The Resolved Distributions of Dust Mass and Temperature in Local Group Galaxies

Abstract: We utilize archival far-infrared maps from the Herschel Space Observatory in four Local Group galaxies (Small and Large Magellanic Clouds, M31, and M33). We model their Spectral Energy Distribution (SED) from 100 to 500 $\mu$m using a single-temperature modified blackbody emission with a fixed emissivity index of $\beta = 1.8$. From the best-fit model, we derive the dust temperature, $T_{\rm d}$, and the dust mass surface density, $\Sigma_{\rm d}$, at 13 parsec resolution for SMC and LMC, and at 167 parsec resolution for all targets. This measurement allows us to build the distribution of dust mass and luminosity as functions of dust temperature and mass surface density. We compare those distribution functions among galaxies and between regions in a galaxy. We find that LMC has the highest mass-weighted average $T_{\rm d}$, while M31 and M33 have the lowest mass-weighted average $T_{\rm d}$. Within a galaxy, star forming regions have higher $T_{\rm d}$ and $\Sigma_{\rm d}$ relative to the overall distribution function, due to more intense heating by young stars and higher gas mass surface density. When we degrade the resolutions to mimic distant galaxies, the mass-weighted mean temperature gets warmer as the resolution gets coarser, meaning the temperature derived from unresolved observation is systematically higher than that in highly resolved observation. As an implication, the total dust mass is lower (underestimated) in coarser resolutions. This resolution-dependent effect is more prominent in clumpy star-forming galaxies (SMC, LMC, and M33), and less prominent in more quiescent massive spiral (M31).