  Spectral Energy Distribution (SED) fitting in the far-infrared (FIR) is
greatly limited by a dearth of data and an excess of free parameters - from
galaxies' dust composition, temperature, mass, orientation, opacity, to heating
from AGN. This paper presents a simple FIR SED fitting technique joining a
modified, single dust temperature greybody, representing the reprocessed
starburst emission in the whole galaxy, to a mid-infrared powerlaw, which
approximates hot-dust emission from AGN heating or clumpy, hot starbursting
regions. This FIR SED can be used to measure infrared luminosities, dust
temperatures and dust masses for both local and high-z galaxies with 3 to 10+
FIR photometric measurements.
  This fitting method is compared to infrared template SEDs in the literature
using photometric data on 65 local luminous and ultraluminous infrared
galaxies, (U)LIRGs. Despite relying only on 2-4 free parameters, the coupled
greybody/powerlaw SED fitting described here produces better fits to
photometric measurements than best-fit literature template SEDs (with residuals
a factor of ~2 lower). A mean emissivity index of beta=1.60+-0.38 and
mid-infrared powerlaw slope of alpha=2.0+-0.5 is measured; the former agrees
with the widely presumed emissivity index of beta=1.5 and the latter is
indicative of an optically-thin dust medium with a shallow radial density
profile, ~r^-0.5. Adopting characteristic dust temperature as the inverse
wavelength where the SED peaks, dust temperatures ~25-45K are measured for
local (U)LIRGs, ~5-15K colder than previous estimates using only simple
greybodies. This comparative study highlights the impact of SED fitting
assumptions on the measurement of physical properties such as infrared
luminosity (and thereby infrared-based star formation rate), dust temperature
and dust mass, for both local and high-redshift galaxies. [abridged]
