Observational tests of the evolution of spheroidal galaxies

In a recent paper, we have elaborated a physically grounded model exploiting the mutual feedback between star-forming spheroidal galaxies and the active nuclei growing in their cores, in order to overcome, in the framework of the hierarchical clustering scenario for galaxy formation, one of the main challenges facing such a scenario (i.e. the fact that massive spheroidal galaxies appear to have formed much earlier and faster than predicted by previous semi-analytical models, while the formation process was slower for less massive objects). After having assessed the values of the two parameters that control the effect of the complex and poorly understood radiative transfer processes on the time-dependent spectral energy distributions, we have compared the model predictions with a variety of infrared to millimetre (mm) data. Our results support a rather strict continuity between objects where stars formed [detected by (sub)mm surveys] and evolved massive early-type galaxies, indicating that large spheroidal galaxies formed most of their stars when they were already assembled as single objects. The model is remarkably successful in reproducing the observed redshift distribution of K less than or equal to 20 galaxies at z > 1, in contrast with both the classical monolithic models (which overestimate the density at high-z) and the semi-analytical models (that are systematically low), as well as the ratio of star-forming to passively evolving spheroids and the counts and redshift distributions of extremely red objects, although the need for a more detailed modelling of the star formation history and of dust geometry is indicated by the data. The model also favourably compares with the ISOCAM 6.7-mum counts, with the corresponding redshift distribution, and with Spitzer/IRAC counts, which probe primarily the passive evolution phase, and with the (sub)mm SCUBA and MAMBO data, probing the active star formation phase. The observed fraction of 24-mum selected sources with no detectable emission in either the 8-mum or R band corresponds well with the predicted surface density of star-forming spheroids with 8-mum fluxes below the detection limit. Finally, we point out distinctive predictions for the redshift distributions of 24-mum sources detected by Spitzer/MIPS surveys.