Neutral gas properties of Lyman continuum emitting galaxies: Column densities and covering fractions from UV absorption lines

Abstract

Context. The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known. Aims. To understand how Lyman continuum (LyC) photons escape galaxies, we constrain the H i covering fractions and column densities using ultraviolet (UV) H i and metal absorption lines of 18 star-forming galaxies that have Lyman series observations. Nine of these galaxies are confirmed LyC emitters Methods. We fit the stellar continuum, dust attenuation, metal, and H i properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen and metals. We used synthetic interstellar absorption lines to explore the systematics of our measurements. Then we applied our method to the observed UV spectra of low-redshift and z ∼ 2 galaxies. Results. The observed H i lines are found to be saturated in all galaxies. An indirect approach using O i column densities and the observed O/H abundances yields H i column densities of log(N H I ) ∼ 18.6 − 20 cm −2 . These columns are too high to allow the escape of ionizing photons. We find that the known LyC leakers have H i covering fractions less than unity. Ionizing photons escape through optically thin channels in a clumpy interstellar medium. Our simulations confirm that the H i covering fractions are accurately recovered. The Si ii and H i covering fractions scale linearly, in agreement with observations from stacked Lyman break galaxy spectra at z ∼ 3. Thus, with an empirical correction, the Si ii absorption lines can also be used to determine the H i coverage. Finally, we show that a consistent fitting of dust attenuation, continuum, and absorption lines is required to properly infer the covering fraction of neutral gas and subsequently to infer the escape fraction of ionizing radiation. Conclusions. These measurements can estimate the LyC escape fraction, as we demonstrate in a companion paper.

Confirmation of double peaked Lyα emission at z = 6.593

“…We note that the shell-model prediction of the systemic velocity of CR7 presented in Dijkstra et al (2016) agrees perfectly with recent [Cii] measurements in Matthee et al (2017b), suggesting that shell-model fitting is a good tool to recover the systemic redshift of a LAE at high redshift. The column density is also significantly lower than the column density inferred from absorption line measurements in local LyC leakers (Gazagnes et al 2018), which lead these authors to conclude that the Hi covering fraction in these galaxies is non-uniform. A relatively high column density in these local LyC leakers is consistent with their larger Lyα peak separation compared to COLA1, and their typical escape fraction of ≈5%.…”

The Herschel Dwarf Galaxy Survey

“…Moreover, Gazagnes et al (2018) show for galaxies with known Lyman continuum leakage that the escape of ionizing photons is related to the covering factor of the H i gas (rather than low H i column densities). Recent modeling of the Local Group dwarf galaxy IC 10 also shows that on scales of individual star-forming regions, clouds tend to be matter-bounded, allowing for the possibility of photons escaping the regions, while on larger spatial scales, clouds tend to be more radiation-bounded (Polles et al 2019).…”

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

“…In the present work we study eleven z ∼ 0.3−0.4 Lyman continuum emitters discovered with the COS spectrograph on-board HST and whose properties have been amply discussed in several papers (Izotov et al 2016a(Izotov et al ,b, 2018aSchaerer et al 2016Schaerer et al , 2018Verhamme et al 2017;Gazagnes et al 2018;Chisholm et al 2018). The metallicities of these galaxies are in the range 12 + log(O/H) = 7.65−8.16, with a median oxygen abundance of 12 + log(O/H) of 7.91.…”

“…On the other hand, the detailed analysis of UV absorption lines (including the Lyman series lines of hydrogen and other low-ionization lines) of the z ∼ 0.3−0.4 leakers of Izotov and collaborators undertaken by Gazagnes et al (2018) and Chisholm et al (2018) revealed the presence of high-column-density gas that is optically thick to LyC radiation and co-exists with lines of sight, which allows the escape of UV ionizing and nonionizing radiation from these galaxies. This implies that the ISM of these galaxies cannot be described by simple, one-zone photoionization models, and that a more complex ISM structure should be accounted for to consistently describe these objects.…”