ARE THE BULK OF z > 2 HERSCHEL GALAXIES PROTO-SPHEROIDS?

We present a backward approach for the interpretation of the evolution of the near-IR and the far-IR luminosity functions (LFs) across the redshift range . In our method, late-type galaxies are treated by means of a parametric phenomenological method based on PEP/HerMES data up to z ∼ 4, whereas spheroids are described by means of a physically motivated backward model. The spectral evolution of spheroids is modeled by means of a single-mass model, associated with a present-day elliptical with a K-band luminosity comparable to the break of the local early-type LF. The formation of proto-spheroids is assumed to occurr across the redshift range . The key parameter is represented by the redshift at which half of all proto-spheroids are already formed. For this parameter, a statistical study indicates values between and . We assume as the fiducial value and show that this assumption allows us to describe accourately the redshift distributions and the source counts. By assuming at the far-IR flux limit of the PEP-COSMOS survey, the PEP-selected sources observed at can be explained as progenitors of local spheroids caught during their formation. We also test the effects of mass downsizing by dividing the spheroids into three populations of different present-day stellar masses. The results obtained in this case confirm the validity of our approach, i.e., that the bulk of proto-spheroids can be modeled by means of a single model that describes the evolution of galaxies at the break of the present-day early-type K-band LF. [ABSTRACT FROM AUTHOR]