2,518 research outputs found
Long-Time Evolution of Gas-Free Disc Galaxies in Binary Systems
We present the results of several detailed numerical N-body simulations of
the dynamical interactions of two equal mass disc galaxies. Both galaxies are
embedded in spherical halos of dark matter and contain central bulges. Our
analysis of the dynamical evolution of the binary system focuses on the
morphological evolution of the stellar distribution of the discs. The satellite
galaxy has coplanar or polar disc orientation in relation to the disc of the
primary galaxy and their initial orbits are prograde eccentric (,
or ). Both galaxies have mass and size comparable to the Milky
Way. We show that the merger of the two disc galaxies, depending on the
relative orientation of the discs, can yield either a disc or lenticular
remnant, instead of an elliptical one. These are the first simulations in the
literature to show the formation of S0-like galaxies from protracted binary
galaxy interactions. Additionally, we demonstrate that the time to merger
increases linearly with the initial apocentric distance between the galaxies,
and decreases with the initial orbital eccentricity. We also show that the
tidal forces of the discs excite transient and wave modes, i.e.,
lopsidedness, spiral arms, and bars. However, after the merging of the discs,
such instabilities fade completely, and the remnant is thicker and bigger than
the original discs. The maximum relative amplitude of these waves is at most
about 15 times greater compared to the control case. of these two wave modes.
Finally, the disc settles down quickly, after the merger, in less than one
outer disc rotation period.Comment: 23 pages, 3 tables and 16 figures. Accepted for publication in A&
A new method for estimating the pattern speed of spiral structure in the Milky Way
In the last few decades many efforts have been made to understand the effect
of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of
the fundamental parameters of the spiral structure is its angular velocity, or
pattern speed , which determines the location of resonances in the
disc and the spirals' radial extent. The most direct method for estimating the
pattern speed relies on backward integration techniques, trying to locate the
stellar birthplace of open clusters. Here we propose a new method based on the
interaction between the spiral arms and the stars in the disc. Using a sample
of around 500 open clusters from the {\it New Catalogue of Optically Visible
Open Clusters and Candidates}, and a sample of 500 giant stars observed by
APOGEE, we find km s kpc, for a local
standard of rest rotation ~km s and solar radius ~kpc.
Exploring a range in and within the acceptable values, 200-240 km
s and 7.5-8.5 kpc, respectively, results only in a small change in our
estimate of , that is within the error. Our result is in close
agreement with a number of studies which suggest values in the range 20-25 km
s kpc. An advantage of our method is that we do not need
knowledge of the stellar age, unlike in the case of the birthplace method,
which allows us to use data from large Galactic surveys. The precision of our
method will be improved once larger samples of disk stars with spectroscopic
information will become available thanks to future surveys such as 4MOST.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA
A new model for gravitational potential perturbations in disks of spiral galaxies. An application to our Galaxy
We propose a new, more realistic, description of the perturbed gravitational
potential of spiral galaxies, with spiral arms having Gaussian-shaped groove
profiles. We investigate the stable stellar orbits in galactic disks, using the
new perturbed potential. The influence of the bulge mass on the stellar orbits
in the inner regions of a disk is also investigated. The new description offers
the advantage of easy control of the parameters of the Gaussian profile of its
potential. We find a range of values for the perturbation amplitude from 400 to
800 km^2 s^{-2} kpc^{-1} which implies a maximum ratio of the tangential force
to the axisymmetric force between 3% and 6%, approximately. Good
self-consistency of arm shapes is obtained between the Inner Lindblad resonance
(ILR) and the 4:1 resonance. Near the 4:1 resonance the response density starts
to deviate from the imposed logarithmic spiral form. This creates bifurcations
that appear as short arms. Therefore the deviation from a perfect logarithmic
spiral in galaxies can be understood as a natural effect of the 4:1 resonance.
Beyond the 4:1 resonance we find closed orbits which have similarities with the
arms observed in our Galaxy. In regions near the center, in the presence of a
massive bulge, elongated stellar orbits appear naturally, without imposing any
bar-shaped potential, but only extending the spiral perturbation a little
inward of the ILR. This suggests that a bar is formed with a half-size around 3
kpc by a mechanism similar to that of the spiral arms. The potential energy
perturbation that we adopted represents an important step in the direction of
self-consistency, compared to previous sine function descriptions of the
potential. Our model produces a realistic description of the spiral structure,
able to explain several details that were not yet understood.Comment: 12 pag., 11 fig. Accepted for publication in A&A, 2012 December 1
Equivalence between the Lovelock-Cartan action and a constrained gauge theory
We show that the four-dimensional Lovelock-Cartan action can be derived from
a massless gauge theory for the group with an additional BRST trivial
part. The model is originally composed by a topological sector and a BRST exact
piece and has no explicit dependence on the metric, the vierbein or a mass
parameter. The vierbein is introduced together with a mass parameter through
some BRST trivial constraints. The effect of the constraints is to identify the
vierbein with some of the additional fields, transforming the original action
into the Lovelock-Cartan one. In this scenario, the mass parameter is
identified with Newton's constant while the gauge field is identified with the
spin-connection. The symmetries of the model are also explored. Moreover, the
extension of the model to a quantum version is qualitatively discussed.Comment: 17 pages. No figures. Final version accepted for publication at the
EPJ
Comparison of the solophenyl-red polarization method and the immunohistochemical analysis for collagen type III
In the present study, we have compared the staining pattern of the Solophenyl-Red 3 BL-method for the visualization of collagen type III with the immunohistochemical staining in serial sections from 7 skin wounds (wound age 3 days up to 4 weeks) to elucidate the specifity of the histochemical staining method. Large amounts of collagen type III were clearly detectable in the investigated wounds using the immunohistochemical technique. In the sections stained with Solophenyl-Red, however, only 3 out of 7 skin lesions showed a significant positive red staining at the wound margin or in the granulation tissue, while the adjacent normal connective tissue revealed a typical intensive staining. Using polarization microscopy no characteristic bright green fibrils, as reported for collagen type 111, could be seen in the wound areas without positive Solophenyl-Red staining. Since the localization of collagen type III detected by immunohistochemistry and the presumed distribution of this collagen type by the Solophenyl-Red method was not identical, the histochemical polarization method has to be regarded as non-specific for visualization of this collagen type
Bimodal chemical evolution of the Galactic disk and the Barium abundance of Cepheids
In order to understand the Barium abundance distribution in the Galactic disk
based on Cepheids, one must first be aware of important effects of the
corotation resonance, situated a little beyond the solar orbit. The thin disk
of the Galaxy is divided in two regions that are separated by a barrier
situated at that radius. Since the gas cannot get across that barrier, the
chemical evolution is independent on the two sides of it. The barrier is caused
by the opposite directions of flows of gas, on the two sides, in addition to a
Cassini-like ring void of HI (caused itself by the flows). A step in the
metallicity gradient developed at corotation, due to the difference in the
average star formation rate on the two sides, and to this lack of communication
between them. In connection with this, a proof that the spiral arms of our
Galaxy are long-lived (a few billion years) is the existence of this step. When
one studies the abundance gradients by means of stars which span a range of
ages, like the Cepheids, one has to take into account that stars, contrary to
the gas, have the possibility of crossing the corotation barrier. A few stars
born on the high metallicity side are seen on the low metallicity one, and
vice-versa. In the present work we re-discuss the data on Barium abundance in
Cepheids as a function of Galactic radius, taking into account the scenario
described above. The [Ba/H] ratio, plotted as a function of Galactic radius,
apparently presents a distribution with two branches in the external region
(beyond corotation). One can re-interpret the data and attribute the upper
branch to the stars that were born on the high metallicity side. The lower
branch, analyzed separately, indicates that the stars born beyond corotation
have a rising Barium metallicity as a function of Galactic radius.Comment: 6 pages, 7 figures, Proceedings of IAU Symposium 29
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