Tracing the Hidden Molecular Gas in the Small Magellanic Cloud with SOFIA and APEX

In low-metallicity environments, reduced dust abundance weakens molecular cloud shielding against far-ultraviolet radiation, enhancing CO photodissociation and producing extended CO-faint or CO-dark molecular gas. The [CII] 158 μ m line is a key tracer of these CO-faint phases. We characterize the multiphase origin of [CII] emission and quantify the CO-faint molecular gas fraction across several regions of the Small Magellanic Cloud (SMC), the nearest low-metallicity (about 0.2 Z) galaxy. We combine high spectral resolution SOFIA/GREAT and upGREAT [CII], APEX CO(2-1), and ASKAP+Parkes H I 21 cm data to decompose [CII] into its atomic and molecular contributions. On average, (78 plus or minus 1)% of [CII] intensity originates from molecular gas and (18 plus or minus 1)% from atomic gas. The [CII]-traced H 2 accounts for (77 plus or minus4)% of the total molecular gas column density, indicating that the SMC molecular reservoir is dominated by CO-faint gas. We derive a H 2-to-CO conversion factor X CO about 8.9 times10 20 cm-2 (K km s-1)-1 on 4 pc scales in CO-emitting regions, 4.5 times the canonical Galactic value. We find no significant correlation between the CO-dark fraction and either total column density or Σ SFR, suggesting that the CO-faint fraction is primarily governed by dust and gas shielding rather than current star formation activity. Our results demonstrate that [CII] is the dominant tracer of molecular gas in the SMC and confirm the prevalence of an extended, stable CO-faint molecular phase in low-metallicity environments.