[Paper Review] Closing in on the sources of cosmic reionization: first results from the GLASS-JWST program

Sara Mascia, L. Pentericci+30 more|arXiv (Cornell University)|Jan 7, 2023

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This study uses JWST/NIRSpec and NIRCam data to analyze 29 gravitationally lensed galaxies at z ≈ 4.5–8 in the Abell 2744 cluster, finding that their physical properties—blue UV slopes, compact sizes, high [O iii]/[O ii] ratios, and high star-formation surface densities—strongly resemble low-redshift Lyman-continuum leakers. The analysis predicts an average LyC escape fraction of 0.12, indicating that low-mass, compact galaxies at z ≥ 6 likely provided a substantial contribution to cosmic reionization.

ABSTRACT

The escape fraction of Lyman-continuum (LyC) photons ($f_{esc}$) is a key parameter for determining the sources of cosmic reionization at $z\geq 6$. 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 [OIII]/[OII] flux ratio, high star-formation surface density, and compact sizes. In this work, we present observations of 29 $4.5 \leq z \leq 8$ 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 $(\log(M_\star)\sim 8.5)$, blue UV spectral slopes ($β\sim -2.1$), compact sizes ($r_e \sim 0.3-0.5$ kpc), and high [OIII]/[OII] 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 $z\geq 6$ have provided a substantial contribution to cosmic reionization.

Motivation & Objective

• To identify the dominant sources of cosmic reionization at z ≥ 6 by probing the escape fraction of ionizing photons (f_esc) in high-redshift galaxies.
• To test whether low-redshift LyC leaker indicators—such as high [O iii]/[O ii] flux ratio, compact size, and blue UV slope—can reliably trace f_esc in the Epoch of Reionization.
• To determine whether low-mass, compact star-forming galaxies at z ≈ 4.5–8 are viable contributors to cosmic reionization by estimating their predicted f_esc values.
• To validate indirect f_esc diagnostics using a calibrated relation derived from low-redshift leaker samples applied to high-redshift lensed galaxies.

Proposed method

• Gravitational lensing in the Abell 2744 cluster magnifies high-redshift galaxies, enabling high-sensitivity JWST/NIRSpec and NIRCam observations.
• Spectroscopic redshifts and emission-line measurements (e.g., [O ii], [O iii], Hβ, Hα) are derived from combined NIRSpec and NIRCam data.
• Physical properties such as stellar mass (M⋆), star-formation rate (SFR), effective radius (re), UV spectral slope (β), and star-formation surface density (ΣSFR) are measured for each galaxy.
• A linear regression model minimizing mean squared error (MSE) is trained on a low-redshift sample of 88 galaxies with known f_esc to derive a predictive relation between f_esc and three key observables: [O iii]/[O ii] (O32), UV slope (β), and effective radius (re).
• The calibrated f_esc relation is applied to 24 high-redshift galaxies with all three parameters measured, yielding predicted escape fractions.
• The MEx diagram is used to confirm the absence of active galactic nuclei (AGN), ensuring the galaxies are purely star-forming.

Experimental results

Research questions

• RQ1 Do high-redshift, lensed galaxies at z ≈ 4.5–8 exhibit physical and spectroscopic properties consistent with low-redshift LyC leakers?
• RQ2 Can the [O iii]/[O ii] flux ratio, UV slope, and compact size serve as reliable indirect indicators of f_esc in the Epoch of Reionization?
• RQ3 What is the predicted average LyC escape fraction (f_esc) for low-mass, compact galaxies at z ≥ 6, and does it support their role as major reionization sources?
• RQ4 How do the observed properties of these galaxies compare to those of typical Lyman break galaxies (LBGs) at similar redshifts and magnitudes?
• RQ5 Is the f_esc prediction from low-redshift diagnostics robust when extrapolated to the high-redshift regime?

Key findings

• The sample consists of purely star-forming galaxies, with no evidence of AGN activity, as confirmed by the MEx diagram.
• The galaxies have a median UV spectral slope of β = -2.08, indicating blue, young stellar populations.
• The effective radii (re) of the galaxies are compact, ranging from 0.2 to 0.5 kpc, consistent with high star-formation surface densities.
• The [O iii]/[O ii] flux ratio (O32) is high, a key indicator of low dust content and potential LyC escape.
• Of the 24 galaxies with all three key parameters (O32, β, re) measured, 20 (83%) have predicted f_esc > 0.05, indicating they would be classified as LyC leakers.
• The average predicted f_esc for the sample is 0.12, suggesting that low-mass, compact star-forming galaxies at z ≥ 6 are strong candidates for driving cosmic reionization.