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.
Highlights
Objects that are extended on the SPIRE beam scale are not expected to be accurately identified with point-source extracting codes, and large galaxies may be misidentified as multiple point sources
Another important point considered here is that ultra-luminous infrared galaxies (ULIRGs) at higher redshifts express a wider variance in dust temperatures compared to their local analogues (e.g. Smith et al 2014; Symeonidis et al 2013)
While we created mock images assuming the colours of sources that have been drawn directly from the simulated catalogues, Asboth et al (2016) and Dowell et al (2014) omitted modelled red sources from their maps, injecting 500 μm-risers with SPIRE colours fixed to the median value measured in their raw maps
Summary
The abundance of dusty galaxies at high-redshifts (z > 4) constrains our theories about early galaxy formation, since it is generally stated that they are the progenitors of massive ellipticals seen in overdense regions of the local Universe (e.g. Eales et al 2017; Toft et al 2014; Simpson et al 2014; Casey et al 2014). To derive a larger number of potentially unlensed 500 μm-risers and analyse them in a more standardised manner, several works used map-search technique (Asboth et al 2016; Dowell et al 2014; Ivison et al 2016). These led to the discovery of the most distant dusty starburst galaxies known to date: SPT0311-58 at z = 6.902 (Strandet et al 2017), HFLS3 at z = 6.34 (Riechers et al 2013) and G09-83808 at z = 6.02 (Zavala et al 2018; Fudamoto et al 2017).
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