We have studied in detail a sample of 967 SPIRE sources with 5σ
detections at 350 and 500 μm and associations with Spitzer-SWIRE 24
μm galaxies in the HerMES-Lockman survey area, fitting their mid- and
far-infrared, and submillimetre, spectral energy distributions (SEDs) in
an automatic search with a set of six infrared templates. For almost 300
galaxies, we have modelled their SEDs individually to ensure the
physicality of the fits. We confirm the need for the new cool and cold
cirrus templates, and also of the young starburst template, introduced
in earlier work. We also identify 109 lensing candidates via their
anomalous SEDs and provide a set of colour-redshift constraints which
allow lensing candidates to be identified from combined Herschel and
Spitzer data. The picture that emerges of the submillimetre galaxy
population is complex, comprising ultraluminous and hyperluminous
starbursts, lower luminosity galaxies dominated by interstellar dust
emission, lensed galaxies and galaxies with surprisingly cold (10-13 K)
dust. 11 per cent of 500 μm selected sources are lensing candidates.
70 per cent of the unlensed sources are ultraluminous infrared galaxies
and 26 per cent are hyperluminous. 34 per cent are dominated by
optically thin interstellar dust (`cirrus') emission, but most of these
are due to cooler dust than is characteristic of our Galaxy. At the
highest infrared luminosities we see SEDs dominated by M82, Arp 220 and
young starburst types, in roughly equal proportions.