28,782 research outputs found
Symmetries in two-dimensional dilaton gravity with matter
The symmetries of generic 2D dilaton models of gravity with (and without)
matter are studied in some detail. It is shown that , one of the
symmetries of the matterless models, can be generalized to the case where
matter fields of any kind are present. The general (classical) solution for
some of these models, in particular those coupled to chiral matter, which
generalizes the Vaidya solution of Einstein Gravity, is also given.Comment: Minor changes have been made; the references have been updated and
some added; 11 pages. To appear in Phys. Rev.
The cosmological origin of the Tully-Fisher relation
We use high-resolution cosmological simulations that include the effects of
gasdynamics and star formation to investigate the origin of the Tully-Fisher
relation in the standard Cold Dark Matter cosmogony. Luminosities are computed
for each model galaxy using their full star formation histories and the latest
spectrophotometric models. We find that at z=0 the stellar mass of model
galaxies is proportional to the total baryonic mass within the virial radius of
their surrounding halos. Circular velocity then correlates tightly with the
total luminosity of the galaxy, reflecting the equivalence between mass and
circular velocity of systems identified in a cosmological context. The slope of
the relation steepens slightly from the red to the blue bandpasses, and is in
fairly good agreement with observations. Its scatter is small, decreasing from
\~0.45 mag in the U-band to ~0.34 mag in the K-band. The particular
cosmological model we explore here seems unable to account for the zero-point
of the correlation. Model galaxies are too faint at z=0 (by about two
magnitudes) if the circular velocity at the edge of the luminous galaxy is used
as an estimator of the rotation speed. The Tully-Fisher relation is brighter in
the past, by about ~0.7 magnitudes in the B-band at z=1, at odds with recent
observations of z~1 galaxies. We conclude that the slope and tightness of the
Tully-Fisher relation can be naturally explained in hierarchical models but
that its normalization and evolution depend strongly on the star formation
algorithm chosen and on the cosmological parameters that determine the
universal baryon fraction and the time of assembly of galaxies of different
mass.Comment: 5 pages, 4 figures included, submitted to ApJ (Letters
Extremal Charged Rotating Dilaton Black Holes in Odd Dimensions
Employing higher order perturbation theory, we find a new class of charged
rotating black hole solutions of Einstein-Maxwell-dilaton theory with general
dilaton coupling constant. Starting from the Myers-Perry solutions, we use the
electric charge as the perturbative parameter, and focus on extremal black
holes with equal-magnitude angular momenta in odd dimensions. We perform the
perturbations up to 4th order for black holes in 5 dimensions and up to 3rd
order in higher odd dimensions. We calculate the physical properties of these
black holes and study their dependence on the charge and the dilaton coupling
constant.Comment: 20 pages, 3 figure
Improving the Angular Resolution of EGRET and New Limits on Supersymmetric Dark Matter Near the Galactic Center
Using the EGRET data and an improved point source analysis, including an
energy dependent point spread function and an unbinned maximum likelihood
technique, we have been able to place considerably lower limits on the gamma
ray flux from the galactic center region. We also test this method on known
sources, the Crab and Vela pulsars. In both cases, we find that our method
improves the angular precision of EGRET data over the 3EG catalog.
This new limit on gamma rays from the galactic center can be used to test
models of annihilating supersymmetric dark matter and galactic halo profiles.
We find that the present EGRET data can limit many supersymmetric models if the
density of the galactic dark matter halo is cuspy or spiked toward the galactic
center. We also discuss the ability of GLAST to test these models.Comment: 6 pages, 4 figure
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