2008MNRAS.383.1131M

This paper investigates the clustering properties of a complete sample of 1041 24-µm-selected sources brighter than F_24µm= 400µJy in the overlapping region between the Spitzer Wide-Area Infrared Extragalactic (SWIRE) and UKIRT Infrared Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS) surveys. With the help of photometric redshift determinations we have concentrated on the two interval ranges z = [0.6-1.2] (low-z sample) and z ≥ 1.6 (high-z sample) as it is in these regions were we expect the mid-infrared (IR) population to be dominated by intense dust-enshrouded activity such as star formation and black hole accretion. Investigations of the angular correlation function produce an amplitude A ∼ 0.010 for the high-z sample and A ∼ 0.0055 for the low-z one. The corresponding correlation lengths are r₀= 15.9^+2.9_–3.4and 8.5^+1.5_–1.8Mpc, showing that the high-z population is more strongly clustered. Comparisons with physical models for the formation and evolution of large-scale structure reveal that the high-z sources are exclusively associated with very massive (M ≳ 10¹³M_☉) haloes, comparable to those which locally host groups-to-clusters of galaxies and are very common within such (rare) structures. Conversely, lower z galaxies are found to reside in smaller haloes (M_min∼ 10¹²M_☉) and to be very rare in such systems. On the other hand, mid-IR photometry shows that the low-z and high-z samples include similar objects and probe a similar mixture of active galactic nucleus (AGN) and star-forming galaxies. While recent studies have determined a strong evolution of the 24-µm luminosity function between z ∼ 2 and 0, they cannot provide information on the physical nature of such an evolution. Our clustering results instead indicate that this is due to the presence of different populations of objects inhabiting different structures, as active systems at z ≲ 1.5 are found to be exclusively associated with low-mass galaxies, while very massive sources appear to have concluded their active phase before this epoch. Finally, we note that the small-scale clustering data seem to require steep (ρ ∝ r^–3) profiles for the distribution of galaxies within their haloes. This is suggestive of close encounters and/or mergers which could strongly favour both AGN and star formation activity.