2024MNRAS.527.9853K

The physical conditions giving rise to high escape fractions of ionizing radiation (LyC f_esc) in star-forming galaxies - most likely protagonists of cosmic reionization - are not yet fully understood. Using the VLT/MUSE observations of ∼1400 Ly α emitters at 2.9 < z < 6.7, we compare stacked rest-frame UV spectra of candidates for LyC leakers and non-leakers selected based on their Ly α profiles. We find that the stacks of potential LyC leakers, i.e. galaxies with narrow, symmetric Ly α profiles with small peak separation, generally show (i) strong nebular O III]λ1666, [Si III]λ1883, and [C III]λ1907 +C III]λ1909 emission, indicating a high-ionization state of the interstellar medium (ISM); (ii) high equivalent widths of He IIλ1640 (∼1 - 3 Å), suggesting the presence of hard ionizing radiation fields; (iii) Si II*λ1533 emission, revealing substantial amounts of neutral hydrogen off the line of sight; (iv) high C IVλλ1548,1550 to [C III]λ1907 +C III]λ1909 ratios (C IV/C III] ≳0.75) , signalling the presence of low column density channels in the ISM. In contrast, the stacks with broad, asymmetric Ly α profiles with large peak separation show weak nebular emission lines, low He IIλ1640 equivalent widths (≲1 Å), and low C IV/C III] (≲0.25), implying low-ionization states and high-neutral hydrogen column densities. Our results suggest that C IV/C III] might be sensitive to the physical conditions that govern LyC photon escape, providing a promising tool for identification of ionizing sources among star-forming galaxies in the epoch of reionization.