The joint formation and evolution of E and high-z QSO: the ABC scenario

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Figure 1:

 The main physical processes driving galaxy formation, in particular the evolution of baryonic matter are extremely complex and still hotly debated. The evolution of ordinary matter, which is highly non linear and is driven mostly by processes occurring well below the resolution of any feasible simulation. These processes are usually referred to as sub-grid physics, include star formation (SF), accretion onto super-massive black holes (SMBH), merging of BH, and are also poorly understood from a physical point of view. In any computation sub-grid physics is treated (if ever) through semi-analytic-like recipes, that is formulae containing many free parameters, which try to describe sub-grid physics by means of relationships between integrated quantities. Unfortunately, the model outcomes are heavily affected, if not driven, by these formulae. The most extensive comparisons between different scenarios and the rich data sets now available are done by means of fully semi-analytic models (SAM) for baryons, possibly in the form of post-processing of gravity-only simulations for DM. Almost all SAMs developed by various groups adopt a-priori what we can call a disk galaxy merger driven sequence of processes leading to galaxy populations at any redshift and in particular locally. Despite the high number of adjustable parameters involved, it has become increasingly clear over the years that calculations based on this general scheme, besides remarkable successes in reproducing several properties of galaxy population, show severe mismatches with some very basic observations. These are in general aspects of the so called cosmic downsizing: star formation and accretion onto SMBH decline more with cosmic time for larger system than for smaller ones, a fact which is at odd with naive expectations of hierarchical growth; Most people think that these difficulties may well be all facets of the same problem, suggesting that some key ingredient is missing or/and the entire scheme needs a substantial revision. It is at present very popular the idea that at least part of the solution could come from an ingredient that only in the very last few years started to be taken into account in some models, i.e. the mutual influence or feed-back (FB) of star formation in galaxies and the development of SMBH-QSO at their centers. This influence is hinted by several empirical and also theoretical facts.

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Figure 2:

The ABC scenario ellqso2 The first SAM in which a key role has been invoked for the reciprocal feed-back between star formation and AGN activity is the "Antihierarchical Baryonic Collapse" (ABC)   (see also Granato et al. 2001 for a more phenomenological treatment). This model, which is embedded in the LambdaCDM hierarchical growth of DM halos and is focused on the formation of spheroids, adopts prescriptions to describe the baryonic physics which reverse the order with which spheroidal galaxies and high-z QSOs complete their formation, as indicated by the various evidences of downsizing. This is obtained by a combination of two ingredients: (i) revised prescriptions for the SF in massive high redshift galactic halos (M_vir \gtrsim 10^12 M_0), which allows SFR as high as thousands of solar masses per year, as implied by observations of sub-mm galaxies, and (ii) the inclusion of a treatment of the growth by accretion of a SMBH promoted by this huge SF activity (positive feed-back between SF and accretion), which at some point becomes so powerful to clean the ISM and quench any further SF and accretion (negative QSO feed-back).

The ABC scenario predicts a well defined evolutionary sequence leading to local ellipticals with dormant SMBH. The sequence begins with a high redshift phase of huge, dust enshrouded SF activity best detectable in the sub-mm spectral region, and lasting about 0.5 Gyr. This phase is ended by the strong feed-back generated by the QSO phase, due to the growth of a SMBH, which is promoted by the huge star formation activity during the previous SMG phase. This feedback sterilizes the system, which then evolve almost passively, thus it predicts a sizeable population of massive and dead galaxies already at high-z. The model leads (in one shot) to predictions in general agreement with the downsizing phenomenon and other observations which are at least disturbing for traditional SAMs, such as the statistical properties of sub-mm detected galaxies, including number counts, z- distribution and their mild AGN activity, the results of deep near-IR surveys, the evolution of qso luminosity function and the local properties of ellipticals (color-magnitude relation and chemical properties).

We are at present working to incorporate this model in a more general and novel model explaining the general dichotomy between disk and spheroidal stellar systems.

People: G. De Zotti, A. Bressan.

Collaboration:A. Lapi, L. Danese, M. Cook (SISSA), G.L. Granato, L. Silva (INAF OA Trieste)  

Publications: Mao et al. (2007), ApJ,667,655; Fan et al. (2008), ApJ,689,L101

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