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

Abstract

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 ($\sim 0.2 Z_\odot$) 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, $(78\pm 1)\%$ of [CII] intensity originates from molecular gas and $(18\pm 1)\%$ from atomic gas. The [CII]-traced H$_2$ accounts for $(77\pm4)\%$ 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 $X_{CO}\approx 8.9\times10^{20}$ 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.