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Galaxies, Volume 13, Issue 1 (February 2025) – 11 articles

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19 pages, 5613 KiB  
Review
Quantum Technologies for the Einstein Telescope
by Mikhail Korobko
Galaxies 2025, 13(1), 11; https://doi.org/10.3390/galaxies13010011 - 5 Feb 2025
Viewed by 515
Abstract
Quantum technology is central to the operation of modern gravitational-wave detectors and will play crucial role in the success of next-generation observatories, such as the Einstein Telescope. There, quantum squeezed light will be utilized to suppress quantum noise across the entire detection band, [...] Read more.
Quantum technology is central to the operation of modern gravitational-wave detectors and will play crucial role in the success of next-generation observatories, such as the Einstein Telescope. There, quantum squeezed light will be utilized to suppress quantum noise across the entire detection band, a task that demands advancements in several areas of quantum technology. This review provides an introduction to the quantum technologies employed in gravitational-wave detection and explores in detail their properties, challenges, and the potential they hold for the Einstein Telescope. Full article
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23 pages, 3450 KiB  
Article
Kinematics of the Milky Way from the Statistical Analysis of the Gaia Data Release 3
by Petr Zavada and Karel Píška
Galaxies 2025, 13(1), 10; https://doi.org/10.3390/galaxies13010010 - 3 Feb 2025
Viewed by 381
Abstract
The aim of the analysis of data from the Gaia Space Observatory is to obtain kinematic parameters of the collective motion of stars in a part of our galaxy. This research is based on a statistical analysis of the motion of [...] Read more.
The aim of the analysis of data from the Gaia Space Observatory is to obtain kinematic parameters of the collective motion of stars in a part of our galaxy. This research is based on a statistical analysis of the motion of 55,038,539 stars selected in different directions from the Sun up to a distance of 3–6 kpc. We developed statistical methods for the analysis working with input data represented by the full astrometric solution (five parameters). Using the proposed statistical methods, we obtained the local velocity of the Sun U,V,W=(9.58,16.25,7.33)±(0.05,0.04,0.02)stat±(0.7,0.9,0.1)syst km/s and the rotation velocity of the galaxy at different radii. For the Sun’s orbit radius, we obtained the velocity of the galaxy rotationVc234±4 km/s. Collective rotation slows down in the region under study linearly with distance from the disk plane: ΔV/ΔZ33.5kms1kpc1. We showed that the different kinematic characteristics and distributions, which depend on the position in the galaxy, can be well described in the studied 3D region by a simple Monte Carlo simulation model, representing an axisymmetric approximation of the galaxy kinematics. The optimal values of the six free parameters were tuned by comparison with the data. Full article
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9 pages, 1558 KiB  
Article
Magnetic Noise Mitigation Strategies for the Einstein Telescope Infrastructure
by Barbara Garaventa, Federico Armato, Andrea Chincarini and Irene Fiori
Galaxies 2025, 13(1), 9; https://doi.org/10.3390/galaxies13010009 - 31 Jan 2025
Viewed by 320
Abstract
The Einstein Telescope (ET) will be a third-generation Gravitational Wave (GW) detector that will tackle cutting-edge technological challenges. The ET will be constructed at a depth of 200–300 m to isolate it from vibrations caused by seismic waves and human activities, which are [...] Read more.
The Einstein Telescope (ET) will be a third-generation Gravitational Wave (GW) detector that will tackle cutting-edge technological challenges. The ET will be constructed at a depth of 200–300 m to isolate it from vibrations caused by seismic waves and human activities, which are sources of noise for GW detection. To meet the ET’s objectives, it will be necessary to improve low-frequency sensitivity by about two orders of magnitude compared to current interferometers (LIGO, Virgo). Magnetic noise is a limiting noise in the frequency range from a few Hz up to around 100 Hz in future GW detectors. This article will discuss the magnetic noise mitigation strategies under development, based on experiences from Virgo. Full article
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7 pages, 215 KiB  
Review
Going Forward to Unveil the Nature of γ Cas Analogs
by Yaël Nazé
Galaxies 2025, 13(1), 8; https://doi.org/10.3390/galaxies13010008 - 24 Jan 2025
Viewed by 283
Abstract
The star γ Cas and its analogs are a subset of Be stars that display particularly hard and bright thermal X-ray emission, which has no equivalent among other massive stars. Here, I will review their characteristics and present the latest results of our [...] Read more.
The star γ Cas and its analogs are a subset of Be stars that display particularly hard and bright thermal X-ray emission, which has no equivalent among other massive stars. Here, I will review their characteristics and present the latest results of our optical and X-ray monitoring campaigns, including an assessment of the links between the circumstellar environment and the high-energy properties. Possible scenarios to explain this phenomenon will be presented in light of these observational results. Full article
(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)
16 pages, 502 KiB  
Review
A Review of Long-Lasting Activities of the Central Engine of Gamma-Ray Bursts
by Bruce Gendre
Galaxies 2025, 13(1), 7; https://doi.org/10.3390/galaxies13010007 - 10 Jan 2025
Viewed by 525
Abstract
Gamma-ray bursts are known to display various features on top of their canonical behavior. In this short review, we will describe and discuss two of them: the ultra-long gamma-ray bursts, which are defined by an extreme duration of their prompt phase, and the [...] Read more.
Gamma-ray bursts are known to display various features on top of their canonical behavior. In this short review, we will describe and discuss two of them: the ultra-long gamma-ray bursts, which are defined by an extreme duration of their prompt phase, and the plateau phase, which is defined by a steady phase of large duration at the start of the afterglow. We will review the main properties of those two phenomena, and will discuss their possible origin, in light of the standard fireball model of gamma-ray bursts. A final section will discuss the future missions, which could bring new evidence to the study of those objects. Full article
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18 pages, 714 KiB  
Article
Implications of the Intriguing Constant Inner Mass Surface Density Observed in Dark Matter Halos
by Jorge Sánchez Almeida
Galaxies 2025, 13(1), 6; https://doi.org/10.3390/galaxies13010006 - 9 Jan 2025
Viewed by 395
Abstract
It has long been known that the observed mass surface density of cored dark matter (DM) halos is approximately constant, independently of the galaxy mass (i.e., ρcrcconstant, with ρc and rc being the central volume [...] Read more.
It has long been known that the observed mass surface density of cored dark matter (DM) halos is approximately constant, independently of the galaxy mass (i.e., ρcrcconstant, with ρc and rc being the central volume density and the radius of the core, respectively). Here, we review the evidence supporting this empirical fact as well as its theoretical interpretation. It seems to be an emergent law resulting from the concentration–halo mass relation predicted by the current cosmological model, where the DM is made of collisionless cold DM particles (CDM). We argue that the prediction ρcrcconstant is not specific to this particular model of DM but holds for any other DM model (e.g., self-interacting) or process (e.g., stellar or AGN feedback) that redistributes the DM within halos conserving its CDM mass. In addition, the fact that ρcrcconstant is shown to allow the estimate of the core DM mass and baryon fraction from stellar photometry alone is particularly useful when the observationally expensive conventional spectroscopic techniques are unfeasible. Full article
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8 pages, 1006 KiB  
Article
Three Stages in Evolution of the Object Th4-4
by Lyudmila Kondratyeva, Eduard Denissyuk, Saule Shomshekova, Inna Reva, Gauhar Aimanova and Maxim Krugov
Galaxies 2025, 13(1), 5; https://doi.org/10.3390/galaxies13010005 - 7 Jan 2025
Viewed by 365
Abstract
The results of observations of the object Th4-4 in 1970–2024 showed that during this period the object went through several stages, when its characteristics corresponded to different classes of objects: Be star, symbiotic star and planetary nebula. Currently, the object appears to be [...] Read more.
The results of observations of the object Th4-4 in 1970–2024 showed that during this period the object went through several stages, when its characteristics corresponded to different classes of objects: Be star, symbiotic star and planetary nebula. Currently, the object appears to be a low-excitation planetary nebula. Full article
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17 pages, 332 KiB  
Article
Black Holes and Baryon Number Violation: Unveiling the Origins of Early Galaxies and the Low-Mass Gap
by Merab Gogberashvili and Alexander S. Sakharov
Galaxies 2025, 13(1), 4; https://doi.org/10.3390/galaxies13010004 - 3 Jan 2025
Viewed by 570
Abstract
We propose that modifications to the Higgs potential within a narrow atmospheric layer near the event horizon of an astrophysical black hole could significantly enhance the rate of sphaleron transitions, as well as transform the Chern–Simons number into a dynamic variable. As a [...] Read more.
We propose that modifications to the Higgs potential within a narrow atmospheric layer near the event horizon of an astrophysical black hole could significantly enhance the rate of sphaleron transitions, as well as transform the Chern–Simons number into a dynamic variable. As a result, sphaleron transitions in this region occur without suppression, in contrast to low-temperature conditions, and each transition may generate a substantially greater baryon number than would be produced by winding around the Higgs potential in Minkowski spacetime. This effect amplifies baryon number violation near the black hole horizon, potentially leading to a considerable generation of matter. Given the possibility of a departure from equilibrium during the absorption of matter and the formation of relativistic jets in supermassive black holes, we conjecture that this process could contribute to the creation of a significant amount of matter around such black holes. This phenomenon may offer an alternative explanation for the rapid growth of supermassive black holes and their surrounding galaxies in the early Universe, as suggested by recent observations from the James Webb Space Telescope. Furthermore, this mechanism may provide insights into the low-mass gap puzzle, addressing the observed scarcity of black holes with masses near the Oppenheimer–Volkoff limit. Full article
51 pages, 15203 KiB  
Review
High-Contrast Imaging: Hide and Seek with Exoplanets
by Riccardo Claudi and Dino Mesa
Galaxies 2025, 13(1), 3; https://doi.org/10.3390/galaxies13010003 - 31 Dec 2024
Viewed by 670
Abstract
So far, most of the about 5700 exoplanets have been discovered mainly with radial velocity and transit methods. These techniques are sensitive to planets in close orbits, not being able to probearge star–planet separations. μ-lensing is the indirect method that allows us [...] Read more.
So far, most of the about 5700 exoplanets have been discovered mainly with radial velocity and transit methods. These techniques are sensitive to planets in close orbits, not being able to probearge star–planet separations. μ-lensing is the indirect method that allows us to probe the planetary systems at the snow-line and beyond, but it is not a repeatable observation. On the contrary, direct imaging (DI) allows for the detection and characterization ofow mass companions at wide separation (≤5–6 au). The main challenge of DI is that a typical planet–star contrast ranges from 10−6, for a young Jupiter in emittedight, to 10−9 for Earth in reflectedight. In theast two decades, aot of efforts have been dedicated to combiningarge (D ≥ 5 m) telescopes (to reduce the impact of diffraction) with coronagraphs and high-order adaptive optics (to correct phase errors induced by atmospheric turbulence), with sophisticated image post-processing, to reach such a contrast between the star and the planet in order to detect and characterize cooler and closer companions to nearby stars. Building on the first pioneering instrumentation, the second generation of high-contrast imagers, SPHERE, GPI, and SCExAO, allowed us to probe hundreds of stars (e.g., 500–600 stars using SHINE and GPIES), contributing to a better understanding of the demography and the occurrence of planetary systems. The DI offers a possible clear vision for studying the formation and physical properties of gas giant planets and brown dwarfs, and the future DI (space and ground-based) instruments with deeper detectionimits will enhance this vision. In this paper, we briefly review the methods, the instruments, the main sample of targeted stars, the remarkable results, and the perspective of this rising technique. Full article
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33 pages, 1413 KiB  
Review
Gamma-Ray Bursts: What Do We Know Today That We Did Not Know 10 Years Ago?
by Asaf Pe’er
Galaxies 2025, 13(1), 2; https://doi.org/10.3390/galaxies13010002 - 31 Dec 2024
Viewed by 660
Abstract
I discuss here the progress made in the last decade on a few of the key open problems in GRB physics. These include (1) the nature of GRB progenitors, and the outliers found to the collapsar/merger scenarios; (2) jet structures, whose existence became [...] Read more.
I discuss here the progress made in the last decade on a few of the key open problems in GRB physics. These include (1) the nature of GRB progenitors, and the outliers found to the collapsar/merger scenarios; (2) jet structures, whose existence became evident following GRB/GW170817; (3) the great progress made in understanding the GRB jet launching mechanisms, enabled by general-relativistic magnetohydrodynamic (GR-MHD) codes; (4) recent studies of magnetic reconnection as a valid energy dissipation mechanism; (5) the early afterglow, which may be highly affected by a wind bubble, as well as recent indication that in many GRBs, the Lorentz factor is only a few tens, rather than a few hundreds. I highlight some recent observational progress, including the major breakthrough in detecting TeV photons and the on-going debate about their origin, polarization measurements, as well as the pair annihilation line recently detected in GRB 221009A, and its implications for prompt emission physics. I probe into some open questions that I anticipate will be at the forefront of GRB research in the next decade. Full article
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31 pages, 1262 KiB  
Article
Distinguishing Black Holes and Superspinars in Spinning Modified Gravity via Generalized Spin Precession Frequency
by Parthapratim Pradhan
Galaxies 2025, 13(1), 1; https://doi.org/10.3390/galaxies13010001 - 31 Dec 2024
Viewed by 440
Abstract
We compute the generalized spin precession frequency (Ωp) of a test gyroscope in a stationary spacetime, specifically for a Kerr–MOG black hole within the framework of scalar–tensor–vector gravity (STVG), also known as modified gravity (MOG). A comprehensive analysis of the [...] Read more.
We compute the generalized spin precession frequency (Ωp) of a test gyroscope in a stationary spacetime, specifically for a Kerr–MOG black hole within the framework of scalar–tensor–vector gravity (STVG), also known as modified gravity (MOG). A comprehensive analysis of the generalized spin frequency was conducted for non-extremal Kerr–MOG black hole, extremal Kerr–MOG black hole, and naked singularity or superspinar, in comparison to non-extremal Kerr black hole, extremal Kerr black hole, and Kerr naked singularity or Kerr superspinar. The generalized spin frequency we derived can be expressed in terms of the black hole mass parameter, the angular momentum parameter, and the MOG parameter. Additionally, we distinguish between non-extremal black hole, extremal black hole, and naked singularity through the computation of the aforementioned precession frequency. Furthermore, we calculate the generalized spin frequency for various angular coordinates, ranging from the polar to the equatorial plane. Lastly, we determine three fundamental epicyclic frequencies, the Keplerian frequency, the radial epicyclic frequency, and the vertical epicyclic frequency, to differentiate these three types of objects. We also compute the periastron frequency and nodal frequency. Utilizing these frequency profiles allows for the distinction of these three compact objects. Full article
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