2018ApJ...853..111J

The Small Magellanic Cloud (SMC) provides the only laboratory to study the structure of molecular gas at high resolution and low metallicity. We present results from the Herschel Spectroscopic Survey of the SMC (HS³), which mapped the key far-IR cooling lines [C II], [O I], [N II], and [O III] in five star-forming regions, and new ALMA 7 m array maps of ¹²CO and ¹³CO(2-1) with coverage overlapping four of the five HS³ regions. We detect [C II] and [O I] throughout all of the regions mapped. The data allow us to compare the structure of the molecular clouds and surrounding photodissociation regions using ¹³CO, ¹²CO, [C II], and [O I] emission at 10^′′ ( 3 pc) scales. We estimate A_V using far-IR thermal continuum emission from dust and find that the CO/[C II] ratios reach the Milky Way value at high A_V in the centers of the clouds and fall to ∼1/5–1/10×the Milky Way value in the outskirts, indicating the presence of translucent molecular gas not traced by bright ¹²CO emission. We estimate the amount of molecular gas traced by bright [C II] emission at low A_V and bright ¹²CO emission at high A_V. We find that most of the molecular gas is at low A_V and traced by bright [C II] emission, but that faint ¹²CO emission appears to extend to where we estimate that the H₂-to-H I transition occurs. By converting our H₂ gas estimates to a CO-to-H₂ conversion factor (X_CO), we show that X_CO is primarily a function of A_V, consistent with simulations and models of low-metallicity molecular clouds.