The Deep SPIRE HerMES Survey: spectral energy distributions and their astrophysical indications at high redshift

We present the cross-identification and source photometry techniques used to process Herschel SPIRE imaging taken as part of the Herschel Multi-Tiered Extragalactic Survey (HerMES). Cross-identifications are performed in map-space so as to minimise source blending effects. We make use of a combination of linear inversion and model selection techniques to produce reliable cross-identification catalogues based on Spitzer MIPS 24 μm source positions. Testing on simulations and real Herschel observations show that this approach gives robust results for even the faintest sources (S250~ 10 mJy). We apply our new technique to HerMES SPIRE observations taken as part of the science demostration phase of Herschel. For our real SPIRE observations we show that, for bright unconfused sources, our flux density estimates are in good agreement with those produced via more traditional point source detection methods (SussEx- tractor; Savage & Oliver et al. 2006) by Smith et al 2010. When compared to the measured number density of sources in the SPIRE bands, we show that our method allows the recovery of a larger fraction of faint sources than these traditional methods. However this completeness is heavily dependant on the relative depth of the existing 24 μm catalogues and SPIRE imaging. Using our deepest multi-wavelength dataset in GOODS-N, we estimate that the use of shallow 24 μm in our other fields introduces an incompleteness at faint levels of between 20–40 per cent at 250 μm.