We use galaxies from the Herschel -ATLAS survey , and a suite of ancillary simulations based on an isothermal dust model , to study our ability to determine the effective dust temperature , luminosity and emissivity index of 250 \mu m selected galaxies in the local Universe ( z < 0.5 ) .
As well as simple far-infrared SED fitting of individual galaxies based on \chi ^ { 2 } minimisation , we attempt to derive the best global isothermal properties of 13,826 galaxies with reliable optical counterparts and spectroscopic redshifts .
Using our simulations , we highlight the fact that applying traditional SED fitting techniques to noisy observational data in the Herschel Space Observatory bands introduces artificial anti-correlation between derived values of dust temperature and emissivity index .
This is true even for galaxies with the most robust statistical detections in our sample , making the results of such fitting difficult to interpret .
We apply a method to determine the best-fit global values of isothermal effective temperature and emissivity index for z < 0.5 galaxies in H -ATLAS , deriving T _ { \mathrm { eff } } = 22.3 \pm 0.1 K and \beta = 1.98 \pm 0.02 ( or T _ { \mathrm { eff } } = 23.5 \pm 0.1 K and \beta = 1.82 \pm 0.02 if we attempt to correct for bias by assuming that T _ { \mathrm { eff } } and \beta _ { \mathrm { eff } } are independent and normally distributed ) .
We use our technique to test for an evolving emissivity index , finding only weak evidence .
The median dust luminosity of our sample is \log _ { 10 } ( L _ { \mathrm { dust } } / L _ { \odot } ) = 10.72 \pm 0.05 , which ( unlike T _ { \mathrm { eff } } ) shows little dependence on the choice of \beta used in our analysis , including whether it is variable or fixed .
In addition , we use a further suite of simulations based on a fixed emissivity index isothermal model to emphasize the importance of the H -ATLAS PACS data for deriving dust temperatures at these redshifts , even though they are considerably less sensitive than the SPIRE data .
Finally , we show that the majority of galaxies detected by H -ATLAS are normal star-forming galaxies , though with a substantial minority ( \sim 31 per cent ) falling in the Luminous Infrared Galaxy category .