Reconciling escape fractions and observed line emission in Lyman-continuum-leaking galaxies

Abstract

Context. Finding and elucidating the properties of Lyman-continuum(LyC)-emitting galaxies is an important step in improving our understanding of cosmic reionization. Aims. Although the z ∼ 0.3 − 0.4 LyC emitters found recently show strong optical emission lines, no consistent quantitative photoionization model taking into account the escape of ionizing photons and inhomogenous interstellar medium (ISM) geometry of these galaxies has yet been constructed. Furthermore, it is unclear to what extent these emission lines can be used to distinguish LyC emitters. Methods. To address these questions we construct one- and two-zone photoionization models accounting for the observed LyC escape, which we compare to the observed emission line measurements. The main diagnostics used include lines of [O III], [O II], and [O I] plus sulfur lines ([S II], [S III]) and a nitrogen line ([N II]), which probe regions of different ionization in the ISM. Results. We find that single (one-zone) density-bounded photoionization models cannot reproduce the emission lines of the LyC leakers, as pointed out by earlier studies, because they systematically underpredict the lines of species of low ionization potential, such as [O I] and [S II]. Introducing a two-zone model, with differing ionization parameter and a variable covering fraction and where one of the zones is density-bounded, we show that the observed emission line ratios of the LyC emitters are well reproduced. Furthermore, our model yields LyC escape fractions, which are in fair agreement with the observations and independent measurements. The [O I] λ6300 excess, which is observed in some LyC leakers, can be naturally explained in this model, for example by emission from low-ionization and low-filling-factor gas. LyC emitters with a high escape fraction (fesc ≳ 38%) are deficient both in [O I] λ6300 and in [S II] λλ6716,6731. We also confirm that a [S II] λλ6716,6731 deficiency can be used to select LyC emitter candidates, as suggested earlier. Finally, we find indications for a possible dichotomy in terms of escape mechanisms for LyC photons between galaxies with relatively low (fesc ≲ 10%) and higher escape fractions. Conclusions. We conclude that two-zone photoionization models are sufficient and required to explain the observed emission line properties of z ∼ 0.3 − 0.4 LyC emitters. This is in agreement with UV absorption line studies, which also show the co-existence of regions with high hydrogen column density (i.e., no escape of ionizing photons) and density-bounded or very low column density regions responsible for the observed escape of LyC radiation. These simple but consistent models provide a first step towards the use of optical emission lines and their ratios as quantitative diagnostics of LyC escape from galaxies.

The synchrony of production and escape: half the bright Lyα emitters at z ≈ 2 have Lyman continuum escape fractions ≈50

“…This ensures consistent Ly𝛼 𝑓 esc comparison, given the spatially extended nature of Ly𝛼 emission (e.g., Hayes et al 2013;Wisotzki et al 2016). We also clarify that this argument does not imply that high Ly𝛼 𝑓 esc selects for high LyC 𝑓 esc -the point is that at high LyC 𝑓 esc , 𝑓 esc,LyC / 𝑓 esc,Ly𝛼 ≈ 1 which is supported by these 𝑓 esc > 20% systems being in "density bounded nebulae" that are transparent to LyC along all lines of sight (e.g., Ramambason et al 2020), thus diminishing the resonance advantage of Ly𝛼 over LyC.…”

“…The optical line ratios and covering fractions of prolific LyC leakers ( 𝑓 esc > 20%) imply they are best described as "density bounded nebulae" (Ramambason et al 2020). That is, the ionization front is surrounded by low column density gas (10 16 − 10 18 cm −2 ) punctuated by entirely transparent (< 10 16 cm −2 ) channels (see Fig.…”

Strong Lyman continuum emitting galaxies show intense C IV λ1550 emission

“…In addition, these observations have served to empirically establish "indirect indi-cators" of LyC escape, which can potentially be used at all redshifts, including for galaxies in the epoch of reionization, where direct observations of the LyC are not possible. Among these indirect indicators are properties of the resolved Lyman alpha (Lyα) line profile (the peak separation), weak UV absorption lines, Mg ii emission, a high ratio of [O iii] λλ4959,5007/[O ii] λ3727, and a deficit of the [S ii] λλ6717,6731 line (see Jaskot & Oey 2013;Verhamme et al 2017;Henry et al 2018;Chisholm et al 2020;Ramambason et al 2020;Wang et al 2021).…”

The Interstellar Medium of Dwarf Galaxies

“…Therefore, by observing z > 6 galaxies, it is not possible to establish a relationship between the leakage of LyC photons into the IGM and the detailed properties of these galaxies. This can only be revealed by observations of galaxies at lower redshift and numerous studies have been published to achieve this goal (e.g., [216][217][218][219][220][221][222]). Several ways have been suggested to find galaxies with high LyC leakages.…”