Determining dust temperatures and masses in the Herschel era: The importance of observations longward of 200 micron

Gordon, K. D., Galliano, F., Hony, S., Bernard, J.-P., Bolatto, A., Bot, C., Engelbracht, C., Hughes, A., Israel, F. P., Kemper, F., Kim, S., Li, A., Madden, S. C., Matsuura, M., Meixner, M., Misselt, K., Okumura, K., Panuzzo, P., Rubio, M., Reach, W. T., Roman-Duval, J., Sauvage, M., Skibba, R., & Tielens, A. G. G. M.
2010, Astronomy and Astrophysics, 518, L89


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. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

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