The Herschel-PEP survey: evidence for downsizing in the hosts of dusty star-forming systems

By making use of Herschel PACS Evolutionary Probe observations of the COSMOS and Extended Groth Strip fields, we have estimated the dependence of the clustering properties of FIR-selected sources on their 100 μm fluxes. Our analysis shows a tendency for the clustering strength to decrease with limiting fluxes. By assuming a power-law slope with >; 1.8 for the two-point correlation function ξ(r) > (r/r0)-γ, we find: r0(S100μm > 8 mJy) >= 4.3+0.7 -0.7 Mpc and r0(S100μm > 5mJy) >=5.8+1.8 -2.0. These values convert into minimum halo masses Mmin ∼ 1011.6 Mȯ for sources brighter than 8 mJy and Mmin ∼ 1012.4 Mȯ for fainter, S100μm > 5 mJy galaxies. We show such an increase of the clustering strength to be due to an intervening population of z ∼ 2 sources, which are very strongly clustered and whose relative contribution, equal to about 10 per cent of the total counts at S100μm > 2 mJy, rapidly decreases for brighter flux cuts. By removing such a contribution, we find that z ≲ 1 far-infrared (FIR) galaxies have approximately the same clustering properties, irrespective of their flux level. The above results were then used to investigate the intrinsic dependence on cosmic epoch of the clustering strength of dusty star-forming galaxies between z∼0 and z∼2.5. Thiswas done by comparing our data set with IRAS in the local universe and with sources selected at 160 μm in the GOODS-South at z ≃ 2. In order to remove any bias in the selection process, the adopted sample only includes galaxies observed at the same rest-frame wavelength, γ ∼ 60 μm, which have comparable luminosities and therefore star formation rates (SFR ≳ 100 MȮ yr-1). Our analysis shows that the same amount of (intense) star-forming activity takes place in extremely different environments at the different cosmological epochs. For z ≲ 1, the hosts of such star-forming systems are small, Mmin ∼ 1011 Mȯ, isolated galaxies. High (z ∼ 2) redshiftstar formation instead seems to uniquely take place in extremely massive/cluster-like haloes, Mmin ∼ 1013.5 Mȯ, which are associated with the highest peaks of the density fluctuation field at those epochs. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.