90 research outputs found
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 -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 near the holographic screen and the . 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
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