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Dust/gas correlations from Herschel Observations

Spitzer and IRAS observations of the LMC suggest an excess of FIR emission with respect to the gas surface density traced by 12CO and HI 21 cm emission lines. This "FIR excess" is noticeable near molecular clouds in the LMC, and has usually been interpreted as the presence of a self-shielded H2 component not traced by CO molecular clouds' envelopes. Based on Herschel observations, we examine the correlation between gas and dust at higher resolution than previously achieved. We consider three additional 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. We examine the structure of NT80 and NT71, two molecular clouds detected in the NANTEN 12CO survey of the LMC. Dust surface density maps are derived from the HERITAGE data. The gas phase is traced by MAGMA 12CO and ATCA HI 21 cm observations of the LMC. 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. 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 (Av = 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 Av = 1-2. We conclude that H2 envelopes not traced by CO and X factor variations close to the CO boundary are more likely to cause these deviations between FIR emission and gas surface density than gas-to-dust ratio or emissivity variations.