The empirical differential oxygen abundance distribution (EDOD) is deduced from subsamples related to two different samples involving solar neighbourhood (SN) thick disk, thin disk, halo, and bulge stars. The EDOD of the SN thick + thin disk is deter
mined by weighting the mass, for assumed SN thick to thin disk mass ratio within the range, 0.1-0.9. Inhomogeneous models of chemical evolution for the SN thick disk, the SN thin disk, the SN thick + thin disk, the SN halo, and the bulge, are computed assuming the instantaneous recycling approximation. The EDOD data are fitted, to an acceptable extent, by their TDOD counterparts provided (i) still undetected, low-oxygen abundance thin disk stars exist, and (ii) a single oxygen overabundant star is removed from a thin disk subsample. In any case, the (assumed power-law) stellar initial mass function (IMF) is universal but gas can be inhibited from, or enhanced in, forming stars at different rates with respect to a selected reference case. Models involving a strictly universal IMF (i.e. gas neither inhibited from, nor enhanced in, forming stars with respect to a selected reference case) can also reproduce the data. The existence of a strictly universal IMF makes similar chemical enrichment within active (i.e. undergoing star formation) regions placed in different environments, but increasing probability of a region being active passing from SN halo to SN thick + thin disk, SN thin disk, SN thick disk, and bulge. On the basis of the results, it is realized that the chemical evolution of the SN thick + thin disk as a whole cannot be excluded.
The empirical differential metallicity distribution (EDMD) is deduced for (i) local thick disk stars; (ii) likely metal-weak thick disk stars; (iii) chemically selected local G dwarfs, with the corrections performed in order to take into account the
stellar scale height; in addition to previous results related to (iv) solar neighbourhood halo subdwarfs; and (v) K-giant bulge stars. The thick disk is conceived as made of two distinct regions: the halo-like and the bulge-like thick disk, and the related EDMD is deduced. Under the assumption that each distribution is typical for the corresponding subsystem, the EDMD of the thick disk, the thick + thin disk, and the Galaxy, is determined by weighting the mass. Models of chemical evolution are computed for each subsystem assuming the instantaneous recycling approximation. The EDMD data are reasonably fitted by simple models implying both homogeneous and inhomogeneous star formation, provided that star formation is inhibited during thick disk evolution. The initial mass function (IMF) is assumed to be a universal power law, which implies an unchanged true yield in different subsystems. The theoretical differential metallicity distribution (TDMD) is first determined for the halo-like thich disk, the bulge-like thick disk, and the thin disk separately, and then for the Galaxy by weighting the mass. An indicative comparison is performed between the EDMD deduced for the disk both in presence and in absence of [O/Fe] plateau, and its counterpart computed for (vi) nearby stars for which the oxygen abundance has been determined both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation. Both distributions are found to exhibit a similar trend, though systematic differences exist.
Unsustained matter distributions unescapely collapse unless fragmentation and centrifugal or pressure support take place. Starting from the above evidence, supermassive compact objects at the centre of large-mass galaxies are conceived as the end-pro
duct of the gravitational collapse of local density maxima around which overdensities are located. At the beginning of evolution, local density maxima are idealized as homogeneous peaks, while the surrounding envelopes are described by a power-law density profile. The dependence of the density profile on a second parameter, chosen to be the ratio between peak and total mass, is analysed. Overdensity evolution is discussed in the context of quintessence cosmological models and further investigation is devoted to a special case with the aim to describe the central collapse. An empirical relation between hole and dark halo mass is translated into a dependence of the fractional hole mass on the overdensity mass. Computations are performed up to the end of central collapse, and density profiles of related configurations are determined together with additional parameters. The central collapse is completed in early times, no longer than a few hundredths of Gyr, which implies hole formation when proto-haloes, proto-bulges, and proto-disks are still expanding or contracting. No appreciable change in evolution is found with regard to different mean peak heights related to equal masses. On the other hand, it is recognized that homogeneous peaks collapse ``faster with respect to surroundings envelopes, in low-mass than in large-mass overdensities. In conclusion, it is inferred that gravitational collapse of homogeneous peaks within overdensities may be a viable mechanism for hole generation.
