Towards a census of high-redshift dusty galaxies with Herschel A selection of «500 mu m-risers»

D. Donevski, V. Buat, F. Boone, C. Pappalardo, M. Bethermin, C. Schreiber, F. Mazyed, J. Álvarez-Márquez, S. Duivenvoorden. 2018. Towards a census of high-redshift dusty galaxies with Herschel A selection of «500 mu m-risers». Astronomy and Astrophysics 614, DOI: 10.1051/0004-6361/201731888

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 mu m to 500 mu m (so-called «500 mu m-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 mu m-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 mu m-risers over 55 deg(2), imposing the criteria: S (500) > S (350) > S (250), S (250) > 13 : 2 mJy and S 500 > 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(-5)(+65) % based on models.

Conclusions. We present the faintest sample of 500 mu m-risers down to S (250) = 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 mu m-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.

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