the signature of reionization in existing LAE data. We find that for stellar reionization scenarios the angular correlation function of the 58 LAEs in the Subaru Deep Field 2 = 6.6 photometric sample is more consistent with a fully ionized universe (mean volume ionized fraction x i ≈ 1) than a universe with x i 2σ confidence level. Measurements in the next year on Subaru will increase their z = 6.6 LAE sample by a factor of 5 and tighten these limits. If the clustering signature of reionization is detected in an LAE survey, a comparison with a Lyman-break or an Ha survey in the same field would confirm the reionization hypothesis. We discuss the optimal LAE survey specifications for detecting reionization, with reference to upcoming programmes.

The impact of The IGM on high-redshift Ly emission lines

We calculate the impact of the intergalactic medium (IGM) on the observed Lyα lines emitted by galaxies in an ionized IGM at z 4. Our model accounts for gas clumping in the IGM and for the fact that high-redshift galaxies reside in overdense regions, which causes the velocity field of the IGM to depart from the Hubble flow. The observed shape of the Lyα line varies widely, with dependence on the intrinsic width and systemic velocity of the line, a galaxies star formation rate and the local extragalactic UV background. For large star formation rates and levels of the UV background, absorption in the IGM does not result in a Lyα line that is asymmetric as is common among known high-redshift Lyα emitters (LAEs). For models in which the lines do show the observed strong asymmetry, the IGM typically transmits only 10‐30 per cent of the Lyα flux. The increase in the ionizing background that accompanied the completion of reionization barely increased the IGM transmission, which suggests that LAEs of comparable luminosity should not appear to be significantly dimmer prior to overlap. In this light, we briefly discuss the potential of LAEs as a probe into the epoch of reionization.

Spectroscopic Confirmation of Three Redshift [CLC][ITAL]z[/ITAL][/CLC] ≈ 5.7 L[CLC]y[/CLC]α Emitters from the Large-Area Lyman Alpha Survey

Narrowband searches for Lyα emission are an efficient way of identifying star-forming galaxies at high redshifts. We present Keck Telescope spectra confirming redshifts z ≈ 5.7 for three objects discovered in the Large-Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory. All three spectra show strong, narrow emission lines with the asymmetric profile that is characteristically produced in high-redshift Lyα emitters by preferential H I absorption in the blue wing of the line. These objects are undetected in deep BW, V, R, and λ ≈ 6600 A narrowband images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyα forest absorption at redshift z ≈ 5.7. All three objects show large equivalent widths (150 A in the rest frame), suggesting at least one of the following: a top-heavy initial mass function, very low stellar metallicity, or the presence of an active nucleus. We consider the case for an active nucleus to be weak in all three objects due to the limited width of the Lyα emission line (less than 500 km s-1) and the absence of any other indicator of quasar activity. The three confirmed high-redshift objects were among four spectroscopically observed targets drawn from the sample of 18 candidates presented by Rhoads & Malhotra. Thus, these spectra support the Lyα emitter population statistics from our earlier photometric study, which imply little evolution in number density from z = 5.7 to 4.5 and provide strong evidence that the reionization redshift is zr > 5.7.