Evolution and instabilities of disks harboring super massive black holes

The bar formation is still an open problem in modern astrophysics. In this paper we present numerical simulation performed with the aim of analyzing the growth of the bar instability inside stellar-gaseous disks, where the star formation is triggered, and a central black hole is present. The aim of this paper is to point out the impact of such a central massive black hole on the growth of the bar. We use N-body-SPH simulations of the same isolated disk-to-halo mass systems harboring black holes with different initial masses and different energy feedback on the surrounding gas. We compare the results of these simulations with the one of the same disk without black hole in its center. We make the same comparison (disk with and without black hole) for a stellar disk in a fully cosmological scenario. A stellar bar, lasting 10 Gyrs, is present in all our simulations. The central black hole mass has in general a mild effect on the ellipticity of the bar but it is never able to destroy it. The black holes grow in different way according their initial mass and their feedback efficiency, the final values of the velocity dispersions and of the black hole masses are near to the phenomenological constraints.Comment: 10 pages, 8 figures, accepted for pubblication in "Astrophysics and Space Science

The radial metallicity gradients in the Milky Way thick disk as fossil signatures of a primordial chemical distribution

In this letter we examine the evolution of the radial metallicity gradient induced by secular processes, in the disk of an $N$-body Milky Way-like galaxy. We assign a [Fe/H] value to each particle of the simulation according to an initial, cosmologically motivated, radial chemical distribution and let the disk dynamically evolve for 6 Gyr. This direct approach allows us to take into account only the effects of dynamical evolution and to gauge how and to what extent they affect the initial chemical conditions. The initial [Fe/H] distribution increases with R in the inner disk up to R ~ 10 kpc and decreases for larger R. We find that the initial chemical profile does not undergo major transformations after 6 Gyr of dynamical evolution. The final radial chemical gradients predicted by the model in the solar neighborhood are positive and of the same order of those recently observed in the Milky Way thick disk. We conclude that: 1) the spatial chemical imprint at the time of disk formation is not washed out by secular dynamical processes, and 2) the observed radial gradient may be the dynamical relic of a thick disk originated from a stellar population showing a positive chemical radial gradient in the inner regions.Comment: 10 pages, 5 figures, Accepted for publication on Astrophysical Journal Letter

Evolution of stellar-gaseous disks in cosmological haloes

We explore the growth and the evolution of the bar instability in stellar-gaseous disks embedded in a suitable dark matter halo evolving in a fully consistent cosmological framework. The aim of this paper is to point out the impact of different gas fractions on the bar formation, inside disks of different disk-to-halo mass ratio, and the role of the cosmological framework. We perform cosmological simulations with the same disk-to-halo mass ratios as in a previous work where the gas was not taken into account. We compare results of the new simulations with the previous ones to investigate the effect of the gas by analysing the morphology of the stellar and gaseous components, the stellar bar strength and the behaviour of its pattern speed. In our cosmological simulations, inside dark-matter dominated disks, a stellar bar, lasting 10 Gyr, is still living at z=0 even if the gaseous fraction exceeds half of the disk mass. However, in the most massive disks we find a threshold value (0.2) of the gas fraction able to destroy the bar. The stellar bar strength is enhanced by the gas and in the more massive disks higher gas fractions increase the bar pattern speed

The Asymmetric Thick Disk: A Star Count and Kinematic Analysis. II The Kinematics

We report a kinematic signature associated with the observed asymmetry in the distribution of thick disk/inner halo stars interior to the Solar circle described in Paper I. In that paper we found a statistically significant excess (20% to 25 %) of stars in quadrant I (l ~ 20 deg to 55 deg) both above and below the plane (b ~ +/- 25 deg to +/- 45 deg) compared to the complementary region in quadrant IV. We have measured Doppler velocities for 741 stars, selected according to the same magnitude and color criteria, in the direction of the asymmetry and in the corresponding fields in quadrant IV. We have also determined spectral types and metallicities measured from the same spectra. We not only find an asymmetric distribution in the V_LSR velocities for the stars in the two regions, but the angular rate of rotation, w, for the stars in quadrant I reveals a slower effective rotation rate compared to the corresponding quadrant IV stars. We use our [Fe/H] measurements to separate the stars into the three primary population groups, halo, thick disk, and disk, and conclude that it is primarily the thick disk stars that show the slower rotation in quadrant I. A solution for the radial, tangential and vertical components of the V_LSR velocities, reveals a significant lag of ~ 80 to 90 km/s in the direction of Galactic rotation for the thick disk stars in quadrant I, while in quadrant IV, the same population has only a ~ 20 km/s lag. The results reported here support a rotational lag among the thick disk stars due to a gravitational interaction with the bar as the most likely explanation for the asymmetry in both the star counts and the kinematics. The affected thick disk stars, however, may be associated with the recently discovered Canis Major debris stream or a similar merger event (abridged).Comment: Accepted for publication in the Astronomical Journa

Star formation and bar instability in cosmological haloes

This is the third of a series of papers presenting the first attempt to analyze the growth of the bar instability in a consistent cosmological scenario. In the previous two articles we explored the role of the cosmology on stellar disks, and the impact of the gaseous component on a disk embedded in a cosmological dark matter halo. The aim of this paper is to point out the impact of the star formation on the bar instability inside disks having different gas fractions. We perform cosmological simulations of the same disk-to-halo mass systems as in the previous works where the star formation was not triggered. We compare the results of the new simulations with the previous ones to investigate the effect of the star formation by analysing the morphology of the stellar components, the bar strength, the behaviour of the pattern speed. We follow the gas and the central mass concentration during the evolution and their impact on the bar strength. In all our cosmological simulations a stellar bar, lasting 10 Gyr, is still living at z=0. The central mass concentration of gas and of the new stars has a mild action on the ellipticity of the bar but is not able to destroy it; at z=0 the stellar bar strength is enhanced by the star formation. The bar pattern speed is decreasing with the disk evolution.Comment: 10 pages, 21 figures, A&A accepte

The extensions of gravitational soliton solutions with real poles

We analyse vacuum gravitational "soliton" solutions with real poles in the cosmological context. It is well known that these solutions contain singularities on certain null hypersurfaces. Using a Kasner seed solution, we demonstrate that these may contain thin sheets of null matter or may be simple coordinate singularities, and we describe a number of possible extensions through them.Comment: 14 pages, LaTeX, 6 figures included using graphicx; to appear in Gen. Rel. Gra

Bar instability in cosmological halos

Aims: We want to investigate the growth of bar instability in stellar disks embedded in a suitable dark matter halo evolving in a fully consistent cosmological framework. Methods: We perform seven cosmological simulations to emphasise the role of both the disk-to-halo mass ratio and of the Toomre parameter, Q, on the evolution of the disk.We also compare our fully cosmological cases with corresponding isolated simulations where the same halo, is extracted from the cosmological scenario and evolved in physical coordinates. Results: A long living bar, lasting about 10 Gyr, appears in all our simulations. In particular, disks expected to be stable according to classical criteria, form indeed weak bars. We argue that such a result is due to the dynamical properties of our cosmological halo which is far from stability and isotropy, typical of the classical halos used in literature; it is dynamically active, endowed of substructures and infall. Conclusions: At least for mild self-gravitating disks, the study of the bar instability using isolated isotropic halos, in gravitational equilibrium, can lead to misleading results. Furthermore, the cosmological framework is needed for quantitatively investigating such an instability.Comment: Astronomy & Astrophysics, accepted, 19 pages, 21 figure

Time varying gravitational constant G via the entropic force

If the uncertainty principle applies to the Verlinde entropic idea, it leads to a new term in the Newton's second law of mechanics in the Planck's scale. This curious velocity dependence term inspires a frictional feature of the gravity. In this short letter we address that this new term modifies the effective mass and the Newtonian constant as the time dependence quantities. Thus we must have a running on the value of the effective mass on the particle mass $m$ near the holographic screen and the $G$. This result has a nigh relation with the Dirac hypothesis about the large numbers hypothesis (L.N.H.) [1]. We propose that the corrected entropic terms via Verlinde idea can be brought as a holographic evidence for the authenticity of the Dirac idea.Comment: Accepted for publication in "Communications in Theoretical Physics (CTP)",Major revisio

Entropy of three-dimensional asymptotically flat cosmological solutions

The thermodynamics of three-dimensional asymptotically flat cosmological solutions that play the same role than the BTZ black holes in the anti-de Sitter case is derived and explained from holographic properties of flat space. It is shown to coincide with the flat-space limit of the thermodynamics of the inner black hole horizon on the one hand and the semi-classical approximation to the gravitational partition function associated to the entropy of the outer horizon on the other. This leads to the insight that it is the Massieu function that is universal in the sense that it can be computed at either horizon.Comment: 16 pages Latex file, v2: references added, cosmetic changes, v3: 1 reference adde

The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A

We present the results of stellar photometry of polar-ring galaxies NGC 2685 and NGC 4650A, using the archival data obtained with the Hubble Space Telescope's Wide Field Planetary Camera 2. Polar rings of these galaxies were resolved into ~800 and ~430 stellar objects in the B, V and Ic bands, considerable part of which are blue supergiants located in the young stellar complexes. The stellar features in the CM-diagrams are best represented by isochrones with metallicity Z = 0.008. The process of star formation in the polar rings of both galaxies was continuous and the age of the youngest detected stars is about 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.Comment: 21 pages, 9 figures, AJ 2004 February, accepte