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Galaxies, Volume 2, Issue 2 (June 2014), Pages 189-291

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Research

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Open AccessArticle Mass Distribution in Rotating Thin-Disk Galaxies According to Newtonian Dynamics
Galaxies 2014, 2(2), 199-222; doi:10.3390/galaxies2020199
Received: 13 February 2014 / Revised: 31 March 2014 / Accepted: 8 April 2014 / Published: 14 April 2014
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Abstract
An accurate computational method is presented for determining the mass distribution in a mature spiral galaxy from a given rotation curve by applying Newtonian dynamics for an axisymmetrically rotating thin disk of finite size with or without a central spherical bulge. The governing
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An accurate computational method is presented for determining the mass distribution in a mature spiral galaxy from a given rotation curve by applying Newtonian dynamics for an axisymmetrically rotating thin disk of finite size with or without a central spherical bulge. The governing integral equation for mass distribution is transformed via a boundary-element method into a linear algebra matrix equation that can be solved numerically for rotation curves with a wide range of shapes. To illustrate the effectiveness of this computational method, mass distributions in several mature spiral galaxies are determined from their measured rotation curves. All the surface mass density profiles predicted by our model exhibit approximately a common exponential law of decay, quantitatively consistent with the observed surface brightness distributions. When a central spherical bulge is present, the mass distribution in the galaxy is altered in such a way that the periphery mass density is reduced, while more mass appears toward the galactic center. By extending the computational domain beyond the galactic edge, we can determine the rotation velocity outside the cut-off radius, which appears to continuously decrease and to gradually approach the Keplerian rotation velocity out over twice the cut-off radius. An examination of circular orbit stability suggests that galaxies with flat or rising rotation velocities are more stable than those with declining rotation velocities especially in the region near the galactic edge. Our results demonstrate the fact that Newtonian dynamics can be adequate for describing the observed rotation behavior of mature spiral galaxies. Full article
Open AccessArticle Satellite Galaxy Velocity Dispersions in the SDSS and Modified Gravity Models
Galaxies 2014, 2(2), 263-274; doi:10.3390/galaxies2020263
Received: 28 March 2014 / Revised: 22 April 2014 / Accepted: 24 April 2014 / Published: 5 May 2014
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Abstract
The Sloan Digital Sky Survey (SDSS) provides data on several hundred thousand galaxies. The precise location of these galaxies in the sky, along with information about their luminosities and line-of-sight (Doppler) velocities, allows one to construct a three-dimensional map of their location and
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The Sloan Digital Sky Survey (SDSS) provides data on several hundred thousand galaxies. The precise location of these galaxies in the sky, along with information about their luminosities and line-of-sight (Doppler) velocities, allows one to construct a three-dimensional map of their location and estimate their line-of-sight velocity dispersion. This information, in principle, allows one to test dynamical gravity models, specifically models of satellite galaxy velocity dispersions near massive hosts. A key difficulty is the separation of true satellites from interlopers. We sidestep this problem by not attempting to derive satellite galaxy velocity dispersions from the data, but instead incorporate an interloper background into the mathematical models and compare the result to the actual data. We find that due to the presence of interlopers, it is not possible to exclude several gravitational theories on the basis of the SDSS data. Full article
Open AccessArticle Quantum Gravity and Cosmological Density Perturbations
Galaxies 2014, 2(2), 275-291; doi:10.3390/galaxies2020275
Received: 14 February 2014 / Revised: 10 May 2014 / Accepted: 27 May 2014 / Published: 11 June 2014
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Abstract
We explore the possible cosmological consequences of a running Newton’s constant, G(⎕), as suggested by the non-trivial ultraviolet fixed point scenario for Einstein gravity with a cosmological constant term. Here, we examine what possible effects a scale-dependent coupling might have on large-scale
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We explore the possible cosmological consequences of a running Newton’s constant, G(⎕), as suggested by the non-trivial ultraviolet fixed point scenario for Einstein gravity with a cosmological constant term. Here, we examine what possible effects a scale-dependent coupling might have on large-scale cosmological density perturbations. Starting from a set of manifestly covariant effective field equations, we develop the linear theory of density perturbations for a non-relativistic perfect fluid. The result is a modified equation for the matter density contrast, which can be solved and thus provides an estimate for the corrections to the growth index parameter, ɤ. Full article
(This article belongs to the Special Issue Particle Physics and Quantum Gravity Implications for Cosmology)

Review

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Open AccessReview String Phenomenology: Past, Present and Future Perspectives
Galaxies 2014, 2(2), 223-258; doi:10.3390/galaxies2020223
Received: 28 February 2014 / Revised: 10 April 2014 / Accepted: 15 April 2014 / Published: 22 April 2014
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Abstract
The observation of a scalar resonance at the Large Hadron Collider (LHC), compatible with perturbative electroweak symmetry breaking, reinforces the Standard Model (SM) parameterisation of all subatomic data. The logarithmic evolution of the SM gauge and matter parameters suggests that this parameterisation remains
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The observation of a scalar resonance at the Large Hadron Collider (LHC), compatible with perturbative electroweak symmetry breaking, reinforces the Standard Model (SM) parameterisation of all subatomic data. The logarithmic evolution of the SM gauge and matter parameters suggests that this parameterisation remains viable up to the Planck scale, where gravitational effects are of comparable strength. String theory provides a perturbatively consistent scheme to explore how the parameters of the Standard Model may be determined from a theory of quantum gravity. The free fermionic heterotic string models provide concrete examples of exact string solutions that reproduce the spectrum of the Minimal Supersymmetric Standard Model. Contemporary studies entail the development of methods to classify large classes of models. This led to the discovery of exophobic heterotic-string vacua and the observation of spinor-vector duality, which provides an insight to the global structure of the space of (2,0) heterotic-string vacua. Future directions entail the study of the role of the massive string states in these models and their incorporation in cosmological scenarios. A complementary direction is the formulation of quantum gravity from the principle of manifest phase space duality and the equivalence postulate of quantum mechanics, which suggest that space is compact. The compactness of space, which implies intrinsic regularisation, may be tightly related to the intrinsic finite length scale, implied by string phenomenology. Full article
(This article belongs to the Special Issue Particle Physics and Quantum Gravity Implications for Cosmology)

Other

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Open AccessShort Communication Musings on Firewalls and the Information Paradox
Galaxies 2014, 2(2), 189-198; doi:10.3390/galaxies2020189
Received: 31 December 2013 / Revised: 3 April 2014 / Accepted: 8 April 2014 / Published: 14 April 2014
Cited by 2 | HTML Full-text
Abstract
The past year has seen an explosion of new and old ideas about black hole physics. Prior to the firewall paper, the dominant picture was the thermofield model apparently implied by anti-de Sitter conformal field theory duality. While some seek a narrow responce
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The past year has seen an explosion of new and old ideas about black hole physics. Prior to the firewall paper, the dominant picture was the thermofield model apparently implied by anti-de Sitter conformal field theory duality. While some seek a narrow responce to Almheiri, Marolf, Polchinski, and Sully (AMPS) , there are a number of competing models. One problem in the field is the ambiguity of the competing proposals. Some are equivalent while others incompatible. This paper will attempt to define and classify a few models representative of the current discussions. Full article
(This article belongs to the Special Issue Aspects of Black Hole Physics)
Open AccessLetter The Lingering Anomalous Secular Increase of the Eccentricity of the Orbit of the Moon: Further Attempts of Explanations of Cosmological Origin
Galaxies 2014, 2(2), 259-262; doi:10.3390/galaxies2020259
Received: 18 April 2014 / Accepted: 25 April 2014 / Published: 2 May 2014
Cited by 5 | HTML Full-text
Abstract
New analyses of extended data records collected with the Lunar Laser Ranging (LLR) technique performed with improved tidal models were not able to resolve the issue of the anomalous rate e˙ of the eccentricity e of the orbit of the Moon, which
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New analyses of extended data records collected with the Lunar Laser Ranging (LLR) technique performed with improved tidal models were not able to resolve the issue of the anomalous rate e ˙ of the eccentricity e of the orbit of the Moon, which is still in place with a magnitude of e ˙ =(5±2)×10 12 y r 1 . Some possible cosmological explanations are offered in terms of the post-Newtonian effects of the cosmological expansion and of the slow temporal variation of the relative acceleration rate S ¨ S 1 of the cosmic scale factor S. None of them is successful since their predicted secular rates of the lunar eccentricity are too small by several orders of magnitude. Full article
(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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