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Several studies have detected Lyman-alpha (Ly α) from bright ($M_{\small UV}\lesssim -21.5$) galaxies during the early stages of reionization despite the significantly neutral intergalactic medium. To explain these detections, it has been suggested that z > 7 Ly α emitters (LAEs) inhabit physical Mpc (pMpc)-scale ionized regions powered by overdensities of faint galaxies; however, systematic searches for these overdensities near LAEs have been challenging. Here, we use Cosmic Evolution Early Release Science JWST/Near Infrared Camera imaging to search for large-scale galaxy overdensities near two very ultraviolet (UV)-bright, z = 8.7 LAEs in the Extended Groth Strip (EGS) field. We colour select 27 z = 8.4–9.1 candidates, including the one LAE in the footprint (EGSY8p7). From spectral energy distribution models, we infer moderately faint UV luminosities ($-21.2\lesssim {M_{\small UV}}\lesssim -19.1$) and stellar masses of M* ≈ 107.5–8.8 M⊙. All are efficient ionizing agents ($\xi _{\text{ion}}^{*}\approx 10^{25.5-26.0}$ Hz erg−1) and are generally morphologically simple with only one compact (re ≲ 140 to ∼650 pc) star-forming component. 13 candidates lie within 5 arcmin of EGSY8p7, leading to a factor-of-four galaxy overdensity at ≲5 arcmin (∼1.4 projected pMpc at z ∼ 8.7) separations from EGSY8p7. Separations of 10–15 arcmin (∼2.7–4.1 projected pMpc) are consistent with an average field. The spatial distribution of our sample may qualitatively suggest an R ≥ 2 pMpc ionized bubble encompassing both LAEs in EGS, which is theoretically unexpected but may be possible for a galaxy population four times more numerous than the average to create with moderate escape fractions (fesc ≳ 0.15) over long times (≳ 200 Myr). Upcoming spectroscopic follow-up will characterize the size of any ionized bubble that may exist and the properties of the galaxies powering such a bubble.

 

We describe JWST/NIRSpec prism measurements of Ly α emission in z ≳ 5 galaxies. We identify Ly α detections in 10 out of 69 galaxies with robust rest-optical emission-line redshift measurements at 5 ≤ z < 7 in the Cosmic Evolution Early Release Science (CEERS) and DDT-2750 observations of the Extended Groth Strip field. Galaxies at z ≃ 6 with faint continuum (F150W=27–29 mag) are found with extremely large rest-frame Ly α equivalent widths (EWs; ranging up to 286 Å). Likely Ly α detections are also seen in two new z > 7 galaxies (z = 7.49 and 7.17) from the second epoch of CEERS observations, both showing large Ly α EWs that likely indicate significant transmission through the intergalactic medium (IGM). We measure high Ly α escape fractions in the 12 Ly α emitters in our sample (median 0.28), two of which show $f_{\rm esc}^{ {\rm Ly}\alpha }$ near unity (>0.80). We find that $50_{-11}^{+11}$ per cent of z ≃ 6 galaxies with [O iii] + H β EW>1000 Å have $f_{\rm esc}^{ {\rm Ly}\alpha }$ >0.2, consistent with the fractions found in lower redshift samples with matched [O iii] + H β EWs. While uncertainties are still significant, we find that only $10_{-5}^{+9}$ per cent of z > 7 galaxies with similarly strong rest optical emission lines show such large $f_{\rm esc}^{ {\rm Ly}\alpha }$, as may be expected if IGM attenuation of Ly α increases towards higher redshifts. We identify photometric galaxy overdensities near the z ≳ 7 Ly α emitters, potentially providing the ionizing flux necessary to create large ionized sightlines that facilitate Ly α transmission. Finally, we investigate the absence of Ly α emission in a comparable (and spectroscopically confirmed) galaxy overdensity at z = 7.88 in the Abell 2744 field, discussing new prism spectra of the field obtained with the UNCOVER program.

 

We present a Bayesian inference on the neutral hydrogen fraction of the intergalactic medium (IGM), $\overline{x}_{\small HI}$, at z ∼ 6–8 using the properties of Lyman break galaxies (LBGs) during the epoch of reionization. We use large samples of LBG candidates at 5.5 ≤ z ≤ 8.2 with spectroscopy from Keck/DEIMOS and Keck/MOSFIRE. For each galaxy, we incorporate either the Lyman-α (Lyα) equivalent width (EW) for detections or the EW limit spectrum for non-detections to parametrize the EW distribution at various ultraviolet brightnesses for a given redshift. Using our reference sample of galaxy candidates from the ionized universe at z ∼ 6.0, we are able to infer $\overline{x}_{\small HI}$ at two redshifts: z ∼ 6.7 and z ∼ 7.6. This work includes intrinsically faint, gravitationally lensed galaxies at z ∼ 6.0 in order to constrain the intrinsic faint-end Lyα EW distribution and provide a comparable population of galaxies to counterparts in our sample that are at higher redshift. The inclusion of faint galaxy candidates, in addition to a more sophisticated modelling framework, allows us to better isolate effects of the interstellar medium and circumgalactic medium on the observed Lyα distribution from those of the IGM. We infer an upper limit of $\overline{x}_{\small HI}$ ≤ 0.25 (0.44) at z = 6.7 ± 0.2 and a neutral fraction of $\overline{x}_{\small HI}$ = $0.83^{+0.08}_{-0.11}$ (0.83$^{+0.11}_{-0.21}$) at z = 7.6 ± 0.6, both within 68 per cent (95 per cent) uncertainty, results that favour a moderately late and fairly rapid reionization.

 

The formation of the first galaxies during cosmic dawn and reionization (at redshifts z = 5–30), triggered the last major phase transition of our universe, as hydrogen evolved from cold and neutral to hot and ionized. The 21-cm line of neutral hydrogen will soon allow us to map these cosmic milestones and study the galaxies that drove them. To aid in interpreting these observations, we upgrade the publicly available code 21cmFAST. We introduce a new, flexible parametrization of the additive feedback from: an inhomogeneous, H2-dissociating (Lyman–Werner; LW) background; and dark matter – baryon relative velocities; which recovers results from recent, small-scale hydrodynamical simulations with both effects. We perform a large, ‘best-guess’ simulation as the 2021 installment of the Evolution of 21-cm Structure (EOS) project. This improves the previous release with a galaxy model that reproduces the observed UV luminosity functions (UVLFs), and by including a population of molecular-cooling galaxies. The resulting 21-cm global signal and power spectrum are significantly weaker, primarily due to a more rapid evolution of the star formation rate density required to match the UVLFs. Nevertheless, we forecast high signal-to-noise detections for both HERA and the SKA. We demonstrate how the stellar-to-halo mass relation of the unseen, first galaxies can be inferred from the 21-cm evolution. Finally, we show that the spatial modulation of X-ray heating due to relative velocities provides a unique acoustic signature that is detectable at z ≈ 10–15 in our fiducial model. Ours are the first public simulations with joint inhomogeneous LW and relative-velocity feedback across the entire cosmic dawn and reionization, and we make them available at this link https://scholar.harvard.edu/julianbmunoz/eos-21.

 

ABSTRACT The reionization of hydrogen is closely linked to the first structures in the Universe, so understanding the timeline of reionization promises to shed light on the nature of these early objects. In particular, transmission of Lyman alpha (Ly α) from galaxies through the intergalactic medium (IGM) is sensitive to neutral hydrogen in the IGM, so can be used to probe the reionization timeline. In this work, we implement an improved model of the galaxy UV luminosity to dark matter halo mass relation to infer the volume-averaged fraction of neutral hydrogen in the IGM from Ly α observations. Many models assume that UV-bright galaxies are hosted by massive dark matter haloes in overdense regions of the IGM, so reside in relatively large ionized regions. However, observations and N-body simulations indicate that scatter in the UV luminosity–halo mass relation is expected. Here, we model the scatter (though we assume the IGM topology is unaffected) and assess the impact on Ly α visibility during reionization. We show that UV luminosity–halo mass scatter reduces Ly α visibility compared to models without scatter, and that this is most significant for UV-bright galaxies. We then use our model with scatter to infer the neutral fraction, $\overline{x}_{\mathrm{ H}\,{\small I}}$, at z ∼ 7 using a sample of Lyman-break galaxies in legacy fields. We infer $\overline{x}_{\mathrm{ H}\,{\small I}} = 0.55_{-0.13}^{+0.11}$ with scatter, compared to $\overline{x}_{\mathrm{ H}\,{\small I}} = 0.59_{-0.14}^{+0.12}$ without scatter, a very slight decrease and consistent within the uncertainties. Finally, we place our results in the context of other constraints on the reionization timeline and discuss implications for future high-redshift galaxy studies.