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Towards a census of high-redshift dusty galaxies with Herschel
A selection of “500 μm-risers”

D. Donevski, V. Buat, F. Boone, C. Pappalardo, M. Bethermin, C. Schreiber, F. Mazyed, J. Alvarez-Marquez, S. Duivenvoorden

Abstract
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.

Astronomy and Astrophysics
Volume 614, Article Number A33, Number of pages 26
2018 June

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia