Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. I. Dust Properties and Insights into the Origin of the Submm Excess Emission

TL;DR

This study analyzes dust properties in the Magellanic Clouds using Herschel data, exploring models to explain the submm excess and suggesting emissivity variations as the likely cause.

Contribution

It compares simple dust emission models to Herschel observations, identifying the broken power-law emissivity model as most effective in explaining the submm excess.

Findings

BEMBB model yields lowest residuals and explains submm excess.

Submm excess correlates with other dust properties.

Emissivity variations are favored over colder dust populations.

Abstract

The dust properties in the Large and Small Magellanic Clouds are studied using the HERITAGE Herschel Key Project photometric data in five bands from 100 to 500 micron. Three simple models of dust emission were fit to the observations: a single temperature blackbody modified by a power- law emissivity (SMBB), a single temperature blackbody modified by a broken power-law emissivity (BEMBB), and two blackbodies with different temperatures, both modified by the same power-law emissivity (TTMBB). Using these models we investigate the origin of the submm excess; defined as the submillimeter (submm) emission above that expected from SMBB models fit to observations < 200 micron. We find that the BEMBB model produces the lowest fit residuals with pixel-averaged 500 micron submm excesses of 27% and 43% for the LMC and SMC, respectively. Adopting gas masses from previous works, the gas-to-dust ratios calculated from our the fitting results shows that the TTMBB fits require significantly more dust than are available even if all the metals present in the interstellar medium (ISM) were condensed into dust. This indicates that the submm excess is more likely to be due to emissivity variations than a second population of colder dust. We derive integrated dust masses of (7.3 +/- 1.7) x 10^5 and (8.3 +/- 2.1) times 10^4 M(sun) for the LMC and SMC, respectively. We find significant correlations between the submm excess and other dust properties; further work is needed to determine the relative contributions of fitting noise and ISM physics to the correlations.