A GLIMPSE into the very faint-end of the H$β$+[OIII]$λλ$4960,5008 luminosity function at z=7-9 behind Abell S1063

TL;DR

This study uses ultra-deep JWST/NIRCam imaging of Abell S1063 to push the Hβ+[OIII] λλ4960,5008 luminosity function to L ≳ 10^{39} erg s^{-1} at z ≈ 7–9, exploiting strong gravitational lensing to access extremely faint galaxies. By combining Lyman-break selection, robust lens modeling, and SED-based line flux measurements with realistic completeness corrections, the authors find a relatively flat faint-end slope for the nebular LF, α_neb ≈ -1.55 to -1.78, in contrast to the steeper UV LF (α_UV ≈ -2 to -2.2). They show that bursty star formation and metallicity evolution can reconcile nebular and UV LFs, and they quantify the ionising photon budget, finding that faint galaxies contribute modestly to reionisation under standard f_esc, while GLIMPSE data indicate that the bulk of Hβ+[OIII] emission is already detected. The results imply a flattened contribution from very faint galaxies to the cosmic star formation rate density and ionising photon production during the EoR. Overall, GLIMPSE demonstrates the power of combining JWST depth with gravitational lensing to constrain the faint galaxy population and their role in reionisation and early star formation.

Abstract

We use the ultra-deep GLIMPSE JWST/NIRCam survey to constrain the faint-end of the H$β$+[OIII]$λλ$4960,5008 luminosity function (LF) down to $10^{39}$ erg/s at z=7-9 behind the lensed Hubble Frontier Field Abell S1063. We perform SED fitting on a Lyman-Break Galaxy sample, measuring combined H$β$+[OIII] fluxes to construct the emission-line LF. The resulting LF ($α$=-1.55 to -1.78) is flatter than the UV LF ($α<-2$), indicating a lower number density of low H$β$+[OIII] emitters at fixed MUV. We explore three explanations: (i) bursty star formation histories reducing the H$β$+[OIII]-to-UV ratio, (ii) metallicity effects on [OIII]/H$β$, or (iii) a faint-end turnover in the UV LF. Assuming an evolving [OIII]/H$β$ ratio, we derive a flatter [OIII]$λ$5008 LF ($α$=-1.45 to -1.66) and a steeper H$β$ LF ($α$=-1.68 to -1.95). The combination of decreasing metallicity and bursty star formation can reconcile the UV and H$β$+[OIII] LF differences. Converting the LF to the ionising photon production rate, we find that galaxies with H$α$ flux $>10^{39}$ erg/s (SFR(H$α$)$>5\times10^{-3} M_\odot$/yr) contribute 21-61% and 24-104% of the ionising photon budget at 7<z<8 and 8<z<9, respectively (for $f_{esc}=0.1$). The LF shape suggests faint galaxies contribute minimally to the ionising photon production rate. Our cosmic star formation rate density (CSFRD) estimates align with previous work, but GLIMPSE's sensitivity to low SFRs confirms that very faint galaxies are minor contributors to both the ionising photon production rate and the CSFRD. Our results suggest that GLIMPSE has detected the bulk of the total H$β$+[OIII] emission from star-forming galaxies, with fainter sources playing a limited role in cosmic reionisation.

Figures (18)

• Figure 1: Relationship between the absolute UV magnitude _UV $M_\text{UV}$ of the GLIMPSE F090W dropouts selected in this work and their SL magnification $\mu$. Beyond $M_{\mathrm{UV}}=-16$, we need magnification to detect these objects. This is shown by the black line, which is the minimum magnification required for a $5\sigma$ detection at $z=7$.
• Figure 2: Evolution of the effective volumed probed for each UV magnitude M$_\text{UV}$ for our two redshift bins 7<z<8 (blue) and 8<z<9 (orange) with 1$\sigma$ uncertainties in shaded area.
• Figure 3: Histogram of the dust attenuation parameter $E(B-V)_l$ sampled from the SED fitting of our sample of $7<z<9$ galaxies. $N=10'000$ realisations of the measurement are performed for each galaxy, following the distributions of the SED fitting, and the histogram is normalised by $N$. The histogram shows that dust attenuation is negligible in our sample.
• Figure 4: Correlation between the [O iii] [O iii]+Hβ H$\beta$ line flux against $M_{UV}$ for our GLIMPSE sample (circles) and measurements from meyerJWSTFRESCOComprehensive2024a. To match our observations, we only kept sources within the same $7 < z < 9$ redshift range. Bayesian error estimates using CIGALE are also shown for the GLIMPSE measurements. Despite the scatter, the quantities correlate well, therefore the completeness for _UV $M_\text{UV}$ approximately translates to [O iii] [O iii]+Hβ H$\beta$ line flux.
• Figure 5: Luminosity function of Hβ H$\beta$+λλ4960,5008 [O iii] [O iii]$\lambda\lambda4960,5008$ for our two redshift ranges. The GLIMPSE data points give the mean and standard deviation of the luminosity function, while the fits provide the median value as a solid line, and the 16-84 percentile in the shaded area. We added previous studies of the LF using JWST/NIRCam GRISM slitless spectroscopy meyerJWSTFRESCOComprehensive2024amatthee_eiger_2023sun_first_2023, a JWST/NIRCam medium band survey by wold_uncovering_2025 and a former Spitzer study by de_barros_greats_2019. Note that all the JWST surveys specifically study the [O iii] [O iii]$\lambda5007$Å, so we converted them to [O iii] [O iii]+Hβ H$\beta$ by considering their respective measured R3 factor as well as $[O~{\sc iii}]5008 [O~{\sc iii}]$λ5008$/[O~{\sc iii}]4960 [O~{\sc iii}]$λ4960$= 2.98$storey_theoretical_2000. The data can be found in Tab. \ref{['tab:O3LF']} and the parametrisation can be found in Tab. \ref{['tab:schechter']}.