The PEP survey: clustering of infrared-selected galaxies and structure formation at z ∼ 2 in GOODS-South

This paper presents the first direct estimate of the 3D clustering properties of far-infrared sources up to z similar to 3. This has been possible thanks to the PACS Evolutionary Probe (PEP) survey of the GOODS-South field performed with the PACS instrument on board the Herschel satellite. 550 and 502 sources were detected respectively in the 100- and 160-mu m channels down to fluxes S-100 mu m = 2 mJy and S-160 mu m = 5 mJy, cuts that ensure >80 per cent completeness of the two catalogues. More than 65 per cent of these sources have an (either photometric or spectroscopic) redshift determination from the MUSIC catalogue; this percentage rises to similar to 95 per cent in the inner portion of GOODS-South which is covered by data at other wavelengths. An analysis of the deprojected two-point correlation function w(theta) over the whole redshift range spanned by the data reports for the (comoving) correlation length, r(0) similar to 6.3 and similar to 6.7Mpc, respectively at 100 and 160 mu m, corresponding to dark matter halo masses M greater than or similar to 10(12.4) M-circle dot, in an excellent agreement with previous estimates obtained for mid-IR selected sources in the same field. Objects at z similar to 2 instead seem to be more strongly clustered, with r(0) similar to 19 and similar to 17 Mpc in the two considered PACS channels. This dramatic increase of the correlation length between z similar to 1 and similar to 2 is connected with the presence, more visible at 100 mu m than in the other band, of a wide (at least 4 Mpc across in projection), M greater than or similar to 10(14) M-circle dot, filamentary structure which includes more than 50 per cent of the sources detected at z similar to 2. An investigation of the properties of such sources indicates the possibility of a boosted star-forming activity in those which reside within the overdense environment with respect to more isolated galaxies found in the same redshift range. If confirmed by larger data sets, this result can be explained as due to the combined effect of large reservoirs of gas available at high redshifts in deep potential wells such as those associated with large overdensities and the enhanced rate of encounters between sources favoured by their relative proximity. Lastly, we also present our results on the evolution of the relationship between luminous and dark matter in star-forming galaxies between z similar to 1 and similar to 2. We find that the increase in (average) stellar mass in galaxies < M*> between z similar to 1 and similar to 2 is about a factor of 10 lower than that of the dark matter haloes hosting such objects (< M*>(z similar to 1)/< M*>(z similar to 2) similar to 4 x 10(-1) versus M-halo(z similar to 1)/M-halo(z similar to 2) similar to 4 x 10(-2)). When compared with recent results taken from the literature, our findings agree with the evolutionary picture of downsizing whereby massive galaxies at z similar to 2 were more actively forming stars than their z similar to 1 counterparts, while at the same time they contained a lower fraction of their mass in the form of luminous matter.