2007A&A...475..801R

Multi-wavelength galaxy number counts provide clues to the nature of galaxy evolution. The interpretation per galaxy type of the mid-IR faint counts obtained with ISO and Spitzer, consistent with the analysis of deep UV-optical-near IR galaxy counts, provide new constraints on the dust and stellar emission. Discovering the nature of new populations, such as high redshift ultra-luminous (≥10¹²L_☉) infrared galaxies (ULIRGs), is also crucial for understanding galaxy evolution at high redshifts. We first present the faint galaxy counts at 12µm from the catalogue of the ISO-ESO-Sculptor Survey (ISO-ESS) published in a companion article (Seymour et al., 2007A&A...475..791S). They go down to 0.31mJy after corrections for incompleteness. We verify the consistency with the existing ISO number counts at 15µm. Then we analyse the 12µm (ISO-ESS) and the 24µm (Spitzer) faint counts, to constrain the nature of ULIRGs, the cosmic star formation history and time scales for mass buildup. We show that the ``normal'' scenarios in our evolutionary code PEGASE, which had previously fitted the deep UV-optical-near IR counts, are unsuccessful at 12µm and 24µm. We thus propose a new ULIRG scenario adjusted to the observed cumulative and differential 12µm and 24µm counts and based on observed 12µm and 25µm IRAS luminosity functions and evolutionary optical/mid-IR colours from PEGASE. We succeed in simultaneously modelling the typical excess observed at 12µm, 15µm (ISO), and 24µm (Spitzer) in the cumulative and differential counts by only changing 9% of normal galaxies (1/3 of the ellipticals) into ultra-bright dusty galaxies evolving as ellipticals, and interpreted as distant ULIRGs. These objects present similarities with the population of radio-galaxy hosts at high redshift. No number density evolution is included in our models even if minor starbursts due to galaxy interactions remain compatible with our results. Higher spectral and spatial resolution in the mid-IR, together with submillimeter observations using the future Herschel observatory, will be useful to confirm these results.