22,141 research outputs found
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Modifying gravity with the Aether: an alternative to Dark Matter
There is evidence that Newton and Einstein's theories of gravity cannot
explain the dynamics of a universe made up solely of baryons and radiation. To
be able to understand the properties of galaxies, clusters of galaxies and the
universe on the whole it has become commonplace to invoke the presence of dark
matter. An alternative approach is to modify the gravitational field equations
to accommodate observations. We propose a new class of gravitational theories
in which we add a new degree of freedom, the Aether, in the form of a vector
field that is coupled covariantly, but non-minimally, with the space-time
metric. We explore the Newtonian and non-Newtonian limits, discuss the
conditions for these theories to be consistent and explore their effect on
cosmology.Comment: Updated version: Notation improved - TG
Remnant Vegetation Inventory on the Southern Agricultural Areas of Western Australia.
Remnant vegetation mapping and its incorporation into a Geographical Information System (GIS) data base is the first stage in an overall documentation and promotion of conservation of remnant vegetation in Western Australia. The primary objective of this project being to determine the amount, distribution and regional location of native vegetation remaining on private land in the agricultural area of south-west Western Australia
Extreme Galactic-Winds and Starburst in IR Mergers and IR QSOs
We report -as a part of a long-term study of mergers and IR QSOs- detailed
spectroscopic evidences for outflow (OF) and/or Wolf Rayet features in: (i) low
velocity OF in the ongoing mergers NGC 4038/39 and IRAS 23128-5919; (ii)
extreme velocity OF (EVOF) in the QSOs IRAS 01003-2238 and IRAS 13218+0552;
(iii) OF and EVOF in a complete sample of ultra-luminous IR galaxies/QSOs ("The
IRAS 1 Jy MKO-KPNO Survey", of 118 objects). We found EVOF in IRAS 11119+3257,
14394+5332, 15130+1958 and 15462-0450. The OF components detected in these
objects were mainly associated to starburst processes: i.e., to galactic-winds
generated in multiple type II SN explosions and massive stars. The EVOF were
detected in objects with strong starburst plus obscured IR QSOs; which suggest
that interaction of both processes could generate EVOF. In addition, we analyze
the presence of Wolf Rayet features in the large sample of Bright PG-QSOs
(Boroson and Green 1992), and nearby mergers and galactic-wind galaxies. We
found clear WR features in the Fe II QSOs (type I): PG 1244+026, 1444+407,
1448+273, 1535+547; and in the IR merger Arp 220. HST archive images of IR+BAL
QSOs show in practically all of these objects "arc or shell" features probably
associated to galactic-winds (i.e., to multiple type II SN explosions) and/or
merger processes. Finally, we discuss the presence of extreme starburst and
galactic wind as a possible evolutive link between IR merger and IR QSOs; where
the relation between mergers and extreme starburst (with powerful
galactic-winds) plays in important role, in the evolution of galaxies.Comment: 14 pages, 6 figure
Forming Galaxies with MOND
Beginning with a simple model for the growth of structure, I consider the
dissipationless evolution of a MOND-dominated region in an expanding Universe
by means of a spherically symmetric N-body code. I demonstrate that the final
virialized objects resemble elliptical galaxies with well-defined relationships
between the mass, radius, and velocity dispersion. These calculations suggest
that, in the context of MOND, massive elliptical galaxies may be formed early
(z > 10) as a result of monolithic dissipationless collapse. Then I reconsider
the classic argument that a galaxy of stars results from cooling and
fragmentation of a gas cloud on a time scale shorter than that of dynamical
collapse. Qualitatively, the results are similar to that of the traditional
picture; moreover, the existence, in MOND, of a density-temperature relation
for virialized, near isothermal objects as well as a mass-temperature relation
implies that there is a definite limit to the mass of a gas cloud where this
condition can be met-- an upper limit corresponding to that of presently
observed massive galaxies.Comment: 9 pages, 9 figures, revised in response to comments of referee. Table
added, extended discussion, accepted MNRA
Persistence effects in deterministic diffusion
In systems which exhibit deterministic diffusion, the gross parameter
dependence of the diffusion coefficient can often be understood in terms of
random walk models. Provided the decay of correlations is fast enough, one can
ignore memory effects and approximate the diffusion coefficient according to
dimensional arguments. By successively including the effects of one and two
steps of memory on this approximation, we examine the effects of
``persistence'' on the diffusion coefficients of extended two-dimensional
billiard tables and show how to properly account for these effects, using walks
in which a particle undergoes jumps in different directions with probabilities
that depend on where they came from.Comment: 7 pages, 7 figure
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