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Galaxies

Galaxies is an international, peer-reviewed, open access journal on astronomy, astrophysics, and cosmology published bimonthly online by MDPI.

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All Articles (1,249)

  • Article
  • Open Access

We investigate the fundamental kinematic parameters of stellar motions in the Milky Way using a statistical sample of 27,485,720 stars with measured three-dimensional velocities from the Gaia DR3 catalogue. From these data, we determine the local solar velocity , the average rotation velocity V0=228±4 km/s, and the circular velocity Vc=234±4 km/s, both at the radius R⊙. Next, within the range of 3−14 kpc, we derive the Galactic circular velocity curve Vc(R) and determine the average spherical components of the stellar velocities as functions of both Galactocentric radius R and distance from the Galactic plane Z. Two-dimensional R−Z maps of these components reveal a complex three-dimensional kinematic structure. In particular, the pattern observed at R≲5 kpc can be associated with the influence of the Galactic bar. To facilitate the description of the observations, we employ a simplified axisymmetric Monte Carlo model with several free parameters that reproduces the main kinematic trends.

Galaxies

13 November 2026

Distribution of distances from the Sun (left) and from the galactic plane (right). Units: 
  
    r
    ,
    z
  
 [kpc]. Bin widths: 0.024 and 0.008 kpc.
  • Article
  • Open Access

We examine the family-separated seesaw (FSS) ansatz proposed in arXiv:2605.27049v2. Combining its paired cancellations with exact partial unitarity fixes the light and heavy mixing blocks in terms of one unitary matrix and the paired mass ratios. We classify their unitary completions and reconstruct the Dirac and singlet Majorana mass matrices, distinguishing the physical heavy masses from the singlet mass parameters. This identifies FSS as an exact realization of form dominance whose conventional one-loop leptogenesis cancellation is already established. The particular sum of lepton-number-violating invariants entering the proposed decay asymmetry therefore vanishes; individual CP invariants need not vanish. We also correct the heavy-mass reconstruction formulas and quantify their errors for both light-mass orderings. These formulas are conditional algebraic relations, not independent predictions of the heavy spectrum. Numerical mass–mixing illustrations, explicit chiral projections for double-beta decay, and a leading-logarithmic estimate of radiative misalignment clarify the physical reach and limitations. The contribution is an exact consistency analysis and correction of specific proposed consequences rather than a new mass-generation mechanism or a general prohibition of leptogenesis.

Galaxies

13 November 2026

  • Article
  • Open Access

Black Hole Thermodynamics from Macroscopic Vacuum Elasticity

  • Dimitris M. Christodoulou,
  • Demosthenes Kazanas and
  • Silas G. T. Laycock

For the past fifty years, the physical foundations of black hole (BH) thermodynamics have remained widely contested because their original derivations rest at the theoretical intersection of general relativity and quantum field theory (QFT). This work provides an alternative macroscopic formulation by treating the physical vacuum in and around a BH as an elastic continuum strained by the presence of mass. Utilizing Hooke’s law of elasticity, we find that the radial strain at the event horizon of every Schwarzschild BH saturates at the same constant value. Under this principle of universal maximum strain, the laws of BH thermodynamics can be formulated within macroscopic continuum mechanics. This dispenses with the conventional QFT derivation in curved spacetime, paralleling the recent macroscopic descriptions of the Casimir effect. Our calculations are carried out using the Reformulated Planck System (RPS), which relies exclusively on Planck’s constant h. This h-native approach prevents the dimensional contamination of 3D continuum mechanics caused by the 2D geometric factor inherently embedded in Dirac’s reduced constant ℏ, a pivotal misstep first recognized in the iconic definitions of the fine-structure constant α and the gravitational coupling constant αg, which long precluded the identification of the α-dependence of the weak coupling constant αw=α. In BH thermodynamics, this RPS-based approach yields higher Hawking temperatures and lower Bekenstein entropies, both by a factor of 2π.

Galaxies

29 October 2026

  • Article
  • Open Access

We study the Kretschmann invariant K of the post-Newtonian spacetime of a slowly rotating, axially symmetric clump of collisionless matter with a prescribed Miyamoto–Nagai potential, whose metric and matter content to second order in the compactness β were constructed in a companion paper. At O(β2), K is a quadratic form in the Hessian of the potential, with closed-form central and asymptotic values. At O(β3) we show that the static part of K is a closed-form local function of the potential and of the matter fields at every point: the second-order metric functions, which solve a Poisson-type problem and encode the global mass distribution, enter only through the Ricci tensor, that is, only through the local matter. This follows from an exact expression of K for static spacetimes in terms of the tidal tensor of static observers and the local stresses, and holds for any regular prescribed potential. At a centre with isotropic stresses it reduces to , also for composite bulge–disc models. The only non-local contribution at this order is gravitomagnetic; it vanishes at the centre and, for an isotropic rotator, reaches within six scale radii about 10% (round clump) to 40% (thin disc) of the static third-order term. For galaxy-scale compactness, β∼10−6, all relativistic corrections are of relative order 10−7–10−5; the third-order term exceeds 10% of K only for compactness ≳0.04, the regime of relativistic star clusters. A central black hole dominates K within about 25 pc of the centre of a Milky-Way-like bulge–disc model.

Galaxies

21 October 2026

Featured Articles of Last Quarter

Pulse profiles from the NuSTAR observation, with each panel corresponding to a selected energy band indicated in the figure. The gray-shaded regions denote the locations of the wing structures.
Outflow velocity (in v/c) from an ionized disk wind versus black hole mass for jetted and non-jetted AGN suggesting a stronger wind/jet tension in spiral AGN compared to elliptical AGN.

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Galaxies - ISSN 2075-4434