A highly obscured and strongly clustered galaxy population discovered with the Spitzer Space Telescope

The ~800 optically unseen (R > 25.5) 24-μm selected sources in the complete Spitzer First Look Survey sample with F24μm >= 0.35mJy are found to be very strongly clustered. If, as indicated by several lines of circumstantial evidence, they are ultraluminous far-infrared galaxies at z ~= 1.6-2.7, the amplitude of their spatial correlation function is very high. The associated comoving clustering length is estimated to be r0 = 14.0+2.1-2.4 Mpc, value which puts these sources amongst the most strongly clustered populations of our known Universe. Their 8-24μm colours suggest that the active galactic nucleus contribution dominates above F24μm ~= 0.8 mJy, consistent with earlier analyses. The properties of these objects (number counts, redshift distribution, clustering amplitude) are fully consistent with those of proto-spheroidal galaxies in the process of forming most of their stars and of growing their active nucleus, as described by the Granato et al. model. In particular, the inferred space density of such galaxies at z ~= 2 is much higher than what is expected from most semi-analytic models. Matches of the observed projected correlation function w(θ) with models derived within the so-called halo occupation scenario show that these sources have to be hosted by haloes more massive than ~=1013.4Msolar. This value is significantly higher than that for the typical galactic haloes hosting massive elliptical galaxies, suggesting a duration of the starburst phase of massive high-redshift dusty galaxies of TB ~ 0.5 Gyr.