[Paper Review] Low-mass bursty galaxies in JADES efficiently produce ionising photons and could represent the main drivers of reionisation

Charlotte Simmonds, Sandro Tacchella+22 more|arXiv (Cornell University)|Oct 2, 2023

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy3 citations

TL;DR

This study analyzes 677 high-redshift galaxies from the JADES survey to show that low-mass, bursty star-forming galaxies efficiently produce ionizing photons, with ionizing photon production efficiency (ξ_ion) increasing with redshift and ultraviolet magnitude. The authors find that these galaxies could be the primary drivers of cosmic reionization, requiring only low to moderate escape fractions (as low as ~2% at z≈9), challenging previous assumptions about high escape fractions being necessary.

ABSTRACT

We study galaxies in JADES Deep to study the evolution of the ionising photon production efficiency, $ξ_{ m{ion}}$, observed to increase with redshift. We estimate $ξ_{ m{ion}}$ for a sample of 677 galaxies at $z \sim 4 - 9$ using NIRCam photometry. Specifically, combinations of the medium and wide bands F335M-F356W and F410M-F444W to constrain emission lines that trace $ξ_{ m{ion}}$: H$α$ and [OIII]. Additionally, we use the spectral energy distribution fitting code exttt{Prospector} to fit all available photometry and infer galaxy properties. The flux measurements obtained via photometry are consistent with FRESCO and NIRSpec-derived fluxes. Moreover, the emission-line-inferred measurements are in tight agreement with the exttt{Prospector} estimates. We also confirm the observed $ξ_{ m{ion}}$ trend with redshift and M$_{ m{UV}}$, and find: $\log ξ_{ m{ion}} (z, ext{M}_{ m{UV}}) = (0.05 \pm 0.02)z + (0.11 \pm 0.02) ext{M}_{ m{UV}} + (27.33 \pm 0.37)$. We use exttt{Prospector} to investigate correlations of $ξ_{ m{ion}}$ with other galaxy properties. We see a clear correlation between $ξ_{ m{ion}}$ and burstiness in the star formation history of galaxies, given by the ratio of recent to older star formation, where burstiness is more prevalent at lower stellar masses. We also convolve our $ξ_{ m{ion}}$ relations with luminosity functions from the literature, and constant escape fractions of 10 and 20\%, to place constraints on the cosmic ionising photon budget. By combining our results, we find that if our sample is representative of the faint low-mass galaxy population, galaxies with bursty star formation are efficient enough in producing ionising photons and could be responsible for the reionisation of the Universe.

Motivation & Objective

• To investigate the ionizing photon production efficiency (ξ_ion) of high-redshift galaxies and its evolution with redshift and UV magnitude.
• To determine whether bursty star formation histories in low-mass galaxies enhance ξ_ion and make them strong candidates for reionization.
• To constrain the cosmic ionizing photon budget by combining ξ_ion relations with luminosity functions and constant or magnitude-dependent escape fractions.
• To validate emission-line-based ξ_ion estimates using SED fitting with Prospector and compare with independent flux measurements.

Proposed method

• Measured fluxes in medium and wide NIRCam bands (F335M–F356W and F410M–F444W) to trace Hα and [O iii] emission lines, which are proxies for ξ_ion.
• Used the Prospector SED fitting code to infer galaxy properties, including ξ_ion, from all available photometry across the sample.
• Performed a 2D fit of log ξ_ion as a function of redshift (z) and absolute UV magnitude (M_UV), yielding a quantitative relation: log ξ_ion = (0.05±0.02)z + (0.11±0.02)M_UV + (27.33±0.37).
• Compared emission-line-derived ξ_ion estimates with Prospector-inferred values, confirming consistency across methods.
• Convolved the derived ξ_ion relation with luminosity functions from the literature and applied constant (10%, 20%) and M_UV-dependent escape fractions to estimate the total cosmic ionizing photon budget.
• Assessed the physical drivers of ξ_ion by correlating it with galaxy properties, particularly burstiness in the star formation history (SFRH).

Experimental results

Research questions

• RQ1 Does the ionizing photon production efficiency (ξ_ion) of high-redshift galaxies increase with redshift and UV magnitude, and is this trend physical rather than observational bias?
• RQ2 Can emission-line equivalent widths (e.g., EW[O iii]) reliably estimate ξ_ion in high-redshift galaxies?
• RQ3 What is the role of bursty star formation histories in enhancing ξ_ion, and how does it correlate with galaxy mass and redshift?
• RQ4 What escape fraction of ionizing photons is required for low-mass, bursty galaxies to account for the cosmic ionizing budget during reionization?
• RQ5 Can the observed ξ_ion evolution and galaxy properties explain the reionization of the intergalactic medium without requiring extremely high escape fractions?

Key findings

• The observed evolution of ξ_ion with redshift and M_UV is physically driven, not an artifact of selection or measurement bias.
• The 2D fit of ξ_ion yields the relation: log ξ_ion(z, M_UV) = (0.05±0.02)z + (0.11±0.02)M_UV + (27.33±0.37), showing strong dependence on both redshift and UV magnitude.
• Burstiness in the star formation history—defined as the ratio of recent to older star formation—shows the strongest correlation with ξ_ion, especially in low-mass galaxies.
• When combined with luminosity functions and constant escape fractions of 10% and 20%, the evolving ξ_ion relation reduces the required average escape fraction to less than 10%, dropping to ~2% at z≈9.
• Low-mass, bursty galaxies are efficient enough in producing ionizing photons to account for the cosmic ionizing budget required to reionize the Universe by z≈6.
• The study confirms that EW([O iii]) is a reliable proxy for ξ_ion in high-redshift galaxies, with emission-line-based estimates tightly consistent with Prospector SED fitting results.