2024A&A...687A..73L

The geometry of the neutral gas in and around galaxies is a key regulator of the escape of ionizing photons. We present the first statistical study aimed at linking the neutral and ionized gas distributions to the Lyman continuum (LyC) escape fraction ($ f_{\mathrm{esc}}^{\mathrm{LyC}} $) in a sample of 22 confirmed LyC leakers and non-leakers at z ≈ 0.35 using the Keck Cosmic Web Imager (Keck/KCWI) and the Low Resolution Spectrograph 2 (HET/LRS2). Our integral field unit data enable the detection of neutral and low-ionization gas, as traced by Mg II, and ionized gas, as traced by [O II], extending beyond the stellar continuum for seven and ten objects, respectively. All but one object with extended Mg II emission also show extended [O II] emission; in this case, Mg II emission is always more extended than [O II] by a factor 1.2 on average. Most of the galaxies with extended emission are non or weak LyC leakers ($ f_{\mathrm{esc}}^{\mathrm{LyC}} < 5\% $), but we find a large diversity of neutral and low-ionization gas configurations around these weakly LyC-emitting galaxies. Conversely, the strongest leakers ($ f_{\mathrm{esc}}^{\mathrm{LyC}} > 5\% $) appear uniformly compact in both Mg II and [O II] with exponential scale lengths ≲1 kpc. Most are unresolved at the resolution of our data. We also find a trend between $ f_{\mathrm{esc}}^{\mathrm{LyC}} $ and the spatial offsets of the nebular gas and the stellar continuum emission. Moreover, we find significant anticorrelations between the spatial extent of the neutral and/or low-ionization gas and the [O III]/[O II] ratio, and Hβ equivalent width, as well as positive correlations with metallicity and UV size, suggesting that galaxies with more compact neutral and/or low-ionization gas sizes are more highly ionized. The observations suggest that strong LyC emitters do not have extended neutral and/or low-ionization gas halos and ionizing photons may be emitted in many directions. Combined with high ionization diagnostics, we propose that the Mg II, and potentially [O II], spatial compactness are indirect indicators of LyC emitting galaxies at high redshift.