Galaxies

17 pages, 320 KiB

Open AccessArticle

CMB Multipole Expansion in a Frame Dragging-Sustained Milky Way

by Federico Re, Marco Galoppo and Massimo Dotti

Galaxies 2025, 13(3), 71; https://doi.org/10.3390/galaxies13030071 - 13 Jun 2025

We study the impact on the cosmic microwave background (CMB) landscape of peculiar rotational general relativistic effects. These effects, on galactic scales, do not possess a Newtonian analogue, and therefore could a priori impact CMB analysis. We find that the velocity inferred from [...] Read more.

We study the impact on the cosmic microwave background (CMB) landscape of peculiar rotational general relativistic effects. These effects, on galactic scales, do not possess a Newtonian analogue, and therefore could a priori impact CMB analysis. We find that the velocity inferred from the CMB dipole, under the kinematic interpretation, coincides with that measured by a stationary observer within the Milky Way and not with the one measured by the zero angular momentum observer. We show that the galaxy peculiar frame-dragging effects do not impact the standard CMB analysis, as these modify the multipole coefficients only at higher orders with respect to the dominant terms. Moreover, we prove that no general relativistic framework at the galactic scale patched within the standard cosmological model can account for the current tension on the CMB quadrupole amplitude. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum—2nd Edition)

7 pages, 1181 KiB

Open AccessCommunication

The Enigmatic, Highly Variable, High-Mass Young Stellar Object Mol 12: A New Extreme Herbig Be (Proto)star

by Mauricio Tapia, Paolo Persi, Jesús Hernández and Nuria Calvet

Galaxies 2025, 13(3), 70; https://doi.org/10.3390/galaxies13030070 - 13 Jun 2025

Abstract

We report new medium-resolution spectroscopy covering the wavelength range from 0.6 to 2.4

μ

m, as well as multi-epoch, multi-wavelength photometry, of the Class I high-mass embedded young stellar object Mol 12 (IRAS 05373+2349). It is embedded (

A_{V} ≃ 12

) [...] Read more.

We report new medium-resolution spectroscopy covering the wavelength range from 0.6 to 2.4

μ

m, as well as multi-epoch, multi-wavelength photometry, of the Class I high-mass embedded young stellar object Mol 12 (IRAS 05373+2349). It is embedded (

A_{V} ≃ 12

) in the centre of a dense core at a distance of 1.59 kpc from the Sun and has a total luminosity of

1.74 \times 10^{3} L_{⊙}

. The spectra show a large number of permitted atomic emission lines, mostly for Fe, H, C, N, and Ca, that originate in the inner zones of a very active protoplanetary disc and no photospheric absorption lines. Conspicuously, the He I line at 1.0830

μ

m displays a complex P-Cygni profile. Also, the first overtone CO emission band-heads at 2.3

μ

m are seen in emission. From the strengths of the principal emission lines, we determined the accretion rate and luminosity to be

\dot{M} \sim 10^{- 5} M_{⊙}

y⁻¹ and

L_{acc} \sim 10^{3} L_{⊙}

, respectively. Decade-long light curves show a series of irregular brightness dips of more than four magnitudes in r, becoming shallower as the wavelength increases and disappearing at

λ > 3 μ

m. The colour–magnitude diagrams suggest the occurrence of a series of eclipses caused by the passage of small dust cloudlets in front of the star, producing more than 10 magnitudes of extra extinction. Full article

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

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28 pages, 1004 KiB

Open AccessArticle

Numerical Solutions and Stability Analysis of White Dwarfs with a Generalized Anisotropic Factor

by Ayazhan Orazymbet, Aray Muratkhan, Daniya Utepova, Nurzada Beissen, Gulzada Baimbetova and Saken Toktarbay

Galaxies 2025, 13(3), 69; https://doi.org/10.3390/galaxies13030069 - 12 Jun 2025

Abstract

This study examines the equilibrium structure and stability of white dwarfs, incorporating both isotropic and anisotropic pressure distributions. The Tolman–Oppenheimer–Volkoff (TOV) equation is numerically solved using the Chandrasekhar equation of state (EoS) to analyze the effects of pressure anisotropy. A general anisotropy function [...] Read more.

This study examines the equilibrium structure and stability of white dwarfs, incorporating both isotropic and anisotropic pressure distributions. The Tolman–Oppenheimer–Volkoff (TOV) equation is numerically solved using the Chandrasekhar equation of state (EoS) to analyze the effects of pressure anisotropy. A general anisotropy function is introduced to close and solve the system of differential equations. The results indicate that anisotropy remains negligible at the center and increases toward the stellar surface. Stability is assessed using the speed of sound criterion,

v_{s}^{2} = d p / d ρ

, and the Buchdahl bound,

2 M / R < 8 / 9

, confirming that white dwarfs remain within stability limits. We performed a sensitivity analysis to examine how variations in the anisotropy parameter

α_{0}

and central density affect the mass, radius, and compactness of white dwarfs. Additionally, we calculated the gravitational redshift at the stellar surface and found that it varies with anisotropy, ranging from

z_{s} \sim 3.15 \times 10^{- 3}

in isotropic cases to

z_{s} \sim 0.2 \times 10^{- 3}

in highly anisotropic models. These results link anisotropy to potentially observable features. The findings suggest that while anisotropy does not significantly affect the overall equilibrium structure, it may play a role in astrophysical scenarios involving strong magnetic fields, rotational deformations, or accretion processes in binary systems. Full article

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10 pages, 1248 KiB

Open AccessArticle

Post-AGB Binaries as Interacting Systems

by Hans Van Winckel

Galaxies 2025, 13(3), 68; https://doi.org/10.3390/galaxies13030068 - 12 Jun 2025

Abstract

We present recent progress in our understanding of the physical interaction mechanisms at work in evolved binaries of low-to-intermediate initial mass, which are surrounded by a stable disc of gas and dust. These systems are known as post-asymptotic giant-branch (post-AGB) binaries, but recently, [...] Read more.

We present recent progress in our understanding of the physical interaction mechanisms at work in evolved binaries of low-to-intermediate initial mass, which are surrounded by a stable disc of gas and dust. These systems are known as post-asymptotic giant-branch (post-AGB) binaries, but recently, it has been shown that some systems are too low in luminosity and should be considered as post-red-giant branch (post-RGB) instead. While the systems are currently well within their Roche lobe, they still show signs of active ongoing interaction between the different building blocks. We end this contribution with some future research plans. Full article

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

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15 pages, 922 KiB

Open AccessArticle

Searching for New Objects with the B[e] Phenomenon

by Aisha Zh. Naurzbayeva, Nazgul Sh. Alimgazinova, Anatoly S. Miroshnichenko, Corinne Rossi, Inna V. Reva, Raushan I. Kokumbayeva, Chingis T. Omarov, Sergei V. Zharikov, Nadine Manset, Ashish Raj, Arti Joshi, Richard J. Rudy, Richard C. Puetter, Raleigh B. Perry and Kunduz M. Turekhanova

Galaxies 2025, 13(3), 67; https://doi.org/10.3390/galaxies13030067 - 11 Jun 2025

Viewed by 83

Abstract

Objects with the B[e] phenomenon, whose defining features are the presence of forbidden emission lines and infrared excess coming from circumstellar dust, represent a broad range of evolutionary stages from pre-main-sequence to planetary nebulae. They are important for understanding mechanisms of the circumstellar [...] Read more.

Objects with the B[e] phenomenon, whose defining features are the presence of forbidden emission lines and infrared excess coming from circumstellar dust, represent a broad range of evolutionary stages from pre-main-sequence to planetary nebulae. They are important for understanding mechanisms of the circumstellar matter formation and evolution. However, it is not easy to discover them, especially among faint stars, as forbidden emission lines are usually weak and hardly noticeable in low-resolution spectra. We developed photometric criteria to search for candidate objects with this phenomenon based on a combination of optical and near-infrared color indices and found nearly 40 objects that satisfy these criteria. Spectroscopy of the candidates allows us to make more confident conclusions on their classification. We present the results of our photometric and spectroscopic observations of six objects, which are part of a large list of ∼40 objects that satisfy our photometric selection criteria for candidate objects with the B[e] phenomenon. Forbidden lines of neutral oxygen were clearly detected in the optical spectrum of one object (VES 683) and suspected in three others. One object, AS 415, is most likely a binary system with components that exhibit partial eclipses but without the B[e] phenomenon, while IRAS 20402 + 4638 may be a luminous member of the FS CMa objects group. Full article

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

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19 pages, 2881 KiB

Open AccessReview

Red Supergiants in the Milky Way and Nearby Galaxies

by Alceste Z. Bonanos

Galaxies 2025, 13(3), 66; https://doi.org/10.3390/galaxies13030066 - 5 Jun 2025

Viewed by 347

Abstract

Identifications of red supergiants (RSGs) in the Milky Way and nearby galaxies have experienced an exponential increase in recent years, driven by advancements in selection techniques, the continued expansion of archival datasets, and a steady increase in spectroscopic data. This review describes the [...] Read more.

Identifications of red supergiants (RSGs) in the Milky Way and nearby galaxies have experienced an exponential increase in recent years, driven by advancements in selection techniques, the continued expansion of archival datasets, and a steady increase in spectroscopic data. This review describes the advances in methodologies and selection criteria for identifying RSGs and presents the current census of these stars in our own Galaxy and nearby galaxies. It also describes the insights gained from resolving nearby RSGs and their complex circumstellar material in the Milky Way and from the growing samples of RSGs being discovered in the Local Group and beyond. These advances impact the Humphreys–Davidson limit in the cool part of the Hertzsprung–Russell diagram. Furthermore, they provide insight into extreme RSGs and the role of photometric variability and, in particular, of the newly discovered phenomenon of dimming events. Recent observations have enabled the determination of the binarity fraction among RSGs, offering new constraints to stellar evolution. Looking ahead, the synergy between large-scale surveys, high-resolution observations, and emerging machine-learning tools promises to further transform our understanding of the final evolutionary stages of massive stars in the coming decade. Full article

(This article belongs to the Special Issue The Red Supergiants: Crucial Signposts for the Fate of Massive Stars)

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16 pages, 1893 KiB

Open AccessReview

Estimating Hubble Constant with Gravitational Observations: A Concise Review

by Rosa Poggiani

Galaxies 2025, 13(3), 65; https://doi.org/10.3390/galaxies13030065 - 4 Jun 2025

Viewed by 255

Abstract

The Hubble constant is of paramount importance in astrophysics and cosmology. A large number of methods have been developed with different electromagnetic probes to estimate its value. The most recent results show a tension between values obtained from Cosmic Microwave Background observations and [...] Read more.

The Hubble constant is of paramount importance in astrophysics and cosmology. A large number of methods have been developed with different electromagnetic probes to estimate its value. The most recent results show a tension between values obtained from Cosmic Microwave Background observations and supernovae. The simultaneous detection of gravitational waves and electromagnetic radiation from GW170817 provided a direct estimation of the Hubble constant that did not depend on the astronomical distance ladder. This concise review will present the methods to estimate the Hubble constant with the gravitational observations of compact binary mergers, discussing both bright and dark sirens and reporting the state of the art of the results. Full article

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

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32 pages, 2523 KiB

Open AccessReview

Dust at the Cosmic Dawn

by Yuri A. Shchekinov and Biman B. Nath

Galaxies 2025, 13(3), 64; https://doi.org/10.3390/galaxies13030064 - 23 May 2025

Viewed by 498

Abstract

Observations provided by the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) have revealed a surprising abundance of galaxies at the “cosmic dawn” epoch,

z > 7

. Some of them are found even in a more distant universe at z [...] Read more.

Observations provided by the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) have revealed a surprising abundance of galaxies at the “cosmic dawn” epoch,

z > 7

. Some of them are found even in a more distant universe at z ≃ 14–16. Most of these galaxies appear to be intriguing: they are found to be either super-bright in the rest-frame ultraviolet (UV) band or super-dusty with a heavily reddened stellar population. The transition from the super-bright and super-dusty regimes seems to occur in the redshift range from z∼10.5 to z∼9.5 within a time range of ∼50 Myr. If confirmed, then the origin of this transition is far from being clear. In the review, we discuss possible mechanisms that can make

z > 10

galaxies free of dust and also explain the origin of apparently excessive dust in galaxies at intermediate and lower redshifts

z < 10

. Full article

(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)

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11 pages, 682 KiB

Open AccessArticle

A Spectroscopic and Photometric Study of MWC 342 and Its B[e] Phenomenon over the Last 40 Years

by Aigerim Zh. Akniyazova, Anatoly S. Miroshnichenko, Sergey V. Zharikov, Hans Van Winckel, Nadine Manset, Ashish Raj, Stephen Drew Chojnowski, Serik A. Khokhlov, Inna V. Reva, Raushan I. Kokumbaeva, Chingis T. Omarov, Konstantin N. Grankin, Aldiyar T. Agishev and Nadezhda L. Vaidman

Galaxies 2025, 13(3), 63; https://doi.org/10.3390/galaxies13030063 - 20 May 2025

Viewed by 327

Abstract

MWC 342 (V1972 Cyg) was discovered nearly 90 years ago as an early-type emission-line star. It was among the first hot stars whose strong infrared excess was detected in the early 1970s. Several mostly short-term photometric and spectroscopic studies resulted in contradictory conclusions [...] Read more.

MWC 342 (V1972 Cyg) was discovered nearly 90 years ago as an early-type emission-line star. It was among the first hot stars whose strong infrared excess was detected in the early 1970s. Several mostly short-term photometric and spectroscopic studies resulted in contradictory conclusions about the nature and evolutionary status of MWC 342. It has been classified as a pre-main-sequence Herbig Be star, an evolved suspected binary system, and a long-period variable star. Suggestions on the nature of the secondary component to this B0/B1 primary included a cool M-type giant and an X-ray source. We collected medium- and high-resolution optical spectra of MWC 342 taken in 1994–2024 as well as optical photometric data taken in 1986–2024. Analysis of these data shows strong variations in the object’s brightness and spectral line properties at various time scales, but no strictly periodic phenomena have been found. Inparticular, such a long-term dataset allowed us to reveal the optical brightness variations over a nearly 20-year-long quasi-period, as well as their anti-correlation with the H

α

emission-line strength. Also, we did not confirm the presence of He ii emission lines and absorption lines of the star’s atmosphere that were suspected in previously published studies. Full article

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

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31 pages, 2268 KiB

Open AccessArticle

Early Optical Follow-Up Observations of Einstein Probe X-Ray Transients During the First Year

by Siyu Wu, Ignacio Pérez-García, Alberto J. Castro-Tirado, Youdong Hu, Maria Gritsevich, María D. Caballero-García, Rubén Sánchez-Ramírez, Sergiy Guziy, Emilio J. Fernández-García, Guillermo García Segura, Carlos Pérez-del-Pulgar, Dingrong Xiong and Bin-Bin Zhang

Galaxies 2025, 13(3), 62; https://doi.org/10.3390/galaxies13030062 - 19 May 2025

Viewed by 409

Abstract

We present early follow-up observations of Einstein Probe (EP) X-ray transients, following its first year of operation. EP is a dedicated wide-field X-ray observatory that is transforming our understanding of the dynamic X-ray universe. During its first year, EP successfully detected [...] Read more.

We present early follow-up observations of Einstein Probe (EP) X-ray transients, following its first year of operation. EP is a dedicated wide-field X-ray observatory that is transforming our understanding of the dynamic X-ray universe. During its first year, EP successfully detected a diverse range of high-energy transients—including gamma-ray bursts (GRBs), tidal disruption events (TDEs), and fast X-ray transients (FXTs), besides many stellar flares, disseminating 128 alerts in the aggregate. Ground-based optical follow-up observations, particularly those performed by our BOOTES telescope network, have played a crucial role in multi-wavelength campaigns carried out so far. Out of the 128 events, the BOOTES Network has been able to follow up 58 events, detecting 6 optical counterparts at early times. These complementary optical measurements have enabled rapid identification of counterparts, precise redshift determinations (such as EP250215a at

z = 4.61

), and detailed characterization of the transient phenomena. The synergy between EP’s cutting-edge X-ray monitoring and the essential optical follow-up provided by facilities, such as the above-mentioned BOOTES Global Network or other Spanish ground-based facilities we have access to, underscores the importance and necessity of coordinated observations in the era of time-domain and multi-messenger astrophysics. Full article

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

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11 pages, 887 KiB

Open AccessArticle

Investigation of the Nature of the B[e] Star CI Cam in the Optical Range

by Elena A. Barsukova, Vitaly P. Goranskij, Aleksandr N. Burenkov and Ilya A. Yakunin

Galaxies 2025, 13(3), 61; https://doi.org/10.3390/galaxies13030061 - 19 May 2025

Viewed by 329

Abstract

The B[e] phenomenon is observed in a wide range of stars at various evolutionary stages. Its nature remains uncertain. The B[e] phenomenon is defined as the simultaneous presence of low-excitation forbidden line emission and strong infrared excess in the spectra of early-type stars. [...] Read more.

The B[e] phenomenon is observed in a wide range of stars at various evolutionary stages. Its nature remains uncertain. The B[e] phenomenon is defined as the simultaneous presence of low-excitation forbidden line emission and strong infrared excess in the spectra of early-type stars. Here, we present new spectral observations of a representative of this class: the star CI Cam. A monitoring campaign was carried out for the He II 4686 Å emission line, which serves as an indicator of binarity in this system. The aim was to detect variations in this line not only due to orbital motion but also those associated with the pulsations of the system’s primary component, the B[e] star. Two maxima in the equivalent width were detected over the pulsation period, during which the equivalent width increased by a factor of three. We refine the classification of CI Cam, assigning it to the FS CMa group of B[e] stars by all criteria, and we refer the secondary component of the system to a group of recently discovered “stripped” stars. Full article

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

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15 pages, 2471 KiB

Open AccessArticle

Spectral and Photometric Studies of NGC 3516 in the Optical Range

by Saule Shomshekova, Alexander Serebryanskiy, Ludmila Kondratyeva, Nazim Huseynov, Samira Rahimli, Vitaliy Kim, Laura Aktay and Yerlan Aimuratov

Galaxies 2025, 13(3), 60; https://doi.org/10.3390/galaxies13030060 - 16 May 2025

Viewed by 355

Abstract

This paper presents the results of the photometric and spectral monitoring of the galaxy NGC 3516, which is an active galactic nucleus (AGN) of type Sy

1.5

with a changing look. Observations were carried out at the Fesenkov Astrophysical Institute (FAI, Almaty, Kazakhstan) [...] Read more.

This paper presents the results of the photometric and spectral monitoring of the galaxy NGC 3516, which is an active galactic nucleus (AGN) of type Sy

1.5

with a changing look. Observations were carried out at the Fesenkov Astrophysical Institute (FAI, Almaty, Kazakhstan) and the Shamakhy Astrophysical Observatory (ShAO, Shamakhy, Azerbaijan). Spectral monitoring of this galaxy in the wavelength range 4000–7000 Å began in 2020, while photometric observations have been conducted since 2014. During the observation period, estimates of the galaxy’s brightness in the B, V and

R c

filters were obtained, as well as measurements of the emission line and continuum fluxes. The light curve shows increased brightness of NGC 3516 in 2016 and 2019. The increase of emission line fluxes of H

β

and H

α

and continuum began in 2019 and continued until spring 2020, when these characteristics reached their maximal values. A powerful X-ray flare took place on 1 April 2020. A new phase of brightening began in 2021 and has continued until 2025. After reaching their maxima in 2020, the emission fluxes of H

β

and H

α

decreased by a factor of 1.5–2 and remained at a low level until 2022–2023, when they began to increase again. Medium-resolution spectra obtained on 20 April 2020, with the 1-meter “West” telescope (TSHAO) were used to study the broad components of the H

β

and H

α

emission line profiles. Model calculations showed that the broad profile of the H

α

line consists of a central unshifted component and two (blue and red) components shifted symmetrically relative to the central component by a velocity of

v = 980 \pm 20

km

s^{- 1}

. The H

β

emission line was relatively weak, so the radial velocity of its components was determined with a large uncertainty:

900 \pm 600

km

s^{- 1}

. Full article

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13 pages, 5876 KiB

Open AccessArticle

V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode

by Vitaly P. Goranskij, Elena A. Barsukova, Aleksandr N. Burenkov, Natalia V. Metlova, Alla V. Zharova and Ilya A. Yakunin

Galaxies 2025, 13(3), 59; https://doi.org/10.3390/galaxies13030059 - 14 May 2025

Viewed by 334

Abstract

The phenomenon of runaway mass transfers between components of binary systems on a dynamical timescale has been theoretically predicted. However, this phenomenon has been observed for the first time in the history of astronomy just now in a symbiotic system V694 Mon. We [...] Read more.

The phenomenon of runaway mass transfers between components of binary systems on a dynamical timescale has been theoretically predicted. However, this phenomenon has been observed for the first time in the history of astronomy just now in a symbiotic system V694 Mon. We employed medium- and high-dispersion spectroscopy, along with multicolor photometry, to study this event in detail. Over 6 years, beginning in 2018, we observed the cessation of disk accretion, the filling of the accretor’s Roche lobe, and the subsequent formation of an A-type star within it. The pulsating envelope of the M giant donor was transferred to the accretor down to its base. Thus, the products from the hydrogen-burning layer appeared on the donor’s surface, and a flash of an emission-line spectrum enriched with s-process elements was detected. We discuss discrepancies between theoretical predictions and observations, as well as other phenomena potentially related to dynamical mass transfer. Full article

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

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8 pages, 246 KiB

Open AccessArticle

Studies of Hot Stars and Other Observational Programs Using the 1-Meter Optical Telescope Zeiss-1000 of SAO RAS

by Vladimir V. Komarov, Victoria N. Komarova and Alexander S. Moskvitin

Galaxies 2025, 13(3), 58; https://doi.org/10.3390/galaxies13030058 - 13 May 2025

Viewed by 372

Abstract

Here, we briefly describe the current state of the Zeiss-1000 telescope of the Special Astrophysical Observatory of Russian Academy of Sciences (SAO RAS). Principal attention is given to research programs from recent years. The observations made according to allocated requests both by researchers [...] Read more.

Here, we briefly describe the current state of the Zeiss-1000 telescope of the Special Astrophysical Observatory of Russian Academy of Sciences (SAO RAS). Principal attention is given to research programs from recent years. The observations made according to allocated requests both by researchers from the observatory and scientists from other institutions and organizations are planned within a half-year schedule. The instrumental facilities provide a wide range of methods for studying objects of interest. They include standard photometry, moderate- and high-resolution spectroscopy, and polarimetry, along with unique “guest” methods (e.g., emission line imaging). The research programs cover different fields of astrophysics. The topic of “hot stars” and other important developments have become possible due to the introduction of new research methods at the telescope. Blazars, gamma-ray burst optical transients, massive supernovae, cataclysmic variables, magnetic stars, white dwarfs, luminous blue variables, red dwarfs, and many others are among its targets. Full article

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

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57 pages, 2185 KiB

Open AccessReview

Exploring the GRB–Supernova Connection: Does a Superluminous Hypernova Population Exist?

by Achille Fiore, Ludovica Crosato Menegazzi and Giulia Stratta

Galaxies 2025, 13(3), 57; https://doi.org/10.3390/galaxies13030057 - 6 May 2025

Viewed by 491

Abstract

Observations of several gamma-ray bursts (GRBs) that are temporally and spatially compatible with energetic supernovae (hypernovae) have established their common origin. In one case (GRB 111209A/SN 2011kl), the associated supernova was classified as superluminous (SN 2011kl). The exceptional duration of the observed gamma-ray [...] Read more.

Observations of several gamma-ray bursts (GRBs) that are temporally and spatially compatible with energetic supernovae (hypernovae) have established their common origin. In one case (GRB 111209A/SN 2011kl), the associated supernova was classified as superluminous (SN 2011kl). The exceptional duration of the observed gamma-ray prompt emission of GRB 111209A (about 7 h) is widely considered key to unlocking the physics behind the still mysterious origin of superluminous supernovae (SLSNe). We review the main observational and theoretical findings that may link some ultra-long GRBs to SLSNe. Specifically, we examine notable events and the role of progenitors and host galaxies in shaping these phenomena and focus on the proposed models. While a magnetar central engine is a plausible mechanism for both luminous and long-duration GRBs, a conclusive answer remains elusive, as alternative explanations are still viable. Further observational and theoretical work is required to clarify progenitor pathways and explosion mechanisms, potentially extending the classical GRB-SN connection to rare superluminous hypernovae. Full article

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

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12 pages, 3593 KiB

Open AccessArticle

Spectral Variability of Herbig Ae Star V1295, Aquila

by Hemayil Adigozalzade, Silva Jarvinen, Swetlana Hubrig, Ulviyya Bashirova and Nariman Ismailov

Galaxies 2025, 13(3), 56; https://doi.org/10.3390/galaxies13030056 - 6 May 2025

Viewed by 353

Abstract

This article presents the results of long-term spectral studies of the unusual Herbig Ae star V1295 Aql. The variability in the spectrophotometric parameters of the emission and absorption components of the hydrogen lines Hα, Hβ, and D Na I—and of various metallic lines—is [...] Read more.

This article presents the results of long-term spectral studies of the unusual Herbig Ae star V1295 Aql. The variability in the spectrophotometric parameters of the emission and absorption components of the hydrogen lines Hα, Hβ, and D Na I—and of various metallic lines—is studied. The periodic variability in these parameters over a period of 51.7 days, recently established using mean longitudinal magnetic field measurements, is confirmed. The obtained average value of the radial velocities of metal absorption lines is Vr = 0.75 ± 1.85 km/s. According to our long-term observations, the radial velocities of the star do not change over time. This indicates the absence of a close stellar-mass companion. Full article

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

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17 pages, 1201 KiB

Open AccessArticle

Time Dilation Observed in Type Ia Supernova Light Curves and Its Cosmological Consequences

by Václav Vavryčuk

Galaxies 2025, 13(3), 55; https://doi.org/10.3390/galaxies13030055 - 3 May 2025

Viewed by 736

Abstract

The cosmic time dilation observed in Type Ia supernova light curves suggests that the passage of cosmic time varies throughout the evolution of the Universe. This observation implies that the rate of proper time is not constant, as assumed in the standard FLRW [...] Read more.

The cosmic time dilation observed in Type Ia supernova light curves suggests that the passage of cosmic time varies throughout the evolution of the Universe. This observation implies that the rate of proper time is not constant, as assumed in the standard FLRW metric, but instead is time-dependent. Consequently, the commonly used FLRW metric should be replaced by a more general framework, known as the Conformal Cosmology (CC) metric, to properly account for cosmic time dilation. The CC metric incorporates both spatial expansion and time dilation during cosmic evolution. As a result, it is necessary to distinguish between comoving and proper (physical) time, similar to the distinction made between comoving and proper distances. In addition to successfully explaining cosmic time dilation, the CC metric offers several further advantages: (1) it preserves Lorentz invariance, (2) it maintains the form of Maxwell’s equations as in Minkowski spacetime, (3) it eliminates the need for dark matter and dark energy in the Friedmann equations, and (4) it successfully predicts the expansion and morphology of spiral galaxies in agreement with observations. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum—2nd Edition)

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8 pages, 350 KiB

Open AccessArticle

The Effect of Viscosity on the Temperature of Ae Star Disks

by R. Anusha and T. A. A. Sigut

Galaxies 2025, 13(3), 54; https://doi.org/10.3390/galaxies13030054 - 3 May 2025

Viewed by 377

Abstract

This study explores the impact of viscous heating on decretion disks around Classical Ae (CAe) stars, with a focus on modeling the disk’s thermal structure. While photoionization is the dominant heating mechanism, viscous dissipation can play an important role in shaping the disk [...] Read more.

This study explores the impact of viscous heating on decretion disks around Classical Ae (CAe) stars, with a focus on modeling the disk’s thermal structure. While photoionization is the dominant heating mechanism, viscous dissipation can play an important role in shaping the disk temperature, particularly for cooler CAe subtypes. Our models incorporate viscosity-driven heating and predict that shear heating has a negligible effect for early A-type stars (A0–A1), but it becomes increasingly significant for later spectral types, especially as the viscosity parameter (

α

) increases. This heating also influences the strength of H

α

emission. Furthermore, our models predict a sharp decline in the number of emission-line stars beyond spectral type A2, a trend observed in CAe populations. However, for sufficiently high

α

values (≥0.3), a higher fraction of emission-line objects is expected even among later subtypes, such as A5, despite the lack of well-characterized CAe stars observed beyond the spectral type A4. Full article

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

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14 pages, 644 KiB

Open AccessReview

Very-High-Energy Gamma-Ray Observations as a Probe to the Nature of Dark Matter and Prospects for MACE

by Mani Khurana, Krishna Kumar Singh, Atul Pathania, Pawan Kumar Netrakanti and Kuldeep Kumar Yadav

Galaxies 2025, 13(3), 53; https://doi.org/10.3390/galaxies13030053 - 2 May 2025

Viewed by 441

Abstract

Searching for very-high-energy photons arising from dark matter interactions in selected astrophysical environments is a promising strategy to probe the existence and particle nature of dark matter. Among the many particle candidates, motivated by the extensions of the Standard Model, Weakly Interacting Massive [...] Read more.

Searching for very-high-energy photons arising from dark matter interactions in selected astrophysical environments is a promising strategy to probe the existence and particle nature of dark matter. Among the many particle candidates, motivated by the extensions of the Standard Model, Weakly Interacting Massive Particles (WIMPs) are considered the most compelling candidate for the elusive dark matter in the universe. In this contribution, we report an overview of the important developments in the field of indirect searching for dark matter through cosmic gamma-ray observations. We mainly focus on the role of atmospheric Cherenkov telescopes in probing the dark matter. Finally, we emphasize the opportunities for the Major Atmospheric Cherenkov Experiment (MACE) situated in Hanle, India, to explore WIMPs in the mass range of 200 GeV to 10 TeV for Segue1 and Draco dwarf–spheroidal galaxies. Full article

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20 pages, 3417 KiB

Open AccessFeature PaperArticle

Statistical Classification and an Optimized Red-Sequence Technique for the Determination of Galaxy Clusters

by Dagoberto R. Mares-Rincón, Josué J. Trejo-Alonso, José A. Guerrero-Díaz-de-León and Jorge E. Macías-Díaz

Galaxies 2025, 13(3), 52; https://doi.org/10.3390/galaxies13030052 - 1 May 2025

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Abstract

This study presents a novel method for characterizing galaxy clusters by integrating statistical classification techniques with an optimized adaptation of the red sequence approach. The proposed algorithm employs Gaussian mixture models to analyze the distribution of three key variables: r magnitude, [...] Read more.

This study presents a novel method for characterizing galaxy clusters by integrating statistical classification techniques with an optimized adaptation of the red sequence approach. The proposed algorithm employs Gaussian mixture models to analyze the distribution of three key variables: r magnitude,

g - r

color index, and redshift z. To enhance cluster discrimination, we incorporate Mahalanobis distance metrics and modify the conventional red sequence technique by adopting the principal eigenvector as the slope of the cluster. A sample of 114 galaxy groups and clusters within the redshift range

0.002 < z < 0.45

was used to validate the method. Comparative analyses demonstrate that the proposed approach achieves comparable or, in certain cases, superior performance in cluster characterization relative to the standard red sequence technique. These results highlight the algorithm’s potential as a robust tool for the exploratory identification and initial parameter determination of galaxy clusters, particularly in large-scale surveys. The methodology bridges statistical rigor with established astrophysical techniques, offering a promising avenue for advancing cluster detection in observational cosmology. Full article

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20 pages, 1318 KiB

Open AccessArticle

The Galactic Pizza: Flat Rotation Curves in the Context of Cosmological Time-Energy Coupling

by Artur Novais and André L. B. Ribeiro

Galaxies 2025, 13(3), 51; https://doi.org/10.3390/galaxies13030051 - 27 Apr 2025

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Abstract

The phenomenon of augmented gravity on the scale of galaxies, conventionally attributed to dark matter halos, is shown to possibly result from the incremental growth of galactic masses and radii over time. This approach elucidates the cosmological origins of the acceleration scale [...] Read more.

The phenomenon of augmented gravity on the scale of galaxies, conventionally attributed to dark matter halos, is shown to possibly result from the incremental growth of galactic masses and radii over time. This approach elucidates the cosmological origins of the acceleration scale

a_{0} \approx c H_{0} / 2 π \approx 10^{- 10}

ms⁻² at which galaxy rotation curves deviate from Keplerian behavior, with no need for new particles or modifications to the laws of gravity, i.e., it constitutes a new explanatory path beyond Cold Dark Matter (CDM) and Modified Newtonian Dynamics (MOND). Once one formally equates the energy density of the universe to the critical value (

ρ = ρ_{c}

) and the cosmic age to the reciprocal of the Hubble parameter (

t = H^{- 1}

), independently of the epoch of observation, the result is the Zero-Energy condition for the cosmic fluid’s equation of state, with key repercussions for the study of dark energy since the observables can be explained in the absence of a cosmological constant. Furthermore, this mass-energy evolution framework is able to reconcile the success of CDM models in describing structure assembly at

z ≲ 6

with the unexpected discovery of massive objects at

z ≳ 10

. Models that feature a strong coupling between cosmic time and energy are favored by this analysis. Full article

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

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17 pages, 7779 KiB

Open AccessReview

Betelgeuse, the Prototypical Red Supergiant

by Andrea K. Dupree and Miguel Montargès

Galaxies 2025, 13(3), 50; https://doi.org/10.3390/galaxies13030050 - 24 Apr 2025

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Abstract

The behavior of the bright red supergiant, Betelgeuse, is described with results principally from the past 6 years. The review includes imaging, photometry, and spectroscopy to record the Great Dimming of 2019–2020. This event was followed by a slow ongoing recovery from the [...] Read more.

The behavior of the bright red supergiant, Betelgeuse, is described with results principally from the past 6 years. The review includes imaging, photometry, and spectroscopy to record the Great Dimming of 2019–2020. This event was followed by a slow ongoing recovery from the massive surface mass ejection after which the stellar characteristics changed. Theoretical simulations address the cause of this episodic mass ejection and the optical Dimming. Recent publications evaluating the perplexing 2100 day periodicity in the star’s brightness and radial velocity provide evidence that Betelgeuse may harbor a companion object. Current attempts at direct detection of this companion are discussed. Betelgeuse provides a well-studied and meaningful example for supergiant stars in our Galaxy and others. Full article

(This article belongs to the Special Issue The Red Supergiants: Crucial Signposts for the Fate of Massive Stars)

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24 pages, 629 KiB

Open AccessReview

Symbiotic Stars in the Era of Modern Ground- and Space-Based Surveys

by Jaroslav Merc

Galaxies 2025, 13(3), 49; https://doi.org/10.3390/galaxies13030049 - 23 Apr 2025

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Abstract

Symbiotic stars, interacting binaries composed of a cool giant and a hot compact companion, exhibit complex variability across the electromagnetic spectrum. Over the past decades, large-scale photometric and spectroscopic surveys from ground- and space-based observatories have significantly advanced their discovery and characterization. These [...] Read more.

Symbiotic stars, interacting binaries composed of a cool giant and a hot compact companion, exhibit complex variability across the electromagnetic spectrum. Over the past decades, large-scale photometric and spectroscopic surveys from ground- and space-based observatories have significantly advanced their discovery and characterization. These datasets have transformed the search for new symbiotic candidates, providing extensive time-domain information crucial for their classification and analysis. This review highlights recent observational results that have expanded the known population of symbiotic stars, refined classification criteria, and enhanced our understanding of their variability. Despite these advances, fundamental questions remain regarding their long-term evolution, mass transfer and accretion processes, or their potential role as progenitors of Type Ia supernovae. With ongoing and upcoming surveys, the coming years promise new discoveries and a more comprehensive picture of these intriguing interacting systems. Full article

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

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12 pages, 590 KiB

Open AccessArticle

Necessary Conditions for Earthly Life Floating in the Venusian Atmosphere

by Jennifer J. Abreu, Alyxander R. Anchordoqui, Nyamekye J. Fosu, Michael G. Kwakye, Danijela Kyriakakis, Krystal Reynoso and Luis A. Anchordoqui

Galaxies 2025, 13(3), 48; https://doi.org/10.3390/galaxies13030048 - 22 Apr 2025

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Abstract

Millimeter-waveband spectra of Venus from both the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA) seem to indicate there may be evidence (signal-to-noise ratio of about 15σ) of a phosphine absorption-line profile against the thermal background from deeper, hotter [...] Read more.

Millimeter-waveband spectra of Venus from both the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA) seem to indicate there may be evidence (signal-to-noise ratio of about 15σ) of a phosphine absorption-line profile against the thermal background from deeper, hotter layers of the atmosphere. Phosphine is an important biomarker; e.g., the trace of phosphine in the Earth’s atmosphere is unequivocally associated with anthropogenic activity and microbial life (which produces this highly reducing gas even in an overall oxidizing environment). Motivated by the JCMT and ALMA tantalizing observations, we reexamine whether Venus could accommodate Earthly life. More concretely, we hypothesize that the microorganisms populating the Venusian atmosphere are not free floating but confined to the liquid environment inside cloud aerosols or droplets. Armed with this hypothesis, we generalize a study of airborne germ transmission to constrain the maximum size of droplets that could be floating in the Venusian atmosphere by demanding that their Stokes fallout times to reach moderately high temperatures are pronouncedly larger than the microbe’s replication time. We also comment on the effect of cosmic ray showers on the evolution of aerial microbial life. Full article

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10 pages, 539 KiB

Open AccessArticle

Fundamental Parameters and Evolutionary Scenario of HD 327083

by Nadezhda L. Vaidman, Anatoly S. Miroshnichenko, Sergey V. Zharikov, Serik A. Khokhlov, Aldiyar T. Agishev and Berik S. Yermekbayev

Galaxies 2025, 13(3), 47; https://doi.org/10.3390/galaxies13030047 - 22 Apr 2025

Viewed by 352

Abstract

In this study, we present refined orbital and fundamental parameters of the Galactic B[e] supergiant binary system HD 327083 using the Bayesian Markov Chain Monte Carlo (MCMC) method applied to the radial velocities data of HD 327083. We found that the system is [...] Read more.

In this study, we present refined orbital and fundamental parameters of the Galactic B[e] supergiant binary system HD 327083 using the Bayesian Markov Chain Monte Carlo (MCMC) method applied to the radial velocities data of HD 327083. We found that the system is well described by a circular orbital model with the mass ratio of the components of

q = 1.15 \pm 0.07

. We modeled the evolutionary history of the system using MESA code. Initially, the system was formed by a binary with the orbital period of

P o r b = 108

day, which contained stars with 13.00

\pm 0.05

M_{⊙}

and

11.50 \pm 0.05

M_{⊙}

masses. They had a relatively slow rotation

υ_{r o t} = 0.40 \pm 0.13 υ_{c r i t}

and provided a strong stellar wind. The current system age is

13.6 \pm 0.1

Myr, and the state of the system corresponds to a close filling of the high massive component’s Roche lobe and a beginning of the mass transfer. The mass-transfer event will occur in a short interval of ≲0.1 Myr only. After that, the mass of the post-primary drops to ≈5

M_{⊙}

, the post-secondary mass grows until ≈20

M_{⊙}

, and the binary will convert to a detached system with a long orbital period of ≈700 days. Full article

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

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14 pages, 3588 KiB

Open AccessReview

Intermediate-Mass Mergers: A New Scenario for Several FS CMa Stars

by Daniela Korčáková, Nela Dvořáková, Iris Bermejo Lozano, Gregg A. Wade, Alicia Moranchel Basurto, Pavel Kroupa, Raul Ortega Chametla, Colin Peter Folsom and Ondrej Juhás

Galaxies 2025, 13(3), 46; https://doi.org/10.3390/galaxies13030046 - 22 Apr 2025

Viewed by 397

Abstract

We summarise the properties and nature of a peculiar group of B-type stars called FS CMa stars. These stars show the B[e] phenomenon, i.e., their spectra exhibit both forbidden emission lines and infrared excess. Such properties point to an extended circumstellar gas and [...] Read more.

We summarise the properties and nature of a peculiar group of B-type stars called FS CMa stars. These stars show the B[e] phenomenon, i.e., their spectra exhibit both forbidden emission lines and infrared excess. Such properties point to an extended circumstellar gas and dust component. Although the phenomenon has been explained in most B[e] stars, the origin and nature of FS CMa stars is disputed. Here, we focus on the merger hypothesis, for which evidence has recently been discovered. Full article

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

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