  We examine the infrared properties of 43 high redshift (0.1<z<1.2),
infrared-luminous galaxies in the Extended Groth Strip (EGS), selected by a
deep 70 micron survey with the Multiband Imaging Photometer on Spitzer (MIPS).
In addition and with reference to starburst-type Spectral Energy Distributions
(SEDs), we derive a set of equations for estimating the total infrared
luminosity (L_IR) in the range 8-1000 microns using photometry from at least
one MIPS band. 42 out of 43 of our sources' optical/infrared SEDs
(lambda_observed < 160 microns) are starburst-type, with only one object
displaying a prominent power-law near-infrared continuum. For a quantitative
analysis, models of radiation transfer in dusty media are fit onto the infrared
photometry, revealing that the majority of galaxies are represented by high
extinction, A_v>35 and for a large fraction (~50 per cent) the SED turns over
into the Rayleigh-Jeans regime at wavelengths longward of 90 microns. For
comparison, we also fit semi-empirical templates based on local galaxy data,
however, these underestimate the far-infrared SED shape by a factor of at least
2 and in extreme cases up to 10 for the majority (~70 per cent) of the sources.
Further investigation of SED characteristics reveals that the mid-infrared
(70/24 microns) continuum slope is decoupled from various galaxy properties
such as the total infrared luminosity and far-infrared peak, quantified by the
L_160/L_70 ratio. In view of these results, we propose that these high-redshift
galaxies have different properties to their local counterparts, in the sense
that large amounts of dust cause heavy obscuration and are responsible for an
additional cold emissive component, appearing as a far-infrared excess in their
SEDs.

Astrophysics