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Galaxies, Volume 2, Issue 3 (September 2014), Pages 292-465

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Research

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Open AccessArticle The Inner Regions of Disk Galaxies: A Constant Baryonic Fraction?
Galaxies 2014, 2(3), 292-299; doi:10.3390/galaxies2030292
Received: 10 April 2014 / Revised: 18 June 2014 / Accepted: 19 June 2014 / Published: 10 July 2014
Cited by 6 | PDF Full-text (224 KB) | HTML Full-text | XML Full-text
Abstract
For disk galaxies (spirals and irregulars), the inner circular-velocity gradient dRV0 (inner steepness of the rotation curve) correlates with the central surface brightness ∑*,0 with a slope of ~0.5. This implies that the central dynamical mass density scales [...] Read more.
For disk galaxies (spirals and irregulars), the inner circular-velocity gradient dRV0 (inner steepness of the rotation curve) correlates with the central surface brightness ∑*,0 with a slope of ~0.5. This implies that the central dynamical mass density scales almost linearly with the central baryonic density. Here I show that this empirical relation is consistent with a simple model where the central baryonic fraction ƒbar,0 is fixed to 1 (no dark matter) and the observed scatter is due to differences in the baryonic mass-to-light ratio Mbar / LR (ranging from 1 to 3 in the R-band) and in the characteristic thickness of the central stellar component Δz (ranging from 100 to 500 pc). Models with lower baryonic fractions are possible, although they require some fine-tuning in the values of Mbar/LR and Δz. Regardless of the actual value of ƒbar,0, the fact that different types of galaxies do not show strong variations in ƒbar,0 is surprising, and may represent a challenge for models of galaxy formation in a Λ Cold Dark Matter (ΛCDM) cosmology. Full article
(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)
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Review

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Open AccessReview Monolithic View of Galaxy Formation and Evolution
Galaxies 2014, 2(3), 300-381; doi:10.3390/galaxies2030300
Received: 4 March 2014 / Revised: 19 May 2014 / Accepted: 27 May 2014 / Published: 14 July 2014
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Abstract
We review and critically discuss the current understanding of galaxy formation and evolution limited to Early Type Galaxies (ETGs) as inferred from the observational data and briefly contrast the hierarchical and quasi-monolithic paradigms of formation and evolution. Since in Cold Dark Matter [...] Read more.
We review and critically discuss the current understanding of galaxy formation and evolution limited to Early Type Galaxies (ETGs) as inferred from the observational data and briefly contrast the hierarchical and quasi-monolithic paradigms of formation and evolution. Since in Cold Dark Matter (CDM) cosmogony small scale structures typically collapse early and form low-mass haloes that subsequently can merge to assembly larger haloes, galaxies formed in the gravitational potential well of a halo are also expected to merge thus assembling their mass hierarchically. Mergers should occur all over the Hubble time and large mass galaxies should be in place only recently. However, recent observations of high redshift galaxies tell a different story: massive ETGs are already in place at high redshift. To this aim, we propose here a revision of the quasi-monolithic scenario as an alternative to the hierarchical one, in which mass assembling should occur in early stages of a galaxy lifetime and present recent models of ETGs made of Dark and Baryonic Matter in a Λ-CDM Universe that obey the latter scheme. The galaxies are followed from the detachment from the linear regime and Hubble flow at z ≥ 20 down to the stage of nearly complete assembly of the stellar content (z ∼ 2 − 1) and beyond.  It is found that the total mass (Mh = MDM + MBM ) and/or initial over-density of the proto-galaxy drive the subsequent star formation histories (SFH). Massive galaxies (Mh ~ _1012M) experience a single, intense burst of star formation (with rates ≥ 103M⊙/yr) at early epochs, consistently with observations, with a weak dependence on the initial over-density; intermediate mass haloes (Mh~_ 1010 − 1011M⊙) have star formation histories that strongly depend on their initial over-density; finally, low mass haloes (Mh ~_ 109M⊙) always have erratic, burst-like star forming histories. The present-day properties (morphology, structure, chemistry and photometry) of the model galaxies closely resemble those of the real galaxies. In this context, we also try to cast light on the physical causes of the Stellar Mass-Radius Relation (MRR) of galaxies. The MRR is the result of two complementary mechanisms: i.e., local physical processes that fix the stellar mass and the radius of each galaxy and cosmological global, statistical principles, which shape the distribution of galaxies in the MR-plane. Finally, we also briefly comment on the spectro-photometric properties of the model galaxies and how nicely they match the observational data. The picture emerging from this analysis is that the initial physical conditions of a proto-galaxy, i.e., nature, seem to play the dominant role in building up the ETGs we see today, whereas nurture by recurrent captures of small objects is a secondary actor of the fascinating and intriguing story of galaxy formation and evolution. Full article
(This article belongs to the Special Issue Advances in Our Understanding of the Dynamics of Galaxies)
Open AccessReview Generalized Curvature-Matter Couplings in Modified Gravity
Galaxies 2014, 2(3), 410-465; doi:10.3390/galaxies2030410
Received: 30 May 2014 / Revised: 7 July 2014 / Accepted: 8 July 2014 / Published: 28 July 2014
Cited by 28 | PDF Full-text (450 KB) | HTML Full-text | XML Full-text
Abstract
In this work, we review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature R and the Lagrangian density of matter, induces a non-vanishing covariant derivative [...] Read more.
In this work, we review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature R and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor, implying non-geodesic motion and, consequently, leads to the appearance of an extra force. Applied to the cosmological context, these curvature-matter couplings lead to interesting phenomenology, where one can obtain a unified description of the cosmological epochs. We also consider the possibility that the behavior of the galactic flat rotation curves can be explained in the framework of the curvature-matter coupling models, where the extra terms in the gravitational field equations modify the equations of motion of test particles and induce a supplementary gravitational interaction. In addition to this, these models are extremely useful for describing dark energy-dark matter interactions and for explaining the late-time cosmic acceleration. Full article
(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

Other

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Open AccessConcept Paper Compton Composites Late in the Early Universe
Galaxies 2014, 2(3), 382-409; doi:10.3390/galaxies2030382
Received: 17 April 2014 / Revised: 25 June 2014 / Accepted: 26 June 2014 / Published: 15 July 2014
Cited by 2 | PDF Full-text (1241 KB) | HTML Full-text | XML Full-text
Abstract
Beginning roughly two hundred years after the big-bang, a tresino phase transition generated Compton-scale composite particles and converted most of the ordinary plasma baryons into new forms of dark matter. Our model consists of ordinary electrons and protons that have been bound [...] Read more.
Beginning roughly two hundred years after the big-bang, a tresino phase transition generated Compton-scale composite particles and converted most of the ordinary plasma baryons into new forms of dark matter. Our model consists of ordinary electrons and protons that have been bound into mostly undetectable forms. This picture provides an explanation of the composition and history of ordinary to dark matter conversion starting with, and maintaining, a critical density Universe. The tresino phase transition started the conversion of ordinary matter plasma into tresino-proton pairs prior to the the recombination era. We derive the appropriate Saha–Boltzmann equilibrium to determine the plasma composition throughout the phase transition and later. The baryon population is shown to be quickly modified from ordinary matter plasma prior to the transition to a small amount of ordinary matter and a much larger amount of dark matter after the transition. We describe the tresino phase transition and the origin, quantity and evolution of the dark matter as it takes place from late in the early Universe until the present. Full article
(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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