[Paper Review] The ionising photon production efficiency at z~6 for Lyman-alpha emitters using JEMS and MUSE

Charlotte Simmonds, Sandro Tacchella+22 more|arXiv (Cornell University)|Mar 14, 2023

Electron and X-Ray Spectroscopy TechniquesMaterials Science3 citations

TL;DR

This study measures the ionising photon production efficiency (ξ_ion,0) for 30 Lyman-alpha emitters at z~6 using JWST JEMS medium-band photometry and MUSE spectroscopy. By fitting SEDs with Prospector and correcting for dust, it finds a median ξ_ion,0 of log(ξ_ion,0) = 25.44 Hz erg⁻¹, indicating these galaxies are highly efficient at producing ionising photons, with slightly enhanced values compared to previous literature estimates.

ABSTRACT

We study the ionising photon production efficiency at the end of the Epoch of Reionisation ($z \sim 5.4 - 6.6$) for a sample of 30 Lyman-$α$ emitters. This is a crucial quantity to infer the ionising photon budget of the Universe. These objects were selected to have reliable spectroscopic redshifts, assigned based on the profile of their Lyman-$α$ emission line, detected in the MUSE deep fields. We exploit medium-band observations from the JWST extragalactic medium band survey (JEMS) to find the flux excess corresponding to the redshifted \ha\ emission line. We estimate the ultra-violet (UV) luminosity by fitting the full JEMS photometry, along with several HST photometric points, with exttt{Prospector}. We find a median UV continuum slope of $β= -2.09^{+0.23}_{-0.21}$ for the sample, indicating young stellar populations with little-to-no dust attenuation. Supported by this, we derive $ξ_{ion,0}$ with no dust attenuation and find a median value of log$\frac{ξ_{ion,0}}{ ext{Hz erg}^{-1}} = 25.44^{+0.21}_{-0.15}$. If we perform dust attenuation corrections and assume a Calzetti attenuation law, our values are lowered by $\sim 0.1$ dex. Our results suggest Lyman-$α$ emitters at the Epoch of Reionisation have enhanced $ξ_{ion,0}$ compared to previous estimations from literature, in particular, when compared to the non-Lyman-$α$ emitting population. This initial study provides a promising outlook on the characterisation of ionising photon production in the early Universe. In the future, a more extensive study will be performed on the entire dataset provided by the JWST Advanced Deep Extragalactic Survey (JADES). Thus, for the first time, allowing us to place constraints on the wider galaxy populations driving reionisation.

Motivation & Objective

• To measure the ionising photon production efficiency (ξ_ion,0) for high-redshift Lyman-alpha emitters at z~6, a critical parameter for understanding the ionising budget of the early Universe.
• To characterise the stellar populations and dust content of these galaxies using multi-wavelength photometry and spectroscopy to assess their ionising photon output.
• To evaluate whether Lyman-alpha emitters are more efficient at producing ionising photons than non-emitting galaxies, potentially identifying them as dominant drivers of reionisation.
• To provide a benchmark for future studies using the full JADES dataset, enabling broader constraints on galaxy populations during the Epoch of Reionisation.

Proposed method

• Selected 30 Lyman-alpha emitters from the MUSE deep fields with spectroscopically confirmed redshifts (z~5.4–6.6) based on Lyα line profiles.
• Used medium-band photometry from the JWST JEMS survey to detect redshifted Hα emission lines and estimate Hα luminosities.
• Fitted full SEDs using the Prospector SED fitting code, incorporating JEMS photometry and HST photometric points to derive rest-frame UV luminosity density at 1500 Å.
• Calculated the UV continuum slope β from the rest-frame 1250–2600 Å region to infer stellar population age and dust attenuation.
• Estimated ξ_ion,0 as the ratio of ionising photon production rate to UV luminosity density, with and without dust correction using the Calzetti attenuation law.
• Assessed [N ii] contamination in photometric bands and confirmed negligible impact on results for z~6 galaxies.

Experimental results

Research questions

• RQ1 What is the ionising photon production efficiency (ξ_ion,0) of Lyman-alpha emitters at z~6, and how does it compare to previous estimates from the literature?
• RQ2 To what extent are these Lyman-alpha emitters dust-attenuated, and how does dust correction affect the derived ξ_ion,0 values?
• RQ3 Are Lyman-alpha emitters more efficient at producing ionising photons than non-Lyman-alpha-emitting galaxies at similar redshifts?
• RQ4 How do the stellar population properties (e.g., UV slope β) of these galaxies inform their ionising output and potential role in cosmic reionisation?
• RQ5 Can the JEMS and MUSE data combination provide robust constraints on Hα luminosity and SED fitting for high-redshift galaxies?

Key findings

• The median ionising photon production efficiency for the sample is log(ξ_ion,0) = 25.44 Hz erg⁻¹, with a 1σ confidence interval of +0.21 to -0.15 dex.
• The median UV continuum slope is β = -2.09, indicating young stellar populations with minimal dust attenuation.
• Dust correction using the Calzetti law reduces ξ_ion,0 by approximately 0.1 dex in cases with non-negligible dust, but the uncorrected values remain the primary result.
• The derived ξ_ion,0 values are slightly enhanced compared to the broader literature sample spanning z~0 to z~9, suggesting LAEs may be more efficient ionisers.
• The highest ξ_ion,0 value is found in JADES-GS+53.13859-27.79024, which also shows signs of AGN activity and will be studied in detail in future work.
• The study confirms that [N ii] contamination in JEMS photometric bands is negligible for z~6 galaxies, supporting the reliability of Hα flux measurements.