Galaxies

37 pages, 40033 KB

Open AccessArticle

Late-Time Radio Diagnostics of Magnetar Magnetic Burial and Reemergence in GRB Afterglows

by Nissim Fraija, C. G. Bernal, A. Galván, B. Betancourt Kamenetskaia and M. G. Dainotti

Galaxies 2025, 13(6), 127; https://doi.org/10.3390/galaxies13060127 - 4 Nov 2025

Recent centimeter-to-millimeter monitoring of nearby gamma-ray bursts (GRBs) has revealed late-time (

≳ 10^{2}

–

10^{4}

days) radio rebrightenings and spectral turnovers not explained by standard forward-shock scenarios with steady microphysics. We attribute these features to a buried millisecond magnetar whose [...] Read more.

Recent centimeter-to-millimeter monitoring of nearby gamma-ray bursts (GRBs) has revealed late-time (

≳ 10^{2}

–

10^{4}

days) radio rebrightenings and spectral turnovers not explained by standard forward-shock scenarios with steady microphysics. We attribute these features to a buried millisecond magnetar whose surface dipole, initially submerged by early fallback (hours after birth), re-emerges via Hall–Ohmic diffusion on year–to–decade timescales, partially re-energizing the external shock. We combine a minimally parametric analytic framework with axisymmetric magnetohydrodynamic simulations of the hypercritical fallback phase to characterize burial depths and the initial conditions for reemergence. The growth of the external dipole is modeled as

\dot{E} (t) \propto {\dot{E}}_{0} f_{G} {(t)}^{σ}

and calibrated against physically plausible diffusion timescales

τ_{m} \sim years

–

decades

. Spin-down power couples to the afterglow through the surrounding ejecta via a single effective coupling factor and a causal delay kernel, encapsulating mediation by supernova ejecta/pulsar-wind nebulae in collapsars and by merger ejecta/winds in compact-object mergers. Applied to a representative set of events with late-time radio detections and upper limits, our scheme reproduces the observed rebrightenings and turnovers with modest coupling efficiencies. Within this picture, late-time centimeter–millimeter afterglows provide a practical diagnostic of magnetic-burial depth and crustal conductivity in newborn magnetars powering GRB afterglows, and motivate systematic radio follow-up hundreds to thousands of days after the trigger. Full article

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22 pages, 10394 KB

Open AccessArticle

Applications of the Irbene Single-Baseline Radio Interferometer

by Ivar Shmeld, Vladislavs Bezrukovs, Jānis Šteinbergs, Karina Šķirmante, Artis Aberfelds, Sergey A. Belov, Ross A. Burns, Dmitrii Y. Kolotkov, Valery M. Nakariakov, Dmitrijs Bezrukovs, Matīss Purviņš, Aija Kalniņa, Arturs Orbidans, Marcis Bleiders and Marina Konuhova

Galaxies 2025, 13(6), 126; https://doi.org/10.3390/galaxies13060126 - 3 Nov 2025

Abstract

The Irbene single-baseline radio interferometer (ISBI), operated by the Ventspils International Radio Astronomy Centre (VIRAC), offers a rare and versatile configuration in modern radio astronomy. Combining the 32-m and 16-m fully steerable parabolic radio telescopes separated by an 800-m baseline, this system possesses [...] Read more.

The Irbene single-baseline radio interferometer (ISBI), operated by the Ventspils International Radio Astronomy Centre (VIRAC), offers a rare and versatile configuration in modern radio astronomy. Combining the 32-m and 16-m fully steerable parabolic radio telescopes separated by an 800-m baseline, this system possesses a unique capability for high-sensitivity, time-domain interferometric observations. Unlike large interferometric arrays optimized for sub-arcsecond resolution imaging, the Irbene system is tailored for studies that require high temporal resolution and a strong signal-to-noise ratio. This paper reviews key scientific applications of the Irbene interferometer, including simultaneous methanol maser and radio continuum variability studies, high-cadence monitoring of quasi-periodic pulsations (QPPs) in stellar flares, ionospheric diagnostics using GNSS signals, orbit determination of navigation satellites and forward scatter radar techniques for space object detection. These diverse applications demonstrate the scientific potential of compact interferometric systems in an era dominated by large-scale observatories. Full article

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8 pages, 369 KB

Open AccessArticle

Using Light Curve Derivatives to Estimate the Fill-Out Factor of Overcontact Binaries

by Shinjirou Kouzuma

Galaxies 2025, 13(6), 125; https://doi.org/10.3390/galaxies13060125 - 31 Oct 2025

Abstract

We propose a simple method for estimating the fill-out factor of overcontact binary systems using the derivatives of light curves. We synthesized 74,431 sample light curves, covering the typical parameter space of overcontact binaries. On the basis of a recent study that proposed [...] Read more.

We propose a simple method for estimating the fill-out factor of overcontact binary systems using the derivatives of light curves. We synthesized 74,431 sample light curves, covering the typical parameter space of overcontact binaries. On the basis of a recent study that proposed a new classification scheme using light curve derivatives up to the fourth order, the sample light curves were classified. Among the classified types, for systems exhibiting high mass ratios and high inclinations (i.e., SPf type), we found that the fill-out factor has a strong correlation with the time interval between two local extrema in the third derivatives of their light curves. An empirical formula to estimate the fill-out factor was derived using regression analysis for the identified correlation. Application to real overcontact binary data demonstrated that the proposed method is practical for obtaining reliable estimates of the fill-out factor and its associated uncertainties. Full article

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20 pages, 7428 KB

Open AccessArticle

Reinforcement Learning-Driven Framework for High-Precision Target Tracking in Radio Astronomy

by Tanawit Sahavisit, Popphon Laon, Supavee Pourbunthidkul, Pattharin Wichittrakarn, Pattarapong Phasukkit and Nongluck Houngkamhang

Galaxies 2025, 13(6), 124; https://doi.org/10.3390/galaxies13060124 - 31 Oct 2025

Abstract

Radio astronomy requires precise target localization and tracking to ensure accurate observations. Conventional regulation methodologies, encompassing PID controllers, frequently encounter difficulties due to orientation inaccuracies precipitated by mechanical limitations, environmental fluctuations, and electromagnetic interferences. To tackle these obstacles, this investigation presents a reinforcement [...] Read more.

Radio astronomy requires precise target localization and tracking to ensure accurate observations. Conventional regulation methodologies, encompassing PID controllers, frequently encounter difficulties due to orientation inaccuracies precipitated by mechanical limitations, environmental fluctuations, and electromagnetic interferences. To tackle these obstacles, this investigation presents a reinforcement learning (RL)-oriented framework for high-accuracy monitoring in radio telescopes. The suggested system amalgamates a localization control module, a receiver, and an RL tracking agent that functions in scanning and tracking stages. The agent optimizes its policy by maximizing the signal-to-noise ratio (SNR), a critical factor in astronomical measurements. The framework employs a reconditioned 12-m radio telescope at King Mongkut’s Institute of Technology Ladkrabang (KMITL), originally constructed as a satellite earth station antenna for telecommunications and was subsequently refurbished and adapted for radio astronomy research. It incorporates dual-axis servo regulation and high-definition encoders. Real-time SNR data and streaming are supported by a HamGeek ZedBoard with an AD9361 software-defined radio (SDR). The RL agent leverages the Proximal Policy Optimization (PPO) algorithm with a self-attention actor–critic model, while hyperparameters are tuned via Optuna. Experimental results indicate strong performance, successfully maintaining stable tracking of randomly moving, non-patterned targets for over 4 continuous hours without any external tracking assistance, while achieving an SNR improvement of up to 23.5% compared with programmed TLE-based tracking during live satellite experiments with Thaicom-4. The simplicity of the framework, combined with its adaptability and ability to learn directly from environmental feedback, highlights its suitability for next-generation astronomical techniques in radio telescope surveys, radio line observations, and time-domain astronomy. These findings underscore RL’s potential to enhance telescope tracking accuracy and scalability while reducing control system complexity for dynamic astronomical applications. Full article

(This article belongs to the Special Issue Recent Advances in Radio Astronomy)

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18 pages, 16328 KB

Open AccessReview

Radio Astronomy with NASA’s Deep Space Network

by T. Joseph W. Lazio and Stephen M. Lichten

Galaxies 2025, 13(6), 123; https://doi.org/10.3390/galaxies13060123 - 31 Oct 2025

Abstract

The Deep Space Network (DSN) is the spacecraft tracking and communication infrastructure for NASA’s deep space missions. At three sites, approximately equally separated in (terrestrial) longitude, there are multiple radio antennas outfitted with cryogenic microwave receiving systems both for receiving transmissions from deep [...] Read more.

The Deep Space Network (DSN) is the spacecraft tracking and communication infrastructure for NASA’s deep space missions. At three sites, approximately equally separated in (terrestrial) longitude, there are multiple radio antennas outfitted with cryogenic microwave receiving systems both for receiving transmissions from deep space spacecraft and for conducting radio astronomical observations, particularly in the L band (1350 MHz–1800 MHz), X band (8200 MHz–8600 MHz), and K band (18 GHz–27 GHz). In particular, the 70 m antennas at the Canberra and Madrid DSN Complexes are well-equipped to participate in international very long baseline interferometry (VLBI) observations. Over the past five years, there has been an effort to refurbish and modernize equipment such as receiving and signal transport systems for radio astronomical observations. We summarize current capabilities, on-going refurbishment activities, and possible future opportunities. Full article

(This article belongs to the Special Issue Recent Advances in Radio Astronomy)

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13 pages, 1202 KB

Open AccessArticle

A Century of Studies of the Object with the B[e] Phenomenon HD 50138

by Holly Buroughs, Anatoly S. Miroshnichenko, Steve Danford, Alicia N. Aarnio, Sergei V. Zharikov, Hans Van Winckel, Nadine Manset, Ashish Raj, Stephen Drew Chojnowski, Gregor Rauw and Azamat A. Khokhlov

Galaxies 2025, 13(6), 122; https://doi.org/10.3390/galaxies13060122 - 30 Oct 2025

Abstract

HD 50138 is a 6.6 mag emission-line B–type star, whose nature is still controversial. It has been thought to be a pre-main-sequence Herbig Be star and an evolved object with the B[e] phenomenon, possibly a mass-transferring binary system. However, it has mostly been [...] Read more.

HD 50138 is a 6.6 mag emission-line B–type star, whose nature is still controversial. It has been thought to be a pre-main-sequence Herbig Be star and an evolved object with the B[e] phenomenon, possibly a mass-transferring binary system. However, it has mostly been studied on short timescales. We collected ∼1000 medium- and high-resolution spectra and available optical photometric data, which cover a time frame from 1981 to 2025, and extended the study from emission lines to a range of absorption lines. A few episodes of dramatic emission-line strength variations were uncovered as well as fast variations of the absorption line widths on timescales of several days. We also found a few previously unreported fadings of the star’s optical brightness seemingly associated with the Hα line profile changes. At the same time, it is still unclear whether the object is a single star or a binary system, as no regular variations of its observed parameters have been detected. Full article

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11 pages, 340 KB

Open AccessArticle

EZ Lyn: A Confirmed Period-Bouncer Cataclysmic Variable Below the Period Minimum

by Nadezhda L. Vaidman, Almansur T. Agishev, Serik A. Khokhlov and Aldiyar T. Agishev

Galaxies 2025, 13(6), 121; https://doi.org/10.3390/galaxies13060121 - 30 Oct 2025

Abstract

We model the short-period cataclysmic variable EZ Lyn with MESA binary evolution and infer its present-day parameters through a staged statistical search. First, we compute a coarse grid of tracks in

(M_{1, 0}, P_{0})

at fixed [...] Read more.

We model the short-period cataclysmic variable EZ Lyn with MESA binary evolution and infer its present-day parameters through a staged statistical search. First, we compute a coarse grid of tracks in

(M_{1, 0}, P_{0})

at fixed

M_{2, 0}

and rank snapshots by a profile likelihood. We then resample the neighbourhood of the minimum to build a refined

Δ χ^{2}

surface. Finally, we sample this surface with an affine-invariant MCMC to obtain posteriors, using a likelihood that treats the one-sided constraint on the donor temperature and the ambiguity of component roles in the binary output. The best-fit snapshot reproduces the observables and identifies EZ Lyn as a period bouncer with a substellar donor. We infer

M_{WD} = 0.850 \pm 0.019 M_{⊙}

,

M_{2} = 0.0483 \pm 0.0137 M_{⊙}

,

R_{WD} = 0.0092 \pm 0.0001 R_{⊙}

,

R_{2} = 0.099 \pm 0.005 R_{⊙}

,

T_{WD} = 11, 500 \pm 20 K

, and

T_{2} = 1600 \pm 50 K

. The instantaneous mass-transfer rate at the best-fit snapshot is

\dot{M} = 3.66 \times 10^{- 11} M_{⊙} {yr}^{- 1}

, consistent with the secular range implied by the white-dwarf temperature. Independent checks from the Roche mean-density relation, surface gravities, and the semi-empirical donor sequence support the solution. In population context, EZ Lyn lies in the period-minimum spike and on the low-mass tail of the donor mass–period plane. The classification is robust to modest displacements along the shallow

Δ χ^{2}

valley. We release inlists, tracks, and analysis scripts for reproducibility. Full article

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21 pages, 4883 KB

Open AccessArticle

VERA’s 20 yr Evolution in Science and Technology

by Mareki Honma, Tomoya Hirota, Tomoaki Oyama and Akiharu Nakagawa

Galaxies 2025, 13(6), 120; https://doi.org/10.3390/galaxies13060120 - 27 Oct 2025

Abstract

We review the past 20 yr evolution of VERA (VLBI Exploration of Radio Astrometry) in both science and techinology. VERA is a VLBI array in Japan which consists of four 20 m-diameter telescopes, originally dedicated to phase-referencing VLBI astrometry. Its main observing bands [...] Read more.

We review the past 20 yr evolution of VERA (VLBI Exploration of Radio Astrometry) in both science and techinology. VERA is a VLBI array in Japan which consists of four 20 m-diameter telescopes, originally dedicated to phase-referencing VLBI astrometry. Its main observing bands are K (22 GHz) and Q (43 GHz) for conducting astrometry observations of H₂O and SiO maser sources. In its 20 yr history, VERA has conducted astrometry observations of ∼100 maser sources, revealing the three-dimensional structure of the Milky Way Galaxy. Its long-term observations of Sgr A* resulted in the first parallax detection of the super-massive black hole at the Galaxy center. Observations of maser sources also revealed physical properties of star-forming regions and provided calibration of AGB stars’ distances and their Period–Luminosity relation. In parallel, several upgrades have been carried out in receivers as well as digital back-ends and correlator to extend the frequency bands and the data rate. Full article

(This article belongs to the Special Issue Recent Advances in Radio Astronomy)

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32 pages, 578 KB

Open AccessArticle

Natural Constants Determined to High Precision from Boltzmann’s Constant and Avogadro’s Number—A Challengeto Experiments and Astrophysical Observations to Match the Precision of the Results

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

Galaxies 2025, 13(6), 119; https://doi.org/10.3390/galaxies13060119 - 27 Oct 2025

Abstract

In this investigation, we explore previously unknown relations between natural constants by taking the following steps: (1) We discard Dirac’s constant ℏ from the universal man-made constants of physics, which we redefine in terms of Planck’s constant h. (2) Working in the [...] Read more.

In this investigation, we explore previously unknown relations between natural constants by taking the following steps: (1) We discard Dirac’s constant ℏ from the universal man-made constants of physics, which we redefine in terms of Planck’s constant h. (2) Working in the SI system of units, we determine Newton’s gravitational constant G from Boltzmann’s constant

k_{B}

and the elementary charge e, recognizing the entropy of matter as their common underlying characteristic. (3) By comparing the mass of 1 mole of electrons to the h-defined Planck mass

M_{P}

, we deduce nature’s own molar constant (

≃

0.1 mol) that contains a ‘reduced Avogadro number’

ℵ_{A} = N_{A} / f_{A}

of particles, where

N_{A}

is Avogadro’s number and

f_{A} ≃ 10

is the associated Avogadro factor. (4) From the new effective gravitational constant

G_{★} \equiv 4 π ε_{0} G

, where

ε_{0}

is the vacuum permittivity, we obtain MOND’s universal constant

A_{0}

and its critical acceleration

a_{0}

, recognizing the Newtonian source of gravity as the common underlying characteristic and repudiating the need for a principle of equivalence of masses. (5) We derive the gravitational coupling constant

α_{g}

solely from

ℵ_{A}

. (6) We adopt the measured value of the h-defined fine-structure constant (FSC)

α

and the value of

α_{g}

(or, equivalently, nature’s

ℵ_{A}

), and we determine the relative ratio

β_{g} = α_{g} / α

precise to 10 significant digits. (7) We derive the relative strong ratio

β_{s} = α_{s} / α

directly from the Avogadro factor

f_{A}

. (8) We determine the coupling constants of weak and strong interactions (

α_{w}

and

α_{s}

, respectively) in terms of the FSC

α

. (9) The relation

α_{w} = \sqrt{α}

leads to a determination of the mass of the W boson

m_{W}

from the measured values of

α

and the reduced Fermi constant

G_{F}^{0}

. (10) Using the Planck mass as a principal constant (

M_{P} = ℵ_{A} m_{e}

, where

m_{e}

is the electron mass), we obtain new classical definitions of

h, α

, and the Compton radius

r_{c}

; and we reformulate in a transparent, geometrically clear way several important QED equations, as well as the extended Planck system of units itself. We discuss the implications of these results, and we pave a way forward in exploring the unification of the fundamental forces of nature. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

32 pages, 5647 KB

Open AccessArticle

The Physical and Mathematical Meaning of Temperature and Its Implications for Astronomy

by Robert E. Criss and Anne M. Hofmeister

Galaxies 2025, 13(6), 118; https://doi.org/10.3390/galaxies13060118 - 24 Oct 2025

Abstract

Temperatures reported for astronomical objects are commonly extreme, and all are ascertained indirectly, using spectroscopy. However, narrow spectral peaks record microscopic behavior (transitions), whereas temperature is a macroscopic (bulk) feature of an object. Using macroscopic theories of heat, light, and their transport, we [...] Read more.

Temperatures reported for astronomical objects are commonly extreme, and all are ascertained indirectly, using spectroscopy. However, narrow spectral peaks record microscopic behavior (transitions), whereas temperature is a macroscopic (bulk) feature of an object. Using macroscopic theories of heat, light, and their transport, we show that temperature is best defined in terms of the radiant flux of an object (Stefan–Boltzmann law)—including that from large gas bodies—because this flux defines which objects are hotter or colder, and because relevance to mathematical order is the essential attribute of any measurable quantity. Laboratory examples further show that spectroscopic determinations of temperature require the following: (1) use of a large spectral range relevant to that temperature; (2) observation of the unique peak shape characteristic of thermal emissions; (3) accounting for reflections at surfaces; and, most importantly, (4) that conditions are optically thick, a condition fostered by large object size and high temperatures. Temperature of monatomic gas is accurately described by classical kinetic theory because molecular translations are unaffected by electron dynamics. Inelastic molecular collisions provide continuous thermal emissions under optically thick conditions attained in immense astronomical environments. We show how thermal and non-thermal spectroscopic features can be distinguished. Our findings are applied to star-forming regions, intergalactic media, lightning, the Sun’s surface and the corona. Our results resolve long-standing problems regarding heat sources. Full article

(This article belongs to the Special Issue Alternative Interpretations of Observed Galactic Behaviors)

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22 pages, 1103 KB

Open AccessReview

Dwarf Galaxies in Focus: A Survey of Observational and Theoretical Studies

by Komiljon Tillaboev, Ikram Tadjibaev, Kamolidin Mamadaliyev, Dildor Otajonova, Malokhat Atajonova and Bagzodbek Abdullayev

Galaxies 2025, 13(5), 117; https://doi.org/10.3390/galaxies13050117 - 21 Oct 2025

Abstract

This paper presents provides a comprehensive survey of dwarf galaxies, which represent the most numerous and diverse systems in the Universe. We discuss their definitions and morphological classifications, emphasizing the unique properties that distinguish them from globular clusters and giant galaxies. Special attention [...] Read more.

This paper presents provides a comprehensive survey of dwarf galaxies, which represent the most numerous and diverse systems in the Universe. We discuss their definitions and morphological classifications, emphasizing the unique properties that distinguish them from globular clusters and giant galaxies. Special attention is given to their formation and evolutionary processes in the framework of hierarchical structure formation and ΛCDM cosmology, including the role of environmental mechanisms and stellar feedback. Star formation histories are explored based on observations and simulations, highlighting both bursty and extended activity across different dwarf types. We further examine the crucial role of dark matter in shaping the dynamics and structure of dwarf galaxies, as well as the core–cusp and missing satellites problems. Finally, we summarize insights from numerical simulations and theoretical models, which provide a bridge between observations and cosmological predictions. This synthesis demonstrates that dwarf galaxies remain essential laboratories for testing galaxy formation theories and probing the nature of dark matter. Full article

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19 pages, 274 KB

Open AccessArticle

Stark Broadening of O I Spectral Lines

by Milan S. Dimitrijević and Sylvie Sahal-Bréchot

Galaxies 2025, 13(5), 116; https://doi.org/10.3390/galaxies13050116 - 15 Oct 2025

Abstract

We do not know a priori chemical composition of a star. However, with more high resolution spectra becoming more abundant thanks to the development of space-born observations, atomic data including Stark broadening parameters for various spectral lines for elements in various ionisation stages [...] Read more.

We do not know a priori chemical composition of a star. However, with more high resolution spectra becoming more abundant thanks to the development of space-born observations, atomic data including Stark broadening parameters for various spectral lines for elements in various ionisation stages are becoming more feasible. Particularly are important spectral lines of C-N-O peak in the distribution of abundances of chemical elements. For the calculation of Stark broadening parameters, spectral line full widths at half intensity maximum (FWHM) and shifts, we used semiclassical perturbation method. As the result, Stark widths and shifts for 36 spectral lines of neutral oxygen, broadened by the collisions with electrons, protons and helium ions, have been obtained and compared with other theoretical calculations. These data are of interest for a number of problems in astrophysics, plasma physics, as well as for inertial fusion and various plasmas in technology. Full article

(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)

21 pages, 2190 KB

Open AccessArticle

Evolution of Size, Mass, and Density of Galaxies Since Cosmic Dawn

by Rajendra P. Gupta

Galaxies 2025, 13(5), 115; https://doi.org/10.3390/galaxies13050115 - 10 Oct 2025

Abstract

The formation and evolution of galaxies and other astrophysical objects have become of great interest, especially since the launch of the James Webb Space Telescope in 2021. The mass, size, and density of objects in the early universe appear to be drastically different [...] Read more.

The formation and evolution of galaxies and other astrophysical objects have become of great interest, especially since the launch of the James Webb Space Telescope in 2021. The mass, size, and density of objects in the early universe appear to be drastically different from those predicted by the standard cosmology—the

Λ

CDM model. This work shows that the mass–size–density evolution is not surprising when we use the CCC+TL cosmology, which is based on the concepts of covarying coupling constants in an expanding universe and the tired light effect contributing to the observed redshift. This model is consistent with supernovae Pantheon+ data, the angular size of the cosmic dawn galaxies, BAO, CMB sound horizon, galaxy formation time scales, time dilation, galaxy rotation curves, etc., and does not have the coincidence problem. The effective radii

r_{e}

of the objects are larger in the new model by

r_{e} \propto {(1 + z)}^{0.93}

. Thus, the object size evolution in different studies, estimated as

r_{e} \propto {(1 + z)}^{s}

with

s = - 1.0

±

0.3

, is modified to

r_{e} \propto {(1 + z)}^{s + 0.93}

, the dynamical mass by

{(1 + z)}^{0.93},

and number density by

{(1 + z)}^{- 2.80}

. The luminosity modification increases slowly with

z

to 1.8 at

z = 20

. Thus, the stellar mass increase is modest, and the luminosity and stellar density decrease are mainly due to the larger object size in the new model. Since the aging of the universe is stretched in the new model, its temporal evolution is much slower (e.g., at

z = 10

, the age is about a dex longer); stars, black holes, and galaxies do not have to form at unrealistic rates. Full article

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44 pages, 3213 KB

Open AccessSystematic Review

A Systematic Literature Review of Machine Learning Techniques for Observational Constraints in Cosmology

by Luis Rojas, Sebastián Espinoza, Esteban González, Carlos Maldonado and Fei Luo

Galaxies 2025, 13(5), 114; https://doi.org/10.3390/galaxies13050114 - 9 Oct 2025

Abstract

This paper presents a systematic literature review focusing on the application of machine learning techniques for deriving observational constraints in cosmology. The goal is to evaluate and synthesize existing research to identify effective methodologies, highlight gaps, and propose future research directions. Our review [...] Read more.

This paper presents a systematic literature review focusing on the application of machine learning techniques for deriving observational constraints in cosmology. The goal is to evaluate and synthesize existing research to identify effective methodologies, highlight gaps, and propose future research directions. Our review identifies several key findings: (1) Various machine learning techniques, including Bayesian neural networks, Gaussian processes, and deep learning models, have been applied to cosmological data analysis, improving parameter estimation and handling large datasets. However, models achieving significant computational speedups often exhibit worse confidence regions compared to traditional methods, emphasizing the need for future research to enhance both efficiency and measurement precision. (2) Traditional cosmological methods, such as those using Type Ia Supernovae, baryon acoustic oscillations, and cosmic microwave background data, remain fundamental, but most studies focus narrowly on specific datasets. We recommend broader dataset usage to fully validate alternative cosmological models. (3) The reviewed studies mainly address the

H_{0}

tension, leaving other cosmological challenges—such as the cosmological constant problem, warm dark matter, phantom dark energy, and others—unexplored. (4) Hybrid methodologies combining machine learning with Markov chain Monte Carlo offer promising results, particularly when machine learning techniques are used to solve differential equations, such as Einstein Boltzmann solvers, prior to Markov chain Monte Carlo models, accelerating computations while maintaining precision. (5) There is a significant need for standardized evaluation criteria and methodologies, as variability in training processes and experimental setups complicates result comparability and reproducibility. (6) Our findings confirm that deep learning models outperform traditional machine learning methods for complex, high-dimensional datasets, underscoring the importance of clear guidelines to determine when the added complexity of learning models is warranted. Full article

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7 pages, 477 KB

Open AccessCommunication

Nucleosynthesis of Cobalt and Copper

by Beatriz Barbuy, Amâncio C. S. Friaça and Heitor Ernandes

Galaxies 2025, 13(5), 113; https://doi.org/10.3390/galaxies13050113 - 22 Sep 2025

Abstract

Chemical abundances of cobalt (Co; Z = 27) and copper (Cu; Z = 29) in bulge and halo stars are presented and compared with chemical evolution models. The aim is to distinguish if Co and Cu are dominantly produced by neutron-capture or the [...] Read more.

Chemical abundances of cobalt (Co; Z = 27) and copper (Cu; Z = 29) in bulge and halo stars are presented and compared with chemical evolution models. The aim is to distinguish if Co and Cu are dominantly produced by neutron-capture or the alpha-rich freeze-out processes. Neutron-capture can be identified by a secondary behaviour in the [X/Fe] vs. [Fe/H] plot, and alpha-rich freeze-out would give rather a primary behaviour. Full article

(This article belongs to the Special Issue Neutron Capture Processes in the Universe)

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34 pages, 4140 KB

Open AccessReview

GW170817: A Short Review of the First Multimessenger Event in Gravitational Astronomy

by Rosa Poggiani

Galaxies 2025, 13(5), 112; https://doi.org/10.3390/galaxies13050112 - 19 Sep 2025

Abstract

The first detection of gravitational waves from the binary black merger GW150914 started the era of gravitational astronomy. The observation of the binary neutron star merger GW170817 and of its associated electromagnetic counterpart GRB 170817A started multi-messenger gravitational astronomy. This short review discusses [...] Read more.

The first detection of gravitational waves from the binary black merger GW150914 started the era of gravitational astronomy. The observation of the binary neutron star merger GW170817 and of its associated electromagnetic counterpart GRB 170817A started multi-messenger gravitational astronomy. This short review discusses the discovery of GW170817 and the follow-up of the electromagnetic counterpart, together with the broad range of results in astrophysics and fundamental physics, including the Gamma-Ray Burst field. The GW170817/GRB 170817A observation showed that binary neutron star mergers can explain at least a fraction of short Gamma-Ray Bursts. The optical and infrared evolution of the associated AT 2017gfo transient showed that binary neutron star mergers are sites of r-process nucleo-synthesis. The combination of gravitational and electromagnetic observations has been used to estimate the Hubble parameter, the speed of gravitational waves, and the equation of state of nuclear matter. The increasing sensitivity of interferometric detectors and the forthcoming operation of third generation detectors will lead to an improved statistics of binary neutron star mergers. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts in Multiwavelength: Theory, Observational Correlations and GRB Cosmology)

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41 pages, 12018 KB

Open AccessReview

Timing Analysis of Black Hole X-Ray Binaries with Insight-HXMT

by Haifan Zhu and Wei Wang

Galaxies 2025, 13(5), 111; https://doi.org/10.3390/galaxies13050111 - 19 Sep 2025

Abstract

The Hard X-ray Modulation Telescope (HXMT), China’s first X-ray astronomy satellite, has significantly contributed to the study of fast variability in black hole X-ray binaries through its broad energy coverage (1–250 keV), high timing resolution, and sensitivity to hard X-rays. This review presents [...] Read more.

The Hard X-ray Modulation Telescope (HXMT), China’s first X-ray astronomy satellite, has significantly contributed to the study of fast variability in black hole X-ray binaries through its broad energy coverage (1–250 keV), high timing resolution, and sensitivity to hard X-rays. This review presents a comprehensive overview of timing analysis techniques applied to black hole X-ray binaries using Insight-HXMT data. We introduce the application and comparative strengths of several time-frequency analysis methods, including traditional Fourier analysis, wavelet transform, bicoherence analysis, and Hilbert-Huang transform. These methods offer complementary insights into the non-stationary and nonlinear variability patterns observed in black hole X-ray binaries, particularly during spectral state transitions and quasi-periodic oscillations. We discuss how each technique has been employed in recent Insight-HXMT studies to characterize timing features such as low-frequency QPOs, phase lags, and power spectrum evolution across different energy bands. Moreover, we present novel phenomena revealed by Insight-HXMT observations, including the detection of high-energy QPOs, spectral parameter modulation with QPO phase, and a new classification scheme for QPO types. The integration of multiple analysis methods enables a more nuanced understanding of the accretion dynamics and the geometry of the inner accretion flow, shedding light on fundamental physical processes in relativistic environments. Full article

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27 pages, 1734 KB

Open AccessArticle

Comparative Photometry of the Quiet Quasar PDS 456 and the Radio-Loud Blazar 3C 273

by Alberto Silva Betzler, Ingrid dos Santos Delfino, Agábio Brasil dos Santos, Roberto Mendes Dias and Orahcio Felicio de Sousa

Galaxies 2025, 13(5), 110; https://doi.org/10.3390/galaxies13050110 - 15 Sep 2025

Abstract

A comparative analysis of the photometric variability of the blazar 3C 273 and the quasar PDS 456 using multi-band data from ground- and space-based platforms (2015–2025) reveals contrasting behaviors. For 3C 273, a statistically significant secular dimming was detected in the ATLASc [...] Read more.

A comparative analysis of the photometric variability of the blazar 3C 273 and the quasar PDS 456 using multi-band data from ground- and space-based platforms (2015–2025) reveals contrasting behaviors. For 3C 273, a statistically significant secular dimming was detected in the ATLASc-band light curve

((5.6 \pm 0.2) \times 10^{- 4} mag {day}^{- 1})

and confirmed by Johnson–Cousins V-band photometry. Ten optical flares were identified, two coinciding with Fermi gamma-ray enhancements, suggesting a synchrotron origin linked to jet activity. A significant bluer-when-brighter trend (

ρ = - 0.54

) was found relative to the o-band, and several color extrema align with gamma-ray activity, reinforcing the nonthermal interpretation. In contrast, PDS 456 exhibits a statistically significant secular brightening in the o-band

((- 3.1 \pm 0.2) \times 10^{- 5} mag {day}^{- 1})

and 75 optical flares, four coinciding with UV flares observed by Swift/UVOT. The

c - o

color index displays a non-Gaussian distribution with asymmetric reddening and blueing episodes. An extreme reddening event aligns with a strong UV flare, suggesting transient inner-disk heating. These results indicate jet-dominated variability in 3C 273 and disk-driven variability in PDS 456, highlighting distinct physical mechanisms in radio-loud versus radio-quiet active galactic nuclei. Full article

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17 pages, 1635 KB

Open AccessReview

Optical and X-Ray Variability of Gamma Cas Analogs

by Alexander Kholtygin

Galaxies 2025, 13(5), 109; https://doi.org/10.3390/galaxies13050109 - 15 Sep 2025

Abstract

γ

Cas analogs is an enigmatic group of Be stars with unusually hard X-rays and an X-ray luminosity of

10^{31}

–

10^{33}

erg

s^{- 1}

, which is higher than a typical value for classical Be stars. The evolutionary status [...] Read more.

γ

Cas analogs is an enigmatic group of Be stars with unusually hard X-rays and an X-ray luminosity of

10^{31}

–

10^{33}

erg

s^{- 1}

, which is higher than a typical value for classical Be stars. The evolutionary status of these mysterious objects and the nature of their X-ray emission remains disputable. I suppose that our understanding of this mystery is in the detailed studies of their optical and X-ray variability on the various time scales from very short to very long. In the present paper the optical and X-ray spectral and photometric observations of these stars are reviewed. The contemporary assumptions on the mechanisms of X-ray radiation generation of

γ

Cas analogs are discussed. It is concluded that the analysis of the binarity of

γ

Cas analog helps to understand their inexplicable nature. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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