Detections of [C ii] 158 μm and [O iii] 88 μm in a Local Lyman Continuum Emitter, Mrk 54, and Its Implications to High-redshift ALMA Studies* ^* Based on observations done with FIFI-LS on SOFIA.

We present integral field, far-infrared (FIR) spectroscopy of Mrk 54, a local Lyman continuum emitter, obtained with FIFI-LS on the Stratospheric Observatory for Infrared Astronomy. This is only the second time, after Haro 11, that [C ii] 158 μm and [O iii] 88 μm spectroscopy of the known LCEs have been obtained. We find that Mrk 54 has a strong [C ii] emission that accounts for ∼1% of the total FIR luminosity, whereas it has only moderate [O iii] emission, resulting in the low [O iii]/[C ii] luminosity ratio of 0.22 ± 0.06. In order to investigate whether [O iii]/[C ii] is a useful tracer of f _esc (LyC escape fraction), we examine the correlations of [O iii]/[C ii] and (i) the optical line ratio of O₃₂ ≡ [O iii] 5007 Å/[O ii] 3727 Å, (ii) specific star formation rate, (iii) [O iii] 88 μm/[O i] 63 μm ratio, (iv) gas-phase metallicity, and (v) dust temperature based on a combined sample of Mrk 54 and the literature data from the Herschel Dwarf Galaxy Survey and the LITTLE THINGS Survey. We find that galaxies with high [O iii]/[C ii] luminosity ratios could be the result of high ionization (traced by O₃₂), bursty star formation, high ionized-to-neutral gas volume filling factors (traced by [O iii] 88 μm/[O i] 63 μm), and low gas-phase metallicities, which is in agreement with theoretical predictions. We present an empirical relation between the [O iii]/[C ii] ratio and f _esc based on the combination of the [O iii]/[C ii] and O₃₂ correlation, and the known relation between O₃₂ and f _esc. The relation implies that high-redshift galaxies with high [O iii]/[C ii] ratios revealed by the Atacama Large Millimeter/submillimeter Array may have f _esc ≳ 0.1, significantly contributing to the cosmic reionization.