OutThere Survey: Addressing $\mathrm{ξ_{ion}}$ and $\mathrm{f_{esc}}$ with a population of average galaxies at z$\sim$2

Abstract: Constraining the major contributors to the ionisation of the early universe is an ongoing endeavour of high-redshift galaxy research. We measure the ionising photon production efficiency and Lyman Continuum escape fraction for a sample of 230 intermediate redshift ($1.3&lt;z\&lt;2.6$) sources observed as a part of the \textit{OutThere survey}; a pure-parallel, wide-area JWST/NIRISS survey with accompanying JWST/NIRCam, NIRISS and HST photometry. The low threshold emission selection criteria for this sample makes for a large and robust control, against which other works may be contrasted, particularly for low-mass galaxies above z$&gt;$5. This control sample allows us to verify the correlations between ionising and spectral/physical properties suggested by previous studies. We find no significant correlations between the ionising photon production efficiency ($\mathrm{ξ<em>{ion}}$) with the UV slope, $\mathrm{M</em>{UV}}$, M$<em>*$ or sSFR. We do find that $\mathrm{ξ</em>{ion}}$ correlates with [OIII]5007Å\, equivalent width (EW) (Spearman coefficient $ρ$ =0.24; p$&lt; 4\times10<sup>{-4}$) and H$α$ EW ($ρ$ =0.63; p$&lt;&lt; 1\times10<sup>{-6}$) hold even at low EW albeit with more scatter. We also find that our novel approach to determining the ionising photon escape fraction $\mathrm{f_{esc}}$ results in values within theoretical ranges (0-10\%) though vary substantially in comparison to the empirical results (median $\mathrm{f_{esc}} = 0.9\%<sup>{+1.1}_{-0.5}$ including non-detections, median $\mathrm{f_{esc}} = 1.9\%<sup>{+8.9}_{-1.8}$ above a $0.01\%$ threshold). We find that this escape fraction method has consistently significant correlations with the redshift, SFR and M$_{UV}$ and sample-dependent correlations with [OIII]5007Å\,EW,H$α$ EW and stellar mass.

• The paper measures ξion and fesc in 230 broadly selected galaxies at 1.3 < z < 2.6, providing a control sample beyond extreme emitters and targeted LyC leakers.
• Emission-line equivalent widths reliably trace ξion, with stronger correlations for Hα (ρ=0.65) than [O III]5007 (ρ=0.26), while UV slope, luminosity, mass, and sSFR show no robust trends.
• The inferred median LyC escape fraction is 0.9% including non-detections, and its lack of correlation with ξion suggests photon production and escape are largely independent processes in average galaxies.

The OutThere Survey paper presents measurements of the ionising photon production efficiency ($\xi_{\mathrm{ion}}$) and the Lyman-continuum (LyC) escape fraction ($f_{esc}$) for a sample of 230 galaxies at $1.3 < z < 2.6$, drawn from the pure-parallel JWST/NIRISS program in the Hubble Ultra-Deep Field-North (2608.19687). Its central contribution is a deliberately unremarkable control sample: rather than targeting extreme emitters, mergers, or dust-free systems, the selection admits the full observed range of [OIII]5007 and H$\alpha$ equivalent widths, providing a baseline against which the heavily selected high-redshift populations used to constrain reionisation can be compared.

Motivation and survey design

The Epoch of Reionisation (EoR) requires knowledge of both $\xi_{\mathrm{ion}}$ and $f_{esc}$, yet direct LyC detection is impossible at $z>6$ due to IGM opacity, forcing reliance on indirect tracers calibrated at lower redshift. The authors argue that existing studies over-represent galaxies with bright emission lines, low dust content, unusual continua, or interactions, biasing inferred ionising budgets. The OutThere survey — at the time of writing the largest JWST pure-parallel program, with 120/127 fields and 425 hours observed — supplies 12 pointings (48 sq. arcmin) with F150W/F200W grism spectra deeper than 12.5 hours, complemented by a 24-filter photometric catalogue combining JADES, PANORAMIC, CONGRESS, and SAPPHIRES imaging from F435W to F460M.

Selection required detected [OIII]5007 and H$\alpha$ between $1.6$\alpha$ detection, yielding an initial sample of 277 that was reduced to 230 after removing non-convergent BEAGLE fits ($f_{esc}$0) and 41 sources with poorly reduced slitless spectra identified through visual classification. The spectroscopic redshift requirement inherently favours emission-line galaxies; the authors acknowledge this but note that within it, no additional line-strength cut is imposed.

Methodology

Physical parameters are derived with BEAGLE using BC16 stellar population synthesis, CLOUDY photo-ionisation, CF00 two-component dust, and delayed-tau plus free 10 Myr constant star formation histories. The ionising efficiency follows the standard H$f_{esc}$1 conversion:

$f_{esc}$2

with dust-corrected $f_{esc}$3 measured at 1500Å. This assumes all ionising photons are absorbed by the birth cloud nebulae ($f_{esc}$4), an assumption deliberately retained so that nebular properties are driven by photometry rather than escape assumptions.

H$f_{esc}$5 is corrected for [NII] blending using model-predicted line ratios and for dust using the CF00 birth-cloud attenuation with the Calzetti-style 0.44 stellar-to-nebular conversion factor. The escape fraction is derived through what the authors term a novel two-layer approach: the ratio of dust-corrected H$f_{esc}$6-implied absorbed photons to the BEAGLE-modelled total ionising budget gives the non-escaping fraction, which is then multiplied by a dust-transmission factor integrated over 100–912Å assuming a flat intrinsic LyC spectrum referenced at 600Å:

$f_{esc}$7

A UV-slope-based empirical estimate ($f_{esc}$8, following Chisholm et al.'s LzLCS relation) provides comparison. Of 233 galaxies, 199 yield measurable values after excluding physically inconsistent cases where the modelled ionising budget falls below the H$f_{esc}$9-implied absorption.

Sample characteristics

The sample spans $1.3 < z < 2.6$0 from 7.22 to 11.41 (median 9.33), $1.3 < z < 2.6$1 from −25.9 to −17.3 (median −19.7), and H$1.3 < z < 2.6$2 EW from 12 to 11170Å (median 287). Only ~15% of galaxies lie on the Speagle et al. main sequence at $1.3 < z < 2.6$3, ~24% qualify as recent bursts ($1.3 < z < 2.6$4), and ~23% meet the extreme [OIII]5007 EW definition (>400Å). Notably, most BEAGLE fits indicate low dust attenuation ($1.3 < z < 2.6$5), and the sample systematically underestimates spectral H$1.3 < z < 2.6$6 relative to the models (median fractional difference −0.18).

Ionising photon production efficiency

No significant Spearman correlation is found between $1.3 < z < 2.6$7 and UV slope, $1.3 < z < 2.6$8, stellar mass, or sSFR. This contradicts several high-redshift findings: Harshan et al. report a positive $1.3 < z < 2.6$9 correlation at $\alpha$0, and Castellano et al. find strong sSFR correlations at $\alpha$1. The correlations do strengthen when the reddest/most massive bins are excluded, suggesting that dust degeneracy and scatter at the extremes suppress the trends in this redder, less actively star-forming sample. The implication is that relations extrapolated to faint EoR galaxies may not hold for average intermediate-redshift populations.

In contrast, the emission-line correlations are robust. The [OIII]5007 EW relation yields $\alpha$2 ($\alpha$3), holding within 3σ of Tang et al. and Boyett et al. even below EW<225Å where those works predicted deviations, though with larger scatter. The H$\alpha$4 EW relation is stronger still: $\alpha$5 ($\alpha$6), matching Tang et al. within 1σ. These results validate the use of rest-frame optical EWs as $\alpha$7 proxies across an extended dynamical range down to very low EWs.

Escape fraction results

The median $\alpha$8 including non-detections is 0.9% (+1.1/−0.5), rising to 1.9% (+8.9/−1.8) above the 0.01% threshold — consistent with theoretical ranges and with stacked upper limits from Siana et al. (~2%) and Cowie et al., but far below targeted LyC-leaker samples such as Steidel et al.'s KLCS (12±0.1%). The UV-slope method yields consistently lower values than Cullen et al.'s high-redshift estimates (5% at z<10.5, 19% above), reflecting the redder UV slopes at $\alpha$9. The large scatter in the theoretical method stems from its dependence on modelled dust corrections, dust transmission, and photo-ionisation budgets simultaneously.

Several correlations emerge that depend sensitively on the adopted lower threshold:

Parameter	Sign of correlation with $\xi_{\mathrm{ion}}$0	Threshold dependence
Redshift	negative	consistent at all thresholds
$\xi_{\mathrm{ion}}$1	positive (fainter → higher $\xi_{\mathrm{ion}}$2)	consistent
$\xi_{\mathrm{ion}}$3	negative (strongest, $\xi_{\mathrm{ion}}$4 unthresholded)	consistent
Stellar mass	negative	only without threshold
H$\xi_{\mathrm{ion}}$5 EW	flips sign	negative at 0.1%, positive unthresholded
[OIII]5007 EW	weak positive	only without threshold

The positive $\xi_{\mathrm{ion}}$6 correlation survives a faint-end cut at $\xi_{\mathrm{ion}}$7 ($\xi_{\mathrm{ion}}$8, $\xi_{\mathrm{ion}}$9 unthresholded), supporting the faint-galaxy contribution narrative — though the authors caution it could partly reflect spectroscopic selection toward brighter sources. The anti-correlation with short-timescale SFR and with the internal SED $f_{esc}$0 suggests escape favours systems with established stellar populations whose feedback has cleared channels, consistent with FIRE simulation expectations that young, embedded populations retain their photons. The threshold-dependent H$f_{esc}$1 EW flip indicates a transition from dust-dominated attenuation at low $f_{esc}$2 to geometry/density-bounded leakage at higher values.

Notably absent is any correlation between $f_{esc}$3 and $f_{esc}$4, implying that the conditions producing ionising photons and those enabling their escape are largely decoupled in this population — a result bearing directly on whether single-parameter predictors can characterise reionisation-era contributions.

Limitations and open questions

Several caveats are explicit. The NIRISS reduction pipeline is still maturing; trace and wavelength calibrations remain in progress and contamination modelling required subjective visual vetting of 41 sources. The dust correction implementation is acknowledged as influential — applying stronger attenuation would make some null correlations significant — and the 0.44 stellar-to-nebular $f_{esc}$5 conversion factor is noted as poorly supported by Reddy et al. The flat LyC spectrum assumption matters materially: referencing at 912Å instead of 600Å raises the median $f_{esc}$6 from 1.9% to 3.9%, while power-law prescriptions produce still larger values the authors deem unphysical, leaving the absolute scale uncertain at the factor-of-two level. The theoretical escape fractions show significant negative redshift trends contrary to empirically calibrated expectations, and the authors flag these methods as requiring further testing before application. Finally, the escape fraction derivation is not directly comparable to SED-fit approaches that treat $f_{esc}$7 as a free parameter, limiting cross-study calibration.

Conclusion

This work delivers a large, uniformly selected control sample demonstrating which $f_{esc}$8 and $f_{esc}$9 correlations survive at $z>6$0 with wide dynamical range. Emission-line EW predictors are validated and extended to low EWs; UV slope, luminosity, and sSFR predictors fail for average galaxies despite success in selected high-redshift samples; and the novel H$z>6$1-based escape fraction method produces plausible but highly scattered values whose correlations are threshold-dependent. The central caution — that individual ionising-emission predictors carry strong methodology dependence — argues for treating extrapolations from extreme or high-redshift populations to the general galaxy population with care until comparable control samples exist at the epochs where they are being applied.