De-blending deep Herschel surveys: A multi-wavelength approach

Aims. Cosmological surveys in the far-infrared are known to suffer from confusion. The Bayesian de-blending tool, XID+, currently provides one of the best ways to de-confuse deep Herschel SPIRE images, using a flat flux density prior. This work is to demonstrate that existing multi-wavelength data sets can be exploited to improve XID+ by providing an informed prior, resulting in more accurate and precise extracted flux densities. Methods. Photometric data for galaxies in the COSMOS field were used to constrain spectral energy distributions (SEDs) using the fitting tool CIGALE. These SEDs were used to create Gaussian prior estimates in the SPIRE bands for XID+. The multi-wavelength photometry and the extracted SPIRE flux densities were run through CIGALE again to allow us to compare the performance of the two priors. Inferred ALMA flux densities (F-ALMA(infer)), at 870 mu m and 1250 mu m, from the best fitting SEDs from the second CIGALE run were compared with measured ALMA flux densities (F-ALMA(meas)) as an independent performance validation. Similar validations were conducted with the SED modelling and fitting tool MAGPHYS and modified black-body functions to test for model dependency. Results. We demonstrate a clear improvement in agreement between the flux densities extracted with XID+ and existing data at other wavelengths when using the new informed Gaussian prior over the original uninformed prior. The residuals between F-ALMA(meas) and F-ALMA(infer) were calculated. For the Gaussian priors these residuals, expressed as a multiple of the ALMA error (sigma), have a smaller standard deviation, 7.95 sigma for the Gaussian prior compared to 12.21 sigma for the flat prior; reduced mean, 1.83 sigma compared to 3.44 sigma; and have reduced skew to positive values, 7.97 compared to 11.50. These results were determined to not be significantly model dependent. This results in statistically more reliable SPIRE flux densities and hence statistically more reliable infrared luminosity estimates.