Determining dust temperatures and masses in the Herschel era: the
  importance of observations longward of 200 micron
  The properties of the dust grains (e.g., temperature and mass) can be derived
from fitting far-IR SEDs (>100 micron). Only with SPIRE on Herschel has it been
possible to get high spatial resolution at 200 to 500 micron that is beyond the
peak (~160 micron) of dust emission in most galaxies. We investigate the
differences in the fitted dust temperatures and masses determined using only
<200 micron data and then also including >200 micron data (new SPIRE
observations) to determine how important having >200 micron data is for
deriving these dust properties. We fit the 100 to 350 micron observations of
the Large Magellanic Cloud (LMC) point-by-point with a model that consists of a
single temperature and fixed emissivity law. The data used are existing
observations at 100 and 160 micron (from IRAS and Spitzer) and new SPIRE
observations of 1/4 of the LMC observed for the HERITAGE Key Project as part of
the Herschel Science Demonstration phase. The dust temperatures and masses
computed using only 100 and 160 micron data can differ by up to 10% and 36%,
respectively, from those that also include the SPIRE 250 & 350 micron data. We
find that an emissivity law proportional to lambda^-1.5 minimizes the 100-350
micron fractional residuals. We find that the emission at 500 micron is ~10%
higher than expected from extrapolating the fits made at shorter wavelengths.
We find the fractional 500 micron excess is weakly anti-correlated with MIPS 24
micron flux and the total gas surface density. This argues against a flux
calibration error as the origin of the 500 micron excess. Our results do not
allow us to distinguish between a systematic variation in the wavelength
dependent emissivity law or a population of very cold dust only detectable at
lambda > 500 micron for the origin of the 500 micron excess.
