10,811 research outputs found
Rendez-vous of dwarfs
We present observations of multiple system of dwarf galaxies at the Russian
6-m telescope and the GMRT (Giant Metrewave Radio Telescope). The optical
observations are a part of the programme Study of Groups of Dwarf Galaxies in
the Local Supercluster. The group of galaxies under consideration looks like
filament of 5 dwarfs. Two faint galaxies show peculiar structure. Long slit
spectrum reveals inner motions about 150 km/s in one of them. It suggests that
the galaxy is on stage of ongoing interaction. Probably, we see the group in
moment of its formation.Comment: 2 pages, 3 figures; to appear in the proceedings of the conference "A
Universe of dwarf galaxies" (Lyon, June 14-18, 2010
Cosmological fluctuation growth in bimetric MOND
I look at the growth of weak density inhomogeneities of nonrelativistic
matter, in bimetric-MOND (BIMOND) cosmology. I concentrate on
matter-twin-matter-symmetric versions of BIMOND, and assume that, on average,
the universe is symmetrically populated in the two sectors. MOND effects are
absent in an exactly symmetric universe, apart from the appearance of a
cosmological constant, Lambda~(a0/c)^2. MOND effects-local and cosmological-do
enter when density inhomogeneities that differ in the two sectors appear and
develop. MOND later takes its standard form in systems that are islands
dominated by pure matter. I derive the nonrelativistic equations governing
small-scale fluctuation growth. The equations split into two uncoupled systems,
one for the sum, the other for the difference, of the fluctuations in the two
sectors. The former is governed strictly by Newtonian dynamics. The latter is
governed by MOND dynamics, which entails stronger gravity, and nonlinearity
even for the smallest of perturbations. These cause the difference to grow
faster than the sum, conducing to matter-twin-matter segregation. The
nonlinearity also causes interaction between nested perturbations on different
scales. Because matter and twin matter (TM) repel each other in the MOND
regime, matter inhomogeneities grow not only by their own self gravity, but
also through shepherding by flanking TM overdensitie. The relative importance
of gravity and pressure in the MOND system depends also on the strength of the
perturbation. The development of structure in the universe, in either sector,
thus depends crucially on two initial fluctuation spectra: that of matter alone
and that of the matter-TM difference. I also discuss the back reaction on
cosmology of BIMOND effects that appear as "phantom matter" resulting from
inhomogeneity differences between the two sectors.Comment: 14 pages. Some clarifications added. Version published in Phys. Rev.
Observational biases in Lagrangian reconstructions of cosmic velocity fields
Lagrangian reconstruction of large-scale peculiar velocity fields can be
strongly affected by observational biases. We develop a thorough analysis of
these systematic effects by relying on specially selected mock catalogues. For
the purpose of this paper, we use the MAK reconstruction method, although any
other Lagrangian reconstruction method should be sensitive to the same
problems. We extensively study the uncertainty in the mass-to-light assignment
due to luminosity incompleteness, and the poorly-determined relation between
mass and luminosity. The impact of redshift distortion corrections is analyzed
in the context of MAK and we check the importance of edge and finite-volume
effects on the reconstructed velocities. Using three mock catalogues with
different average densities, we also study the effect of cosmic variance. In
particular, one of them presents the same global features as found in
observational catalogues that extend to 80 Mpc/h scales. We give recipes,
checked using the aforementioned mock catalogues, to handle these particular
observational effects, after having introduced them into the mock catalogues so
as to quantitatively mimic the most densely sampled currently available galaxy
catalogue of the nearby universe. Once biases have been taken care of, the
typical resulting error in reconstructed velocities is typically about a
quarter of the overall velocity dispersion, and without significant bias. We
finally model our reconstruction errors to propose an improved Bayesian
approach to measure Omega_m in an unbiased way by comparing the reconstructed
velocities to the measured ones in distance space, even though they may be
plagued by large errors. We show that, in the context of observational data, a
nearly unbiased estimator of Omega_m may be built using MAK reconstruction.Comment: 29 pages, 21 figures, 6 tables, Accepted by MNRAS on 2007 October 2.
Received 2007 September 30; in original form 2007 July 2
The Balance of Dark and Luminous Mass in Rotating Galaxies
A fine balance between dark and baryonic mass is observed in spiral galaxies.
As the contribution of the baryons to the total rotation velocity increases,
the contribution of the dark matter decreases by a compensating amount. This
poses a fine-tuning problem for \LCDM galaxy formation models, and may point to
new physics for dark matter particles or even a modification of gravity.Comment: 4 pages RevTeX. Phys. Rev. Letters, in pres
Nilpotent normal form for divergence-free vector fields and volume-preserving maps
We study the normal forms for incompressible flows and maps in the
neighborhood of an equilibrium or fixed point with a triple eigenvalue. We
prove that when a divergence free vector field in has nilpotent
linearization with maximal Jordan block then, to arbitrary degree, coordinates
can be chosen so that the nonlinear terms occur as a single function of two
variables in the third component. The analogue for volume-preserving
diffeomorphisms gives an optimal normal form in which the truncation of the
normal form at any degree gives an exactly volume-preserving map whose inverse
is also polynomial inverse with the same degree.Comment: laTeX, 20 pages, 1 figur
The vertical disk structure of the edge-on spiral galaxy NGC 3079
NGC 3079 is an edge-on SB(s)c galaxy at a redshift of 1225 km/s relative to the Local Group. Earlier researchers found a spectacular 'figure-eight' radio structure aligned along the minor axis of the galaxy, centered on the nucleus, and extending 3 kpc above and below the plane. The geometry of this structure and the evidence of unusually high nuclear gas velocities suggest that a wind-type outflow from the nucleus is taking place. The disk of NGC 3079 is also remarkable: it is extremely rich in H 2 regions and is the only unambiguous example of a galaxy outside M31 and our own Galaxy to exhibit 'Heiles-like' shells. Other researchers have also identified a nebulosity with a ragged X-shaped morphology formed by a system of lumpy filaments with individual lengths of 3 - 5 kpc. They suggest that this material is ambient halo gas entrained into the boundary layers of the nuclear outflow. The complex structure of the line emission in NGC 3079 makes this object an ideal target for an imaging spectroscopic study. The present paper reports the preliminary results of such a study
The spacetime structure of MOND with Tully-Fisher relation and Lorentz invariance violation
It is believed that the modification of Newtonian dynamics (MOND) is possible
alternate for dark matter hypothesis. Although Bekenstein's TeVeS supplies a
relativistic version of MOND, one may still wish a more concise covariant
formulism of MOND. In this paper, within covariant geometrical framwork, we
present another version of MOND. We show the spacetime structure of MOND with
properties of Tully-Fisher relation and Lorentz invariance violation.Comment: 6 pages. arXiv admin note: substantial text overlap with
arXiv:1111.1383 and arXiv:1108.344
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