Author: Sara Mascia

The resonant scattering interaction between Ly α photons and neutral hydrogen implies that a partially neutral IGM can significantly impact the detectability of Ly α emission in galaxies. The redshift evolution of the Ly α equivalent width distribution of galaxies thus offers a key probe of the degree of ionization during the Epoch of Reionization (EoR). Previous in-depth investigations at z ≥ 7 were limited by ground-based instrument capabilities. We present an extensive study of Ly α emission from galaxies at 4 < z < 8.5, observed from the CEERS and JADES surveys in the JWST NIRSpec/PRISM configuration. The sample consists of 235 galaxies, among which we identify 65 as Ly α emitters. We first measure Ly α escape fractions from Balmer lines, and explore the correlations with the inferred galaxies’ physical properties, which are similar to those found at lower redshift. We also investigate the possible connection between the escape of Ly α photons and the inferred escape fractions of LyC photons obtained from indirect indicators. We then analyze the redshift evolution of the Ly α emitter fraction, finding lower average values at z = 5 and 6 compared to ground-based observations. At z = 7 we find a very large difference in Ly α visibility between the EGS and GOODS-South fields, possibly due to the presence of early reionized regions in the EGS. Such large variance is also expected in the Cosmic Dawn II radiation-hydrodynamical simulation. Our findings suggest a scenario in which the ending phase of the EoR is characterized by ∼ 1 pMpc ionized bubbles around a high fraction of moderately bright galaxies. Finally, we characterize such two ionized regions found in the EGS at z = 7.18 and z = 7.49 by estimating the radius of the ionized bubble that each of the spectroscopically-confirmed members could have created.

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The Epoch of Reionization (EoR) began when galaxies grew in abundance and luminosity, so their escaping Lyman continuum (LyC) radiation started ionizing the surrounding neutral intergalactic medium (IGM). Despite significant recent progress, the nature and role of cosmic reionizers are still unclear: in order to define them, it would be necessary to directly measure their LyC escape fraction ( fesc). However, this is impossible during the EoR due to the opacity of the IGM. Consequently, many efforts at low and intermediate redshift have been made to determine measurable indirect indicators in high-redshift galaxies so that their fesc can be predicted. This work presents the analysis of the indirect indicators of 62 spectroscopically confirmed star-forming galaxies at 6≤z≤9 from the Cosmic Evolution Early Release Science (CEERS) survey, combined with 12 sources with public data from other JWST-ERS campaigns. From the NIRCam and NIRSpec observations, we measured their physical and spectroscopic properties. We discovered that on average 6<z<9 star-forming galaxies are compact in the rest-frame UV ( re∼0.4 kpc), are blue sources (UV- β slope ∼-2.17), and have a predicted fesc of about 0.13. A comparison of our results to models and predictions as well as an estimation of the ionizing budget suggests that low-mass galaxies with UV magnitudes fainter than M1500=−18 that we currently do not characterize with JWST observations probably played a key role in the process of reionization.

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The physical processes that make a galaxy a Lyman alpha emitter have been extensively studied over the past 25 yr. However, the correlations between physical and morphological properties of galaxies and the strength of the Lyα emission line are still highly debated. Here, we investigate the correlations between the rest-frame Lyα equivalent width and stellar mass, star formation rate, dust reddening, metallicity, age, half-light semi-major axis, Sérsic index, and projected axis ratio in a sample of 1578 galaxies in the redshift range of 2 ≤ z ≤ 7.9 from the GOODS-S, UDS, and COSMOS fields. From the large sample of Lyα emitters (LAEs) in the dataset, we find that LAEs are typically common main sequence (MS) star-forming galaxies that show a stellar mass ≤10⁹ M_⊙, star formation rate ≤ 10^0.5 M_⊙ yr⁻¹, E(B − V)≤0.2, and half-light semi-major axis ≤1 kpc. Building on these findings, we have developed a new method based on a random forest (RF) machine learning (ML) classifier to select galaxies with the highest probability of being Lyα emitters. When applied to a population in the redshift range z ∈ [2.5, 4.5], our classifier holds a (80 ± 2)% accuracy and (73 ± 4)% precision. At higher redshifts (z ∈ [4.5, 6]), we obtained an accuracy of 73% and precision of 80%. These results highlight the possibility of overcoming the current limitations in assembling large samples of LAEs by making informed predictions that can be used for planning future large-scale spectroscopic surveys.

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Recently, intense emission from nebular C III] and C IV emission lines have been observed in galaxies in the epoch of reionization (z > 6) and have been proposed as the prime way of measuring their redshift and studying their stellar populations. These galaxies might represent the best examples of cosmic reionizers, as suggested by recent low-z observations of Lyman continuum emitting galaxies, but it is hard to directly study the production and escape of ionizing photons at such high redshifts. The ESO spectroscopic public survey VANDELS offers the unique opportunity to find rare examples of such galaxies at cosmic noon (z ∼ 3), thanks to the ultra deep observations available. We have selected a sample of 39 galaxies showing C IV emission, whose origin (after a careful comparison to photoionization models) can be ascribed to star formation and not to active galactic nuclei. By using a multiwavelength approach, we determined their physical properties including metallicity and the ionization parameter and compared them to the properties of the parent population to understand what the ingredients are that could characterize the analogs of the cosmic reionizers. We find that C IV emitters are galaxies with high photon production efficiency and there are strong indications that they might also have a large escape fraction: given the visibility of C IV in the epoch of reionization, this could become the best tool to pinpoint the cosmic reioinzers.

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The escape fraction of Lyman-continuum (LyC) photons (f_esc) is a key parameter for determining the sources of cosmic reionization at z ≥ 6. At these redshifts, owing to the opacity of the intergalactic medium, the LyC emission cannot be measured directly. However, LyC leakers during the epoch of reionization could be identified using indirect indicators that have been extensively tested at low and intermediate redshifts. These include a high [O III]/[O II] flux ratio, high star-formation surface density, and compact sizes. In this work, we present observations of 29 4.5 ≤ z ≤ 8 gravitationally lensed galaxies in the Abell 2744 cluster field. From a combined analysis of JWST-NIRSpec and NIRCam data, we accurately derived their physical and spectroscopic properties: our galaxies have low masses (log(M_⋆)∼8.5), blue UV spectral slopes (β ∼ −2.1), compact sizes (r_e ∼ 0.3 − 0.5 kpc), and high [O III]/[O II] flux ratios. We confirm that these properties are similar to those characterizing low-redshift LyC leakers. Indirectly inferring the fraction of escaping ionizing photons, we find that more than 80% of our galaxies have predicted f_esc values larger than 0.05, indicating that they would be considered leakers. The average predicted f_esc value of our sample is 0.12, suggesting that similar galaxies at z ≥ 6 have provided a substantial contribution to cosmic reionization.

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