2018A&A...614A..33D

Context. Over the last decade a large number of dusty star-forming galaxies has been discovered up to redshift z = 2 - 3 and recent studies have attempted to push the highly confused Herschel SPIRE surveys beyond that distance. To search for z >= 4 galaxies they often consider the sources with fluxes rising from 250 µm to 500 µm (so-called "500 µm-risers"). Herschel surveys offer a unique opportunity to efficiently select a large number of these rare objects, and thus gain insight into the prodigious star-forming activity that takes place in the very distant Universe.
Aims. We aim to implement a novel method to obtain a statistical sample of 500 µm-risers and fully evaluate our selection inspecting different models of galaxy evolution.
Methods. We consider one of the largest and deepest Herschel surveys, the Herschel Virgo Cluster Survey. We develop a novel selection algorithm which links the source extraction and spectral energy distribution fitting. To fully quantify selection biases we make end-to-end simulations including clustering and lensing.
Results. We select 133 500 µm-risers over 55 deg², imposing the criteria: S₅₀₀ > S₃₅₀ > S₂₅₀, S₂₅₀ > 13.2 mJy and S₅₀₀ > 30 mJy. Differential number counts are in fairly good agreement with models, displaying a better match than other existing samples. The estimated fraction of strongly lensed sources is 24₊₆^–5% based on models.
Conclusions. We present the faintest sample of 500 µm-risers down to S₂₅₀ = 13.2 mJy. We show that noise and strong lensing have an important impact on measured counts and redshift distribution of selected sources. We estimate the flux-corrected star formation rate density at 4 < z < 5 with the 500 µm-risers and find it to be close to the total value measured in far-infrared. This indicates that colour selection is not a limiting effect to search for the most massive, dusty z > 4 sources.