Tracing the hidden molecular gas in the Small Magellanic Cloud with SOFIA and APEX

Abstract

In low-metallicity environments, the reduced dust abundance debilitates the shielding of molecular clouds against far-ultraviolet (FUV) radiation, enhancing CO photodissociation and producing extended envelopes of “CO-faint” or “CO-dark” molecular gas. The ̧ii 158 μm fine-structure line is one of the brightest coolants of the interstellar medium (ISM) and a key tracer of these CO-faint molecular phases. We aim to characterize the multiphase origin of the ̧ii line emission and quantify the fraction of molecular gas that remains undetected in CO across several regions of the Small Magellanic Cloud (SMC), the nearest low-metallicity (∼0.2 Z_⊙) galaxy that provides an ideal laboratory for testing photodissociation region (PDR) models under conditions that resemble those in early galaxies. We combined high spectral resolution SOFIA/GREAT and upGREAT ̧ii 158 μm observations with APEX CO(2–1) data and the ASKAP+Parkes H I 21 cm map to perform a spectral decomposition of the ̧ii emission into its atomic and molecular contributions. Using the linear decomposition approach, we derived the ̧ii fractions associated with H I and H_2, and converted the molecular component into column densities assuming typical PDR conditions constrained by previous SMC studies. We then compared the resulting molecular gas content with CO-based estimates and with the local star formation rate (SFR) surface density derived from Hα and L_ TIR tracers. The ̧ii line width in the analyzed SMC lines of sight consistently falls between the H I and CO line widths, indicating that the ̧ii emission arises from both molecular and atomic gas. On average, $( 78 ± 1)%$ of the ̧ii intensity originates from the molecular phase, while $( 18 ± 1)%$ is associated with atomic gas. The ̧ii-traced H_2 accounts for approximately $(77 ± 4)%$ of the total molecular gas column density, implying that the SMC’s molecular reservoir is dominated by CO-faint gas. This estimation yields a H_2-to-CO conversion factor on 4 pc scales X_ ̊m CO ≈ 8.9 20 cm -2 $(K km s^-1)^-1 in CO-emitting regions, which is 4.5 times larger than the canonical Galactic conversion factor (X_ ̊m CO,MW 20 cm^-2(K km s^-1)^-1). We find no significant correlation between the CO-dark fraction and either total column density or $Σ_ SFR , suggesting that the abundance of CO-faint gas is primarily governed by the efficiency of dust and gas shielding rather than by current star formation activity. Our results demonstrate that ̧ii is the dominant tracer of molecular gas in the SMC and they confirm the prevalence of an extended, stable CO-faint molecular phase in low-metallicity environments. This has important implications for interpreting ̧ii observations in unresolved galaxies, both locally and at a high redshift, where CO may not fully trace the molecular gas reservoir.