Determining dust temperatures and masses in the Herschel era: The importance of observations longward of 200 micron
Bernard, J. -P.; Reach, W. T.; Okumura, K.; Kim, S.; Madden, S. C.; Galliano, F.; Hony, S.; Sauvage, M.; Panuzzo, P.; Roman-Duval, J.; Gordon, K. D.; Tielens, A. G. G. M.; Meixner, M.; Misselt, K.; Rubio, M.; Israel, F. P.; Matsuura, M.; Engelbracht, C.; Bot, C.; Li, A.; Skibba, R.; Kemper, F.; Bolatto, A.; Hughes, A.
United States, France, Australia, Netherlands, United Kingdom, South Korea, Chile
Abstract
Context. The properties of the dust grains (e.g., temperature and mass) can be derived from fitting far-IR SEDs (≥100 μm). Only with SPIRE on Herschel has it been possible to get high spatial resolution at 200 to 500 μm that is beyond the peak (~160 μm) of dust emission in most galaxies.
Aims: We investigate the differences in the fitted dust temperatures and masses determined using only <200 μm data and then also including >200 μm data (new SPIRE observations) to determine how important having >200 μm data is for deriving these dust properties.
Methods: We fit the 100 to 350 μm 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 μm (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.
Results: The dust temperatures and masses computed using only 100 and 160 μm data can differ by up to 10% and 36%, respectively, from those that also include the SPIRE 250 & 350 μm data. We find that an emissivity law proportional to λ-1.5 minimizes the 100-350 μm fractional residuals. We find that the emission at 500 μm is ~10% higher than expected from extrapolating the fits made at shorter wavelengths. We find the fractional 500 μm excess is weakly anti-correlated with MIPS 24 μm flux and the total gas surface density. This argues against a flux calibration error as the origin of the 500 μm 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 λ ≥ 500 μm for the origin of the 500 μm excess.