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A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (31 October 2023) | Viewed by 20085

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Dr. Vladimir Korchagin

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Guest Editor

Institute of Physics, Southern Federal University, Rostov-on-Don, Russia
Interests: galactic structure and dynamics; interaction of galaxies; dark matter distribution in galactic subsystems

Prof. Dr. Maxim Yu. Khlopov

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Guest Editor

1. Director of Virtual Institute of Astroparticle Physics, Paris, France
2. National Research Nuclear University MEPhI, President of Center for Cosmoparticle Physics Cosmion, Main Researcher of Research Institute of Physics of Southern Federal University, Rostov-on-Don, Russia
Interests: cosmology; particle physics; beyond the standard models; cosmoparticle physics; dark matter; primordial black holes; antimatter
Special Issues, Collections and Topics in MDPI journals

Dr. Orchidea Maria Lecian

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1. Department of Medicine and Pharmacy & Department of Medicine and Dentistry, Sapienza University of Rome, 00185 Rome, Italy
2. ICRA-International Center for Relativistic Astrophysics C/O Physics Department, Sapienza University of Rome, Piazzale Aldo Moro, 5-00185 Rome, Italy
3. ICRANet- International Center for Relativistic Astrophysics Network, Piazza della Repubblica, 65122 Pescara, Italy
Interests: physics; mathematics
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Special Issue Information

Dear Colleagues,

The Milky Way, a typical spiral galaxy, is an important anchor point for shedding light on a number of challenging questions. One fundamental issue is our current lack of understanding regarding the nature of an extremely weakly interacting form of matter—that is, dark matter. The importance of dark matter is supported by a wide range of observations, including large scale structures, the cosmic microwave background, and the isotopic abundances resulting from the primordial nucleosynthesis. The Milky Way galaxy functions as a giant laboratory for studying the interaction of dark-matter particles with high-energy cosmic rays and other forms of matter. Uncovering structural details of the Milky Way’s subsystems is one of the key paths to figuring out the nature of dark matter particles.

Developments in physics beyond the standard model of fundamental interactions predict the existence of exotic components of galactic cosmic rays, new types of compact stars, and macroscopic antimatter configurations in the galaxy. Testing such predictions with multi-messenger astronomical observations will shed light on fundamental physical principles of the evolution of the Universe.

It is known that the Milky Way stellar halo was predominantly formed by merging of a number of dwarf galaxies. Open issues remain, however, such as how many progenitor galaxies have been accreted and which globular clusters and stellar streams among those observed in the Milky Way galaxy have an external origin. Answering these questions is important to understanding the hierarchical buildup of our own and other galaxies. The aim of this Special Issue is to bring together recent works touching on different aspects of Milky Way physics. We welcome contributions on the formation and dynamics of the Milky Way galaxy; physics of dark matter as it pertains to the Milky Way, including its interaction with matter and interstellar magnetic fields; physics of accretion disks; and physics of extreme environments such as that of the galactic center. The aim of this Special Issue is to present the current state of our understanding of the Milky Way’s formation and structure, and is especially timely in light of the start of the James Webb Space Telescope mission, which is expected to bring a new level in our knowledge of the formation, structure and evolution of the Milky Way.

Dr. Vladimir Korchagin
Prof. Dr. Maxim Yu. Khlopov
Prof. Dr. Orchidea Maria Lecian
Guest Editors

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Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Galaxies is an international peer-reviewed open access semimonthly journal published by MDPI.

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Published Papers (9 papers)

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Research

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29 pages, 27073 KiB

Open AccessArticle

Formation of Transitional cE/UCD Galaxies through Massive/Dwarf Disc Galaxy Mergers

by Alexander V. Khoperskov, Sergey S. Khrapov and Danila S. Sirotin

Galaxies 2024, 12(1), 1; https://doi.org/10.3390/galaxies12010001 - 25 Dec 2023

Cited by 2 | Viewed by 1689

Abstract

The dynamics of the merger of a dwarf disc galaxy with a massive spiral galaxy of the Milky Way type were studied in detail. The remnant of such interaction after numerous crossings of the satellite through the disc of the main galaxy was a compact stellar core, the characteristics of which were close to small compact elliptical galaxies (cEs) or large ultra-compact dwarfs (UCDs). Such transitional cE/UCD objects with an effective radius of 100–200 pc arise as a result of stripping the outer layers of the stellar core during the destruction of a dwarf disc galaxy. Numerical models of the satellite before interaction included baryonic matter (stars and gas) and dark mass. We used N-body to describe the dynamics of stars and dark matter, and we used smoothed-particle hydrodynamics to model the gas components of both galaxies. The direct method of calculating the gravitational force between all particles provided a qualitative resolution of spatial structures up to 10 pc. The dwarf galaxy fell onto the gas and stellar discs of the main galaxy almost along a radial trajectory with a large eccentricity. This ensured that the dwarf crossed the disc of the main galaxy at each pericentric approach over a time interval of more than 9 billion years. We varied the gas mass and the initial orbital characteristics of the satellite over a wide range, studying the features of mass loss in the core. The presence of the initial gas component in a dwarf galaxy significantly affects the nature of the formation and evolution of the compact stellar core. The gas-rich satellite gives birth to a more compact elliptical galaxy compared to the merging gas-free dwarf galaxy. The initial gas content in the satellite also affects the internal rotation in the stripped nucleus. The simulated cE/UCD galaxies contained very little gas and dark matter at the end of their evolution. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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22 pages, 10637 KiB

Open AccessArticle

A Study of the Properties and Dynamics of the Disk of Satellites in a Milky-Way-like Galaxy System

by Xinghai Zhao, Grant J. Mathews, Lara Arielle Phillips and Guobao Tang

Galaxies 2023, 11(6), 114; https://doi.org/10.3390/galaxies11060114 - 16 Nov 2023

Cited by 2 | Viewed by 1650

Abstract

The dynamics of the satellite systems of Milky-Way-like galaxies offer a useful means by which to study the galaxy formation process in the cosmological context. It has been suggested that the currently observed anisotropic distribution of the satellites in such galaxy systems is [...] Read more.

Λ C D M

cosmology model on the galactic scale if the observed satellites are random samples of the dark matter (DM) sub-halos that are nearly isotropically distributed around the central galaxy. In this study, we present original high-resolution zoom-in studies of central galaxies and satellite systems based upon initial conditions for the DM distribution from the Aquarius simulations but with substantial high-resolution baryon physics added. We find that the galaxy most like the Milky Way in this study does indeed contain a disk of satellites (DOS). Although one galaxy DOS system does not answer the question of how common such disks are, it does allow the opportunity to explore the properties and dynamics of the DOS system. Our investigation centers on the spatial arrangement (distances, angles, etc.) of satellites in this Milky-Way-like galaxy system with a specific emphasis on identifying and analyzing the disk-like structure along with its dynamical and morphological properties. Among the conclusions from this study, we find that the satellites and DM sub-halos in the galaxy simulations are anisotropically distributed. The dynamical properties of the satellites, however, indicate that the direction of the angular momentum vector of the whole satellite system is different from the normal direction of the fitted DOS and from the normal direction of the velocity dispersion of the system. Hence, the fitted DOS appears to be comprised of infalling sub-halos and is not a rotationally supported system. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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11 pages, 1252 KiB

Open AccessCommunication

On the Possible Asymmetry in Gamma Rays from Andromeda Due to Inverse Compton Scattering of Star Light on Electrons from Dark Matter Annihilation

by Konstantin Belotsky and Maxim Solovyov

Galaxies 2023, 11(6), 109; https://doi.org/10.3390/galaxies11060109 - 7 Nov 2023

Viewed by 1310

Abstract

Dark matter is a popular candidate to a new source of primary-charged particles, especially positrons in cosmic rays, which are proposed to account for observable anomalies. While this hypothesis of decaying or annihilating DM is mostly applied for our Galaxy, it could possibly lead to some interesting phenomena when applied for the other ones. In this work, we look into the hypothetical asymmetry in gamma radiation from the upper and lower hemisphere of the dark matter halo of the Andromeda galaxy due to inverse Compton scattering of starlight on the DM-produced electrons and positrons. While our 2D toy model raises expectations for the possible effect, a more complex approach gives negligible effect for the dark halo case, but shows some prospects for a dark disk model. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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21 pages, 2518 KiB

Open AccessArticle

A Scattered Star Group in the Orion A Region of the Milky Way

by Sergei Vereshchagin, Natalya Chupina, Kristina Lyzenko, Anatoly Kalinkin, Nikolay Kondratev, Dana Kovaleva and Sergei Sapozhnikov

Galaxies 2023, 11(5), 99; https://doi.org/10.3390/galaxies11050099 - 19 Sep 2023

Cited by 1 | Viewed by 1558

Abstract

Using Gaia DR3 data, we identified an extended a ~60 pc group of stars sharing common motion but scattered in space, including from 150 to 300 probable members, named Group V. It can be associated with a group identified by Getman et al. (2019) and by Jerabkova et al. (2019) as a relic of a gas filament, traced by the mutual position of stars after the gas is swept out. We estimate its age to be approximately 16 million years. A combination of methods is applied to select probable members of Group V. We discuss the kinematic characteristics of the stars of Group V and the controversial clues they provide for understanding its nature. Due to the vicinity of a number of open clusters in the space, differentiating between members of the group and of the clusters is problematic, and mutual contamination is inevitable. The pair of clusters Gulliver 6 and UBC 17b is wrapped inside Group V but differs from it in kinematics. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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8 pages, 294 KiB

Open AccessCommunication

Gaia BH1: A Key for Understanding the Demography of Low-q Binaries in the Milky Way Galaxy

by Oleg Malkov

Galaxies 2023, 11(5), 98; https://doi.org/10.3390/galaxies11050098 - 18 Sep 2023

Viewed by 1481

Abstract

The recently discovered Gaia BH1 binary system, a Sun-like star and a dark object (presumably a black hole), may significantly change our understanding of the population of binaries. The paper presents the components mass ratio (q) distributions of binary systems of different observational classes. They all show a significant shortage of low-q systems. In this work, I demonstrate (quantitatively) how our ignorance extends, and point out the importance of discovering and studying systems like Gaia BH1. In addition, an approximate mass–temperature relation and mass ratio–magnitude difference relation for main-sequence stars are presented here. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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11 pages, 271 KiB

Open AccessCommunication

The Formalism of Milky-Way Antimatter-Domains Evolution

by Maxim Yu. Khlopov and Orchidea Maria Lecian

Galaxies 2023, 11(2), 50; https://doi.org/10.3390/galaxies11020050 - 22 Mar 2023

Cited by 2 | Viewed by 1668

Abstract

If baryosynthesis is strongly nonhomogeneous, macroscopic regions with antibaryon excess can be created in the same process from which the baryonic matter is originated. This exotic possibility can become real, if the hints to the existence of antihelium component in cosmic rays are confirmed in the AMS02 experiment, indicating the existence of primordial antimatter objects in our Galaxy. Possible forms of such objects depend on the parameters of models of baryosynthesis and evolution of antimatter domains. We elaborate the formalism of analysis of evolution of antibaryon domain with the account for baryon-antibaryon annihilation at the domain borders and possible “Swiss cheese” structure of the domain structure. We pay special attention to evolution of various forms of high, very high and ultrahigh density antibaryon domains and deduce equations of their evolution in the expanding Universe. The proposed formalism will provide the creation of evolutionary scenarios, linking the possible forms and properties of antimatter bodies in our Galaxy to the mechanisms of nonhomogeneous baryosynthesis. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

15 pages, 11996 KiB

Open AccessArticle

The Influence of the Galactic Bar on the Dynamics of Globular Clusters

by Roman Tkachenko, Vladimir Korchagin, Anna Jmailova, Giovanni Carraro and Boris Jmailov

Galaxies 2023, 11(1), 26; https://doi.org/10.3390/galaxies11010026 - 6 Feb 2023

Cited by 5 | Viewed by 1967

Abstract

We make use of recent estimates for the parameters of the Milky Way’s halo globular clusters and study the influence of the galactic bar on the dynamics of these clusters by computing their orbits. We use both an axisymmetric and non-axisymmetric galactic potentials, which include the rotating elongated bar/bulge structure. We account for observational errors both in the positions and in the velocities of the globular clusters and explore the influence of the bar on clusters’ evolution. This is contained in the angular momentum–total energy plane,

(L_{z}, E)

, which is widely exploited as an indicator of the groups of globular clusters that originated from the same accretion event. Particular attention is devoted to the Gaia-Sausage/Enceladus and Pontus structures identified recently as two independent accretion events. Our study shows that it is not possible to identify GSE and Pontus as different merger events. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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27 pages, 1873 KiB

Open AccessArticle

Spirality: A Novel Way to Measure Spiral Arm Pitch Angle

by Deanna Shields, Benjamin Boe, Casey Pfountz, Benjamin L. Davis, Matthew Hartley, Ryan Miller, Zac Slade, M. Shameer Abdeen, Daniel Kennefick and Julia Kennefick

Galaxies 2022, 10(5), 100; https://doi.org/10.3390/galaxies10050100 - 17 Oct 2022

Cited by 19 | Viewed by 2500

Abstract

We present the MATLAB code Spirality, a novel method for measuring spiral arm pitch angles by fitting galaxy images to spiral templates of known pitch. Computation time is typically on the order of 2 min per galaxy, assuming 8 GB of working memory. We tested the code using 117 synthetic spiral images with known pitches, varying both the spiral properties and the input parameters. The code yielded correct results for all synthetic spirals with galaxy-like properties. We also compared the code’s results to two-dimensional Fast Fourier Transform (2DFFT) measurements for the sample of nearby galaxies defined by DMS PPak. Spirality’s error bars overlapped 2DFFT’s error bars for 26 of the 30 galaxies. The two methods’ agreement correlates strongly with galaxy radius in pixels and also with i-band magnitude, but not with redshift, a result that is consistent with at least some galaxies’ spiral structure being fully formed by

z = 1.2

, beyond which there are few galaxies in our sample. The Spirality code package also includes GenSpiral, which produces FITS images of synthetic spirals, and SpiralArmCount, which uses a one-dimensional Fast Fourier Transform to count the spiral arms of a galaxy after its pitch is determined. All code is freely available. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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Review

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29 pages, 1261 KiB

Open AccessReview

The Effect of the LMC on the Milky Way System

by Eugene Vasiliev

Galaxies 2023, 11(2), 59; https://doi.org/10.3390/galaxies11020059 - 18 Apr 2023

Cited by 35 | Viewed by 4703

Abstract

We review the recent theoretical and observational developments concerning the interaction of the Large Magellanic Cloud (LMC) with the Milky Way and its neighbourhood. An emerging picture is that the LMC is a fairly massive companion (10–20% of the Milky Way mass) and just passed the pericentre of its orbit, likely for the first time. The gravitational perturbation caused by the LMC is manifested at different levels. The most immediate effect is the deflection of orbits of stars, stellar streams, or satellite galaxies passing in the vicinity of the LMC. Less well known but equally important is the displacement (reflex motion) of central regions of the Milky Way about the centre of mass of both galaxies. Since the Milky Way is not a rigid body, this displacement varies with the distance from the LMC, and as a result, the Galaxy is deformed and its outer regions (beyond a few tens kpc) acquire a net velocity with respect to its centre. These phenomena need to be taken into account at the level of precision warranted by current and future observational data, and improvements on the modelling side are also necessary for an adequate interpretation of these data. Full article

(This article belongs to the Special Issue Galactic Structure and Dynamics)

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