Abstract
Context. The Small Magellanic Cloud (SMC) is an ideal laboratory for studying the properties of star-forming regions thanks to its low metallicity, which has an impact on the molecular gas abundance. However, a small number of molecular gas surveys of the entire galaxy have been carried out in the last few years, limiting the measurements of interstellar medium (ISM) properties in a homogeneous manner.
Aims. We present the CO(3-2) APEX survey at a 6 pc resolution of the bar of the SMC, observed with the SuperCAM receiver attached to the APEX telescope. This high-resolution survey has allowed us to study certain properties of the ISM and to identify CO clouds in the innermost parts of the H2 envelopes.
Methods. We adopted the CO analysis in the SMC bar comparing the CO(3–2) survey with that of the CO(2–1) of a similar resolution. We studied the CO(3–2)-to-CO(2–1) ratio (R32), which is very sensitive to the environment properties (e.g., star-forming regions). We analyzed the correlation of this ratio with observational quantities that trace the star formation such as the local CO emission, the Spitzer color [70/160], and the total IR surface brightness measured from the Spitzer and Herschel bands. For the identification of the CO(3–2) clouds, we used the CPROPS algorithm, which allowed us to measure the physical properties of the clouds. We analyzed the scaling relationships of such physical properties.
Results. We obtained R32 = 0.65 ± 0.02 for the SW bar and a slightly higher ratio, R32 = 0.7 ± 0.1, for N66 in the SMC. We found that R32 varies from region to region, depending on the star formation activity. In regions dominated by HII and photo-dissociated regions (e.g., N22, N66) R32 tends to be higher than the median values. Meanwhile, lower values were found toward quiescent clouds. We also found that R32 is correlated with the IR color [70/160] and the total IR surface brightness. This finding indicates that R32 increases with environmental properties, such as the dust temperature, total gas density, and radiation field. We identified 225 molecular clouds with sizes of R > 1.5 pc and signal-to-noise ratios (S/N) of >3, of which only 17 are well resolved CO(3–2) clouds with S/N ≳ 5. These 17 clouds follow consistent scaling relationships to the inner Milky Way clouds but with some departures. For instance, CO(3–2) tends to be less turbulent and less luminous than the inner Milky Way clouds of similar sizes. Finally, we estimated a median virial-based CO(3–2)-to-H2 conversion factor of 12.6−7+10 M⊙ (K km s−1 pc2)−1 for the total sample.