Dust/gas correlations from Herschel observations

Context. Previous Spitzer and IRAS observations of the LMC suggest an excess of FIR emission with respect to the gas surface density traced by (CO)-C-12 rotational emission lines and HI 21 cm emission. This so-called "FIR excess" is especially noticeable near molecular clouds in the LMC, and has usually been interpreted as indicating the presence of a self-shielded H-2 component not traced by CO in the envelopes of molecular clouds. Aims. Based on Herschel HERITAGE observations taken as part of the science demonstration phase, we examine the correlation between gas and dust surface densities at higher resolution than previously achieved. We consider three additional possible causes for the FIR excess: X factor, FIR dust emissivity, and gas-to-dust ratio variations between the diffuse and dense phases of the ISM. Methods. We examine the structure of NT80 and NT71, two molecular clouds detected in the NANTEN (CO)-C-12 survey of the LMC. Dust surface density maps were derived from the HERITAGE data. The gas phase is traced by MAGMA (CO)-C-12 and ATCA+Parkes HI 21 cm observations of the LMC. These data provide unprecedented resolution (1') to examine the structure of molecular clouds. The dust emissivity, gas-to-dust ratio, and X factor required to match the dust and gas surface densities are derived, and their correlations with the dust surface density are examined. Results. We show that the dust surface density is spatially correlated with the atomic and molecular gas phases. The dust temperature is consistently lower in the dense phase of the ISM than in the diffuse phase. We confirm variations in the ratio of FIR emission to gas surface density derived from HI and CO observations. There is an excess of FIR emission, spatially correlated with regions of intermediate Hi and dust surface densities (A(V) = 1-2), and little or no CO. While there is no significant trend in the dust emissivity or gas-to-dust ratio with dust surface density, the X factor is enhanced at A(V) = 1-2. We conclude that H-2 envelopes not traced by CO and X factor variations close to the CO boundary may be more likely to cause these deviations between FIR emission and gas surface density than gas-to-dust ratio or emissivity variations.