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Department of Physics & Astronomy, University of Padua, Vicolo Osservatorio 2, Padua 35122, Italy


National Institute for Astrophysics INAF, Astronomical Observatory of Rome, Via Frascati 33,Monte Porzio Catone RM 00040, Italy


Author to whom correspondence should be addressed.

Abstract We review and critically discuss the current understanding of galaxy formation and evolution limited to Early Type Galaxies ETGs as inferred from the observational data and briefly contrast the hierarchical and quasi-monolithic paradigms of formation and evolution. Since in Cold Dark Matter CDM cosmogony small scale structures typically collapse early and form low-mass haloes that subsequently can merge to assembly larger haloes, galaxies formed in the gravitational potential well of a halo are also expected to merge thus assembling their mass hierarchically. Mergers should occur all over the Hubble time and large mass galaxies should be in place only recently. However, recent observations of high redshift galaxies tell a different story: massive ETGs are already in place at high redshift. To this aim, we propose here a revision of the quasi-monolithic scenario as an alternative to the hierarchical one, in which mass assembling should occur in early stages of a galaxy lifetime and present recent models of ETGs made of Dark and Baryonic Matter in a Λ-CDM Universe that obey the latter scheme. The galaxies are followed from the detachment from the linear regime and Hubble flow at z ≥ 20 down to the stage of nearly complete assembly of the stellar content z ∼ 2 − 1 and beyond. It is found that the total mass Mh = MDM + MBM and-or initial over-density of the proto-galaxy drive the subsequent star formation histories SFH. Massive galaxies Mh ~ 1012M⊙ experience a single, intense burst of star formation with rates ≥ 103M⊙-yr at early epochs, consistently with observations, with a weak dependence on the initial over-density; intermediate mass haloes Mh~ 1010 − 1011M⊙ have star formation histories that strongly depend on their initial over-density; finally, low mass haloes Mh ~ 109M⊙ always have erratic, burst-like star forming histories. The present-day properties morphology, structure, chemistry and photometry of the model galaxies closely resemble those of the real galaxies. In this context, we also try to cast light on the physical causes of the Stellar Mass-Radius Relation MRR of galaxies. The MRR is the result of two complementary mechanisms: i.e., local physical processes that fix the stellar mass and the radius of each galaxy and cosmological global, statistical principles, which shape the distribution of galaxies in the MR-plane. Finally, we also briefly comment on the spectro-photometric properties of the model galaxies and how nicely they match the observational data. The picture emerging from this analysis is that the initial physical conditions of a proto-galaxy, i.e., nature, seem to play the dominant role in building up the ETGs we see today, whereas nurture by recurrent captures of small objects is a secondary actor of the fascinating and intriguing story of galaxy formation and evolution. View Full-Text

Keywords: observational data; early types; formation and evolution; mass-radius relation observational data; early types; formation and evolution; mass-radius relation

Author: Cesare Chiosi 1,* , Emiliano Merlin 1,2, Lorenzo Piovan 1 and Rosaria Tantalo 1



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