The far-ultraviolet continuum slope as a Lyman Continuum escape estimator at high redshift

Abstract

Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high-redshifts. While observations have established that reionization occurred, observational constraints on the high-redshift ionizing emissivity remain elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and literature observations, a combined sample of 89 star-forming galaxies at redshifts near 0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6σ significant) inverse correlation between the continuum slope at 1550 Å (defined as F$_\lambda \propto \lambda ^{\beta ^{1550}_{\rm obs}}$) and both the LyC escape fraction (fesc, LyC) and fesc, LyC times the ionizing photon production efficiency (ξion). On average, galaxies with redder continuum slopes have smaller fesc, LyC than galaxies with bluer slopes mainly due to higher dust attenuation. More than 5% (20%) of the LyC emission escapes galaxies with $\beta _{\rm obs}^{1550}$ <−2.1 (-2.6). We find strong correlations between $\beta _{\rm obs}^{1550}$ and the [O iii]/[O ii] flux ratio (at 7.5σ significance), galaxy stellar mass (at 5.9σ), the gas-phase metallicity (at 4.6σ), and the observed FUV absolute magnitude (at 3.4σ). Using previous observations of $\beta _{\rm obs}^{1550}$ at high-redshift, we estimate the evolution of fesc, LyC with both redshift and galaxy magnitude. The LzLCS observations suggest that fainter and lower mass galaxies dominate the ionizing photon budget at higher redshift, possibly due to their rapidly evolving metal and dust content. Finally, we use our correlation between $\beta _{\rm obs}^{1550}$ and fesc, LyC × ξion to predict the ionizing emissivity of galaxies during the epoch of reionization. Our estimated emissivities match IGM observations, and suggest that star-forming galaxies emit sufficient LyC photons into the IGM to exceed recombinations near redshifts of 7–8.

The connection between the escape of ionizing radiation and galaxy properties at z~3 in the Keck Lyman Continuum Spectroscopic Survey

“…The rapidity of reionization depends strongly on the population of galaxies that dominates the ionizing emissivity over cosmic time. Our results indicate that fainter, less massive galaxies with lower dust content have conditions favorable for escaping ionizing radiation, broadly consistent with other recent LyC observations at z ∼ 3 (Begley et al 2022) and in the local Universe (Flury et al 2022a,b;Chisholm et al 2022). If these trends were present within the epoch of reionization, the process of reionization may have started early and progressed gradually, such that the IGM neutral fraction is 20% at z ∼ 7 (Finkelstein et al 2019), in slight tension with neutral fraction constraints from Lyα damping wing measurements (Bolton et al 2011;Greig et al 2017;Bañados et al 2018).…”

“…Chisholm et al (2022) calculate the ionizing emissivity between z ∼ 4 − 8 using empirical relations between fesc and β1500 found in the LzLCS, which are consistent with our results connecting fesc and E(B-V), and match constraints indicating that the ionizing emissivity flattens out at z < 5.5 (Becker & Bolton 2013;Becker et al 2021). However, as noted in Section 4.1, average fesc values assumed by Chisholm et al (2022) are too low at z ∼ 3 when compared to the KLCS. As an alternative scenario, Matthee et al (2022) instead directly tie fesc to the strength of Lyα emission and build a model that produces rapid reionization and a flattened evolution of the ionizing emissivity at z < 6.…”

“…Both the Chisholm et al (2022) and Matthee et al ( 2022) models highlight important existing relationships found between fesc and galaxy property in our analyses, and present ionizing emissivities that both overcome recombination in the IGM at z ∼ 8 and avoid overproducing ionizing photons at z < 6. The trends between fesc and galaxy properties presented in this work are vital for anchoring assumptions of fesc during the epoch of reionization, where direct constraints on fesc are impossible.…”

“…However, the correlation coefficient between fesc and M * was determined not to be significant, mirroring other local explorations of the two variables (Izotov et al 2021). Augmenting this result, an examination of the LzLCS sample in tandem with archival observations (totaling 89 star-forming galaxies at z ∼ 0.3) found that galaxies at lower M * tend to have both bluer spectral slopes and higher fesc (Chisholm et al 2022). This analysis focused on the strong inverse correlation found between fesc and the UV spectral slope at 1550 Å (β1500) in the expanded sample.…”

“…If these trends were present within the epoch of reionization, the process of reionization may have started early and progressed gradually, such that the IGM neutral fraction is 20% at z ∼ 7 (Finkelstein et al 2019), in slight tension with neutral fraction constraints from Lyα damping wing measurements (Bolton et al 2011;Greig et al 2017;Bañados et al 2018). Chisholm et al (2022) calculate the ionizing emissivity between z ∼ 4 − 8 using empirical relations between fesc and β1500 found in the LzLCS, which are consistent with our results connecting fesc and E(B-V), and match constraints indicating that the ionizing emissivity flattens out at z < 5.5 (Becker & Bolton 2013;Becker et al 2021). However, as noted in Section 4.1, average fesc values assumed by Chisholm et al (2022) are too low at z ∼ 3 when compared to the KLCS.…”