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

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 μ m and [O iii ] 88 μ m 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 O _32 ≡ [O iii ] 5007 Å/[O ii ] 3727 Å, (ii) specific star formation rate, (iii) [O iii ] 88 μ m/[O i ] 63 μ m 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 O _32 ), bursty star formation, high ionized-to-neutral gas volume filling factors (traced by [O iii ] 88 μ m/[O i ] 63 μ m), 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 O _32 correlation, and the known relation between O _32 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.