Galaxies

2024

Jump to: 2023, 2022, 2021, 2020

18 pages, 4170 KiB

Open AccessArticle

Semi-Empirical Estimates of the Cosmic Planet Formation Rate

by Andrea Lapi, Lumen Boco, Francesca Perrotta and Marcella Massardi

Galaxies 2024, 12(4), 49; https://doi.org/10.3390/galaxies12040049 - 18 Aug 2024

Viewed by 743

Abstract

We devise and exploit a data-driven, semi-empirical framework of galaxy formation and evolution, coupling it to recipes for planet formation from stellar and planetary science, to compute the cosmic planet formation rate, and the properties of the planets’ preferred host stellar and galactic [...] Read more.

We devise and exploit a data-driven, semi-empirical framework of galaxy formation and evolution, coupling it to recipes for planet formation from stellar and planetary science, to compute the cosmic planet formation rate, and the properties of the planets’ preferred host stellar and galactic environments. We also discuss how the rates and formation sites of planets are affected when considering their habitability, and when including possible threatening sources related to star formation and nuclear activity. Overall, we conservatively estimate a cumulative number of some

10^{20}

Earth-like planets and around

10^{18}

habitable Earths in our past lightcone. Finally, we find that a few

10^{17}

are older than our own Earth, an occurrence which places a loose lower limit a few

10^{- 18}

to the odds for a habitable world to ever host a civilization in the observable Universe. Full article

► Show Figures

Figure 1

15 pages, 537 KiB

Open AccessArticle

The First VHE Activity of OJ 287 and the Extragalactic Background Light

by Sameer Yadav and Pankaj Kushwaha

Galaxies 2024, 12(4), 34; https://doi.org/10.3390/galaxies12040034 - 30 Jun 2024

Viewed by 912

Abstract

The BL Lacertae (BL Lac) object OJ 287 underwent an intense X-ray activity phase, exhibiting its brightest recorded X-ray flare in 2016–2017, characterized by much softer X-ray spectra and, concurrently, its first-ever recorded very-high-energy (VHE) emission (100–560 GeV), reported by the VERITAS observatory. [...] Read more.

The BL Lacertae (BL Lac) object OJ 287 underwent an intense X-ray activity phase, exhibiting its brightest recorded X-ray flare in 2016–2017, characterized by much softer X-ray spectra and, concurrently, its first-ever recorded very-high-energy (VHE) emission (100–560 GeV), reported by the VERITAS observatory. Broadband spectral energy distribution reveals a new jet emission component similar to high-synchrotron-peaked BL Lac objects, thereby implying the soft X-ray spectrum for the synchrotron emission. Using the advantage of simultaneous X-ray and VHE spectral information, as well as the source being a low-synchrotron-peaked BL Lac object, we systematically explored the extragalactic background light (EBL) spectrum by demanding that the VHE spectrum cannot be harder than the X-ray spectrum. We used three different phenomenological forms of the EBL spectral shape (power-law, parabola, and polynomial) motivated by current constraints on the EBL with the Bayesian Monte Carlo approach to infer the credible EBL range. Our study favors an almost flat power-law spectral shape and is consistent with previous studies. The other spectral forms capable of capturing curvature though result in a better statistics value; the improvement is statistically insignificant given the additional parameters. Full article

► Show Figures

Figure 1

11 pages, 3803 KiB

Open AccessArticle

Wave-Particle Interactions in Astrophysical Plasmas

by Héctor Pérez-De-Tejada

Galaxies 2024, 12(3), 28; https://doi.org/10.3390/galaxies12030028 - 6 Jun 2024

Viewed by 638

Abstract

Dissipation processes derived from the kinetic theory of gases (shear viscosity and heat conduction) are employed to examine the solar wind that interacts with planetary ionospheres. The purpose of this study is to estimate the mean free path of wave-particle interactions that produce [...] Read more.

Dissipation processes derived from the kinetic theory of gases (shear viscosity and heat conduction) are employed to examine the solar wind that interacts with planetary ionospheres. The purpose of this study is to estimate the mean free path of wave-particle interactions that produce a continuum response in the plasma behavior. Wave-particle interactions are necessary to support the fluid dynamic interpretation that accounts for the interpretation of various features measured in a solar wind–planet ionosphere region; namely, (i) the transport of solar wind momentum to an upper ionosphere in the presence of a velocity shear, and (ii) plasma heating produced by momentum transport. From measurements conducted in the solar wind interaction with the Venus ionosphere, it is possible to estimate that in general terms, the mean free path of wave-particle interactions reaches λ_H ≥ 1000 km values that are comparable to the gyration radius of the solar wind particles in their Larmor motion within the local solar wind magnetic field. Similar values are also applicable to conditions measured by the Mars ionosphere and in cometary plasma wakes. Considerations are made in regard to the stochastic trajectories of the plasma particles that have been implied from the measurements made in planetary environments. At the same time, it is as possible that the same phenomenon is applicable to the interaction of stellar winds with the ionosphere of exoplanets, and also in regions where streaming ionized gases reach objects that are subject to rotational motion in other astrophysical problems (galactic flow–plasma interactions, black holes, etc.). Full article

► Show Figures

Figure 1

9 pages, 10540 KiB

Open AccessArticle

Doppler Tomography of the Circumstellar Disk of the Be Star κ Draconis

by Ilfa A. Gabitova, Anatoly S. Miroshnichenko, Sergey V. Zharikov, Ainash Amantayeva and Serik A. Khokhlov

Galaxies 2024, 12(3), 23; https://doi.org/10.3390/galaxies12030023 - 7 May 2024

Viewed by 1041

Abstract

κ

Draconis is a binary system with a classical Be star as the primary component. Its emission-line spectrum consists of hydrogen lines, notably the H

α

line with peak intensity ratio (V/R) variations phase-locked with the orbital period P = 61.55 days. Among [...] Read more.

κ

Draconis is a binary system with a classical Be star as the primary component. Its emission-line spectrum consists of hydrogen lines, notably the H

α

line with peak intensity ratio (V/R) variations phase-locked with the orbital period P = 61.55 days. Among binaries demonstrating the Be phenomenon,

κ

Dra stands out as one of a few systems with a discernible mass of its secondary component. Based on more than 200 spectra obtained in 2014–2023, we verified the physical parameters and constructed the mass function. We used part of these data obtained in 2014–2021 to investigate regions in the circumstellar disk of the primary component that emit the H

α

line using the Doppler tomography method. The results show that the disk has a non-uniform density distribution with a prominent enhancement at

V_{y}

≈ 99 km

s^{- 1}

and

V_{x}

\approx - 6

km

s^{- 1}

that corresponds to a cloud-like source of the double-peaked H

α

line profile. We argue that this enhancement’s motion is responsible for the periodic variations in the H

α

V/R ratio, which is synchronised in orbital phase with the radial velocity (RV) of absorption lines from the atmosphere of the primary component. Full article

► Show Figures

Figure 1

2023

Jump to: 2024, 2022, 2021, 2020

70 pages, 2653 KiB

Open AccessReview

Cosmic Ray Processes in Galactic Ecosystems

by Ellis R. Owen, Kinwah Wu, Yoshiyuki Inoue, H.-Y. Karen Yang and Alison M. W. Mitchell

Galaxies 2023, 11(4), 86; https://doi.org/10.3390/galaxies11040086 - 16 Jul 2023

Cited by 8 | Viewed by 3790

Abstract

Galaxy evolution is an important topic, and our physical understanding must be complete to establish a correct picture. This includes a thorough treatment of feedback. The effects of thermal–mechanical and radiative feedback have been widely considered; however, cosmic rays (CRs) are also powerful [...] Read more.

Galaxy evolution is an important topic, and our physical understanding must be complete to establish a correct picture. This includes a thorough treatment of feedback. The effects of thermal–mechanical and radiative feedback have been widely considered; however, cosmic rays (CRs) are also powerful energy carriers in galactic ecosystems. Resolving the capability of CRs to operate as a feedback agent is therefore essential to advance our understanding of the processes regulating galaxies. The effects of CRs are yet to be fully understood, and their complex multi-channel feedback mechanisms operating across the hierarchy of galaxy structures pose a significant technical challenge. This review examines the role of CRs in galaxies, from the scale of molecular clouds to the circumgalactic medium. An overview of their interaction processes, their implications for galaxy evolution, and their observable signatures is provided and their capability to modify the thermal and hydrodynamic configuration of galactic ecosystems is discussed. We present recent advancements in our understanding of CR processes and interpretation of their signatures, and highlight where technical challenges and unresolved questions persist. We discuss how these may be addressed with upcoming opportunities. Full article

► Show Figures

Figure 1

17 pages, 1987 KiB

Open AccessArticle

Faint Galaxy Number Counts in the Durham and SDSS Catalogues

by John H. Marr

Galaxies 2023, 11(3), 65; https://doi.org/10.3390/galaxies11030065 - 7 May 2023

Cited by 1 | Viewed by 1928

Abstract

Galaxy number counts in the K-, H-, I-, R-, B- and U-bands from the Durham Extragalactic Astronomy and Cosmology catalogue could be well-fitted over their whole range using luminosity function (LF) parameters derived from the SDSS at [...] Read more.

Galaxy number counts in the K-, H-, I-, R-, B- and U-bands from the Durham Extragalactic Astronomy and Cosmology catalogue could be well-fitted over their whole range using luminosity function (LF) parameters derived from the SDSS at the bright region and required only modest luminosity evolution with the steepening of the LF slope (

α

), except for a sudden steep increase in the B-band and a less steep increase in the U-band at faint magnitudes that required a starburst evolutionary model to account for the excess faint number counts. A cosmological model treating Hubble expansion as an Einstein curvature required less correction at faint magnitudes than a standard

Λ

CDM model, without requiring dark matter or dark energy. Data from DR17 of the SDSS in the g, i, r, u and z bands over two areas of the sky centred on the North Galactic Cap (NGC) and above the South Galactic Cap (SGC), with areas of 5954 and 859 sq. deg., respectively, and a combined count of 622,121 galaxies, were used to construct bright galaxy number counts and galaxy redshift/density plots within the limits of redshift

\leq 0.4

and mag

\leq 20

. Their comparative densities confirmed an extensive void in the Southern sky with a deficit of 26% out to a redshift z ≤ 0.15. Although not included in the number count data set because of its incompleteness at fainter magnitudes, extending the SDSS redshift-number count survey to fainter and more distant galaxies with redshift ≤ 1.20 showed a secondary peak in the number counts with many QSOs, bright X-ray and radio sources, and evolving irregular galaxies with rapid star formation rates. This sub-population at redshifts of 0.45–0.65 may account for the excess counts observed in the B-band. Recent observations from the HST and James Webb Space Telescope (JWST) have also begun to reveal a high density of massive galaxies at high redshifts (

z > 7

) with high UV and X-ray emissions, and future observations by the JWST may reveal the assembly of galaxies in the early universe going back to the first light in the universe. Full article

► Show Figures

Figure 1

2022

Jump to: 2024, 2023, 2021, 2020

11 pages, 313 KiB

Open AccessArticle

Anthropic Principle and the Hubble-Lemaître Constant

by Michal Křížek and Lawrence Somer

Galaxies 2022, 10(3), 71; https://doi.org/10.3390/galaxies10030071 - 24 May 2022

Cited by 2 | Viewed by 2458

Abstract

According to the weak formulation of the anthropic principle, all fundamental physical constants have just such values that they enabled the origin of life. In this survey paper, we demonstrate also that the current value of the Hubble–Lemaître constant essentially contributed to the [...] Read more.

According to the weak formulation of the anthropic principle, all fundamental physical constants have just such values that they enabled the origin of life. In this survey paper, we demonstrate also that the current value of the Hubble–Lemaître constant essentially contributed to the existence of humankind. Life on Earth has existed continually for at least 3.5 Gyr, and this requires very stable conditions during this quite long time interval. Nevertheless, as the luminosity of the Sun increases, Earth has receded from the Sun by an appropriate speed such that it received an almost constant solar flux during the last 3.5 Gyr. We introduce several other examples illustrating that the solar system and also our galaxy expand by a speed comparable to the Hubble–Lemaître constant. Full article

► Show Figures

Figure 1

8 pages, 435 KiB

Open AccessArticle

Why Masses of Binary Black Hole Mergers Are Overestimated?

by Michal Křížek and Lawrence Somer

Galaxies 2022, 10(2), 52; https://doi.org/10.3390/galaxies10020052 - 23 Mar 2022

Cited by 3 | Viewed by 2618

Abstract

We show that masses of binary black hole mergers are overestimated, since a large gravitational redshift is not taken into account. Such a phenomenon occurs due to time dilation in a close neighborhood of any black hole. This fact allows us to explain [...] Read more.

We show that masses of binary black hole mergers are overestimated, since a large gravitational redshift is not taken into account. Such a phenomenon occurs due to time dilation in a close neighborhood of any black hole. This fact allows us to explain a high mass gap between observed binary neutron stars and calculated binary black hole mergers. We also present other reasons why masses of black hole mergers are determined incorrectly. Full article

► Show Figures

Figure 1

2021

Jump to: 2024, 2023, 2022, 2020

21 pages, 365 KiB

Open AccessArticle

Bound on Photon Circular Orbits in General Relativity and Beyond

by Sumanta Chakraborty

Galaxies 2021, 9(4), 96; https://doi.org/10.3390/galaxies9040096 - 7 Nov 2021

Cited by 11 | Viewed by 2446

Abstract

The existence of a photon circular orbit can tell us a lot about the nature of the underlying spacetime, since it plays a pivotal role in the understanding of the characteristic signatures of compact objects, namely the quasi-normal modes and shadow radius. For [...] Read more.

The existence of a photon circular orbit can tell us a lot about the nature of the underlying spacetime, since it plays a pivotal role in the understanding of the characteristic signatures of compact objects, namely the quasi-normal modes and shadow radius. For this purpose, determination of the location of the photon circular orbit is of utmost importance. In this work, we derive bounds on the location of the photon circular orbit around compact objects within the purview of general relativity and beyond. As we have explicitly demonstrated, contrary to the earlier results in the context of general relativity, the bound on the location of the photon circular orbit is not necessarily an upper bound. Depending on the matter content, it is possible to arrive at a lower bound as well. This has interesting implications for the quasi-normal modes and shadow radius, the two key observables related to the strong field tests of gravity. Besides discussing the bound for higher dimensional general relativity, we have also considered how the bound on the photon circular orbits gets modified in the braneworld scenario, for pure Lovelock and general Lovelock theories of gravity. Implications of these results for compact objects were also discussed. Full article

29 pages, 5349 KiB

Open AccessReview

On the Origin and Evolution of the Intra-Cluster Light: A Brief Review of the Most Recent Developments

by Emanuele Contini

Galaxies 2021, 9(3), 60; https://doi.org/10.3390/galaxies9030060 - 31 Aug 2021

Cited by 48 | Viewed by 4216

Abstract

Not all the light in galaxy groups and clusters comes from stars that are bound to galaxies. A significant fraction of it constitutes the so-called intracluster or diffuse light (ICL), a low surface brightness component of groups/clusters generally found in the surroundings of [...] Read more.

Not all the light in galaxy groups and clusters comes from stars that are bound to galaxies. A significant fraction of it constitutes the so-called intracluster or diffuse light (ICL), a low surface brightness component of groups/clusters generally found in the surroundings of the brightest cluster galaxies and intermediate/massive satellites. In this review, I will describe the mechanisms responsible for its formation and evolution, considering the large contribution given to the topic in the last decades by both the theoretical and observational sides. Starting from the methods that are commonly used to isolate the ICL, I will address the remarkable problem given by its own definition, which still makes the comparisons among different studies not trivial, to conclude by giving an overview of the most recent works that take advantage of the ICL as a luminous tracer of the dark matter distribution in galaxy groups and clusters. Full article

► Show Figures

Figure 1

15 pages, 5206 KiB

Open AccessArticle

Application of Spatio-Temporal Spectral Analysis for Detection of Seismic Waves in Gravitational-Wave Interferometer

by Robert Szymko, Mateusz Denys, Tomasz Bulik, Bartosz Idźkowski, Adam Kutynia, Krzysztof Nikliborc and Maciej Suchiński

Galaxies 2021, 9(3), 50; https://doi.org/10.3390/galaxies9030050 - 14 Jul 2021

Viewed by 2356

Abstract

Mixed spatio–temporal spectral analysis was applied for the detection of seismic waves passing through the west–end building of the Virgo interferometer. The method enables detection of a passing wave, including its frequency, length, direction, and amplitude. A thorough analysis aimed at improving sensitivity [...] Read more.

Mixed spatio–temporal spectral analysis was applied for the detection of seismic waves passing through the west–end building of the Virgo interferometer. The method enables detection of a passing wave, including its frequency, length, direction, and amplitude. A thorough analysis aimed at improving sensitivity of the Virgo detector was made for the data gathered by 38 seismic sensors, in the two–week measurement period, from 24 January to 6 February 2018, and for frequency range 5–20 Hz. Two dominant seismic–wave frequencies were found: 5.5 Hz and 17.1 Hz. The presented method can be applied for a better understanding of the interferometer seismic environment, and by identifying noise sources, help the noise–hunting and mitigation work that eventually leads to interferometer noise suppression. Full article

► Show Figures

Figure 1

10 pages, 294 KiB

Open AccessArticle

Favored Inflationary Models by Scalar Field Condensate Baryogenesis

by Daniela Kirilova and Mariana Panayotova

Galaxies 2021, 9(3), 49; https://doi.org/10.3390/galaxies9030049 - 10 Jul 2021

Cited by 1 | Viewed by 2229

Abstract

We calculate the baryon asymmetry value generated in the Scalar Field Condensate (SCF) baryogenesis model obtained in several inflationary scenarios and different reheating models. We provide analysis of the baryon asymmetry value obtained for more than 70 sets of parameters of the SCF [...] Read more.

We calculate the baryon asymmetry value generated in the Scalar Field Condensate (SCF) baryogenesis model obtained in several inflationary scenarios and different reheating models. We provide analysis of the baryon asymmetry value obtained for more than 70 sets of parameters of the SCF model and the following inflationary scenarios, namely: new inflation, chaotic inflation, Starobinsky inflation, MSSM inflation, quintessential inflation. We considered both cases of efficient thermalization after inflation and delayed thermalization. We have found that the SFC baryogenesis model produces baryon asymmetry orders of magnitude bigger than the observed one for the following inflationary models: new inflation, new inflation model by Shafi and Vilenkin, MSSM inflation, chaotic inflation with high reheating temperature and the simplest Shafi–Vilenkin chaotic inflationary model. Strong diluting mechanisms are needed for these models to reduce the resultant baryon excess at low energies to its observational value today. We have found that a successful generation of the observed baryon asymmetry is possible by the SCF baryogenesis model in Modified Starobinsky inflation, chaotic inflation with low reheating temperature, chaotic inflation in SUGRA, and Quintessential inflation. Full article

► Show Figures

Figure 1

10 pages, 808 KiB

Open AccessArticle

SS 433 Optical Flares: A New Analysis Reveals Their Origin in L₂ Overflow Episodes

by Michael Bowler

Galaxies 2021, 9(3), 46; https://doi.org/10.3390/galaxies9030046 - 22 Jun 2021

Cited by 1 | Viewed by 2088

Abstract

The microquasar SS 433 exhibits in H

α

intermittent flares. A sequence of observations made in 2004 showed flaring Doppler shifted to both the red and the blue simultaneously. The mean shifts varied from day to day, following the orbital phase of the [...] Read more.

The microquasar SS 433 exhibits in H

α

intermittent flares. A sequence of observations made in 2004 showed flaring Doppler shifted to both the red and the blue simultaneously. The mean shifts varied from day to day, following the orbital phase of the compact object. At the time, this behaviour was interpreted as indicating an origin in the outer rim of the accretion disk. A new analysis of these old data, presented in this paper, shows that the flares are not eclipsed by the Companion that eclipses the photosphere surrounding the compact object. They are therefore not intermittent sightings of an accretion disk. The alternative explanation is plasma expelled through the

L_{2}

point, following the phase of the orbit as it invades the space beyond the system. That space has been mapped with comparatively recent GRAVITY observations of a similar flare in Br

γ

, indeed revealing a strong rotation component. Full article

► Show Figures

Figure 1

8 pages, 384 KiB

Open AccessArticle

Effects of 2HDM in Electroweak Phase Transition

by Arnab Chaudhuri, Maxim Yu. Khlopov and Shiladitya Porey

Galaxies 2021, 9(2), 45; https://doi.org/10.3390/galaxies9020045 - 20 Jun 2021

Cited by 7 | Viewed by 2219

Abstract

The entropy production scenarios due to the electroweak phase transition (EWPT) in the framework of the minimal extension of standard model, namely the two Higgs doublet model (2HDM), are revisited. The possibility of first order phase transition is discussed. Intense parameter scanning was [...] Read more.

The entropy production scenarios due to the electroweak phase transition (EWPT) in the framework of the minimal extension of standard model, namely the two Higgs doublet model (2HDM), are revisited. The possibility of first order phase transition is discussed. Intense parameter scanning was done with the help of BSMPT, a C++ package. We perform numerical calculations in order to calculate the entropy production with numerous benchmark points. Full article

► Show Figures

Figure 1

26 pages, 3058 KiB

Open AccessArticle

Ergosphere, Photon Region Structure, and the Shadow of a Rotating Charged Weyl Black Hole

by Mohsen Fathi, Marco Olivares and José R. Villanueva

Galaxies 2021, 9(2), 43; https://doi.org/10.3390/galaxies9020043 - 18 Jun 2021

Cited by 13 | Viewed by 4234

Abstract

In this paper, we explore the photon region and the shadow of the rotating counterpart of a static charged Weyl black hole, which has been previously discussed according to null and time-like geodesics. The rotating black hole shows strong sensitivity to the electric [...] Read more.

In this paper, we explore the photon region and the shadow of the rotating counterpart of a static charged Weyl black hole, which has been previously discussed according to null and time-like geodesics. The rotating black hole shows strong sensitivity to the electric charge and the spin parameter, and its shadow changes from being oblate to being sharp by increasing in the spin parameter. Comparing the calculated vertical angular diameter of the shadow with that of M87*, we found that the latter may possess about

10^{36}

protons as its source of electric charge, if it is a rotating charged Weyl black hole. A complete derivation of the ergosphere and the static limit is also presented. Full article

► Show Figures

Figure 1

7 pages, 777 KiB

Open AccessArticle

Astronomical Seeing at Maidanak Observatory during the Year 2018

by Yusufjon Tillayev, Azimjon Azimov and Aktam Hafizov

Galaxies 2021, 9(2), 38; https://doi.org/10.3390/galaxies9020038 - 21 May 2021

Cited by 6 | Viewed by 2907

Abstract

Results of a four-month campaign of astronomical seeing measurements at Maidanak astronomical observatory (MAO) are presented. A differential image motion monitor (DIMM) was used for seeing estimations during the period from August to November 2018. The observation was organized within the framework of [...] Read more.

Results of a four-month campaign of astronomical seeing measurements at Maidanak astronomical observatory (MAO) are presented. A differential image motion monitor (DIMM) was used for seeing estimations during the period from August to November 2018. The observation was organized within the framework of a site testing for a new telescope which is going to be installed at the observatory. The median value of seeing for the entire period was determined as 0.70 arcseconds, which agrees well with the results of the period 1996–2003. The comparison of monthly values showed that some monthly median values differ from the seasonal trend of the previous period. Full article

► Show Figures

Figure 1

18 pages, 483 KiB

Open AccessReview

Naturally-Coupled Dark Sectors

by Durmuş Demir

Galaxies 2021, 9(2), 33; https://doi.org/10.3390/galaxies9020033 - 12 May 2021

Cited by 7 | Viewed by 2628

Abstract

The dark sector, composed of fields that are neutral under the standard model (SM) gauge group, can couple to the SM through the Higgs, hypercharge and neutrino portals, and pull the SM towards its scale by loop corrections. This instability, which is not [...] Read more.

The dark sector, composed of fields that are neutral under the standard model (SM) gauge group, can couple to the SM through the Higgs, hypercharge and neutrino portals, and pull the SM towards its scale by loop corrections. This instability, which is not possible to prevent in the known SM completions, such as supersymmetry, due to the sizable couplings to the SM, calls for alternative mechanisms that can neutralize sensitivities of the SM to the dark sector scale and to the ultraviolet cutoff above it. Here we review such a mechanism in which incorporation of gravity into the SM predicts the existence of a dark sector and allows it to be naturally coupled to the SM. We discuss and illustrate salient processes that can probe the naturally coupled dark sectors. Full article

► Show Figures

Figure 1

28 pages, 19436 KiB

Open AccessEditor’s ChoiceArticle

Modeling of Spiral Structure in a Multi-Component Milky Way-Like Galaxy

by Sergey Khrapov, Alexander Khoperskov and Vladimir Korchagin

Galaxies 2021, 9(2), 29; https://doi.org/10.3390/galaxies9020029 - 28 Apr 2021

Cited by 7 | Viewed by 4326

Abstract

Using recent observational data, we construct a set of multi-component equilibrium models of the disk of a Milky Way-like galaxy. The disk dynamics are studied using collisionless-gaseous numerical simulations, based on the joined integration of the equations of motion for the collision-less particles [...] Read more.

Using recent observational data, we construct a set of multi-component equilibrium models of the disk of a Milky Way-like galaxy. The disk dynamics are studied using collisionless-gaseous numerical simulations, based on the joined integration of the equations of motion for the collision-less particles using direct integration of gravitational interaction and the gaseous SPH-particles. We find that after approximately one Gyr, a prominent central bar is formed having a semi-axis length of about three kpc, together with a multi-armed spiral pattern represented by a superposition of

m =

2-, 3-, and 4-armed spirals. The spiral structure and the bar exist for at least 3 Gyr in our simulations. The existence of the Milky Way bar imposes limitations on the density distributions in the subsystems of the Milky Way galaxy. We find that a bar does not form if the radial scale length of the density distribution in the disk exceeds 2.6 kpc. As expected, the bar formation is also suppressed by a compact massive stellar bulge. We also demonstrate that the maximum value in the rotation curve of the disk of the Milky Way galaxy, as found in its central regions, is explained by non-circular motion due to the presence of a bar and its orientation relative to an observer. Full article

► Show Figures

Figure 1

53 pages, 947 KiB

Open AccessArticle

History of Astronomy in Australia: Big-Impact Astronomy from World War II until the Lunar Landing (1945–1969)

by Alister W. Graham, Katherine H. Kenyon, Lochlan J. Bull, Visura C. Lokuge Don and Kazuki Kuhlmann

Galaxies 2021, 9(2), 24; https://doi.org/10.3390/galaxies9020024 - 31 Mar 2021

Viewed by 5749

Abstract

Radio astronomy commenced in earnest after World War II, with Australia keenly engaged through the Council for Scientific and Industrial Research. At this juncture, Australia’s Commonwealth Solar Observatory expanded its portfolio from primarily studying solar phenomena to conducting stellar and extragalactic research. Subsequently, [...] Read more.

Radio astronomy commenced in earnest after World War II, with Australia keenly engaged through the Council for Scientific and Industrial Research. At this juncture, Australia’s Commonwealth Solar Observatory expanded its portfolio from primarily studying solar phenomena to conducting stellar and extragalactic research. Subsequently, in the 1950s and 1960s, astronomy gradually became taught and researched in Australian universities. However, most scientific publications from this era of growth and discovery have no country of affiliation in their header information, making it hard to find the Australian astronomy articles from this period. In 2014, we used the then-new Astrophysics Data System (ADS) tool Bumblebee to overcome this challenge and track down the Australian-led astronomy papers published during the quarter of a century after World War II, from 1945 until the lunar landing in 1969. This required knowledge of the research centres and facilities operating at the time, which are briefly summarised herein. Based on citation counts—an objective, universally-used measure of scientific impact—we report on the Australian astronomy articles which had the biggest impact. We have identified the top-ten most-cited papers, and thus also their area of research, from five consecutive time-intervals across that blossoming quarter-century of astronomy. Moreover, we have invested a substantial amount of time researching and providing a small tribute to each of the 62 scientists involved, including several trail-blazing women. Furthermore, we provide an extensive list of references and point out many interesting historical connections and anecdotes. Full article

► Show Figures

Figure 1

24 pages, 391 KiB

Open AccessArticle

Neutrino Charge in a Magnetized Media

by Avijit K. Ganguly, Venktesh Singh, Damini Singh and Ankur Chaubey

Galaxies 2021, 9(2), 22; https://doi.org/10.3390/galaxies9020022 - 30 Mar 2021

Cited by 1 | Viewed by 2764

Abstract

In the presence of a thermal medium or an external electro-magnetic field, neutrinos can interact with photon, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold—to induce an effective

ν γ

vertex [...] Read more.

In the presence of a thermal medium or an external electro-magnetic field, neutrinos can interact with photon, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold—to induce an effective

ν γ

vertex and to modify the dispersion relations of all the particles involved to render the processes kinematically viable. It has already been noted that, in a medium, neutrinos acquire an effective charge, which in the standard model of electroweak interaction comes from the vector type vertex of weak interaction. On the other hand, in a magnetized plasma, the axial vector part also starts contributing to the effective charge of a neutrino. This contribution corresponding to the axial vector part in the interaction Lagrangian is denoted as the axial polarisation tensor. In this note, we outline the calculation of the axial polarization tensor to odd and even powers in

e B

. We further show its gauge invariance properties. Finally, we infer upon the zero external momentum limit of this axial polarisation tensor. Full article

► Show Figures

Figure 1

12 pages, 471 KiB

Open AccessArticle

A Possible Quasi-Periodic Oscillation in the X-ray Emission of 3C 120

by Aditi Agarwal, Priyanka Rani, Raj Prince, C. S. Stalin, G. C. Anupama and Vipul Agrawal

Galaxies 2021, 9(2), 20; https://doi.org/10.3390/galaxies9020020 - 25 Mar 2021

Cited by 6 | Viewed by 3344

Abstract

We present here the detection of a possible quasi-periodic oscillation (QPO) signal in the X-ray light curve of the active galactic nucleus 3C 120, a broad line radio galaxy at z = 0.033. The hint of a QPO at the 3

σ

level [...] Read more.

We present here the detection of a possible quasi-periodic oscillation (QPO) signal in the X-ray light curve of the active galactic nucleus 3C 120, a broad line radio galaxy at z = 0.033. The hint of a QPO at the 3

σ

level at 7.1 × 10

^{- 6}

Hz (∼1.65 days) was detected based on the analysis of X-ray data acquired in the 3–79 keV band by the Nuclear Spectroscopic Telescope Array (NuSTAR). The data, when processed separately in the soft (3–10 keV), hard (10–79 keV) and the total (3–79 keV) bands using four different techniques, namely discrete correlation function, Lomb Scargle periodogram, structure–function, and power spectral density indicated the presence of a QPO. 3C 120 very well fits in the negative correlation in the frequency of the QPO versus the black hole mass (F

_{Q P O}

versus M

_{B H}

) diagram known for stellar-mass and supermassive black hole sources. Considering the observed signs of QPO to represent the innermost stable orbit of the accretion disk, we found a black hole mass of

1.9 \times 10^{9}

M

_{⊙}

for a Kerr black hole and

3.04 \times 10^{8}

M

_{⊙}

for a Schwarzschild black hole. This deduced black hole mass from QPO measurement is a few times larger than the black hole mass obtained from reverberation mapping observations. Full article

► Show Figures

Figure 1

15 pages, 632 KiB

Open AccessArticle

The Dirac Electron Consistent with Proper Gravitational and Electromagnetic Field of the Kerr–Newman Solution

by Alexander Burinskii

Galaxies 2021, 9(1), 18; https://doi.org/10.3390/galaxies9010018 - 17 Mar 2021

Cited by 2 | Viewed by 3065

Abstract

The Dirac electron is considered as a particle-like solution consistent with its own Kerr–Newman (KN) gravitational field. In our previous works we considered the regularized by López KN solution as a bag-like soliton model formed from the Higgs field in a supersymmetric vacuum [...] Read more.

The Dirac electron is considered as a particle-like solution consistent with its own Kerr–Newman (KN) gravitational field. In our previous works we considered the regularized by López KN solution as a bag-like soliton model formed from the Higgs field in a supersymmetric vacuum state. This bag takes the shape of a thin superconducting disk coupled with circular string placed along its perimeter. Using the unique features of the Kerr–Schild coordinate system, which linearizes Dirac equation in KN space, we obtain the solution of the Dirac equations consistent with the KN gravitational and electromagnetic field, and show that the corresponding solution takes the form of a massless relativistic string. Obvious parallelism with Heisenberg and Schrödinger pictures of quantum theory explains remarkable features of the electron in its interaction with gravity and in the relativistic scattering processes. Full article

► Show Figures

Figure 1

6 pages, 227 KiB

Open AccessArticle

Black Holes and Other Clues to the Quantum Structure of Gravity

by Steven B. Giddings

Galaxies 2021, 9(1), 16; https://doi.org/10.3390/galaxies9010016 - 25 Feb 2021

Cited by 6 | Viewed by 2837

Abstract

Bringing gravity into a quantum-mechanical framework is likely the most profound remaining problem in fundamental physics. The “unitarity crisis” for black hole evolution appears to be a key facet of this problem, whose resolution will provide important clues. Investigating this raises the important [...] Read more.

Bringing gravity into a quantum-mechanical framework is likely the most profound remaining problem in fundamental physics. The “unitarity crisis” for black hole evolution appears to be a key facet of this problem, whose resolution will provide important clues. Investigating this raises the important structural question of how to think about subsystems and localization of information in quantum gravity. Paralleling field theory, the answer to this is expected to be an important ingredient in the mathematical structure of the theory. Perturbative gravity results indicate a structure different from that of quantum field theory, but suggest an avenue to defining subsystems. If black holes do behave similarly to familiar subsystems, unitarity demands new interactions that transfer entanglement from them. Such interactions can be parameterized in an effective approach, without directly addressing the question of the fundamental dynamics, whether that is associated with quantum spacetime, wormholes, or something else. Since such interactions need to extend outside the horizon, that raises the question of whether they can be constrained, or might be observed, by new electromagnetic or gravitational wave observations of strong gravity regions. This note overviews and provides connections between these developments. Full article

41 pages, 27864 KiB

Open AccessReview

Survey of CO₂ Radiation Experimental Data in Relation with Planetary Entry

by Philippe Reynier

Galaxies 2021, 9(1), 15; https://doi.org/10.3390/galaxies9010015 - 23 Feb 2021

Cited by 8 | Viewed by 3028

Abstract

This paper focuses on a survey of experimental data related to radiation into CO₂ plasma flows, which are encountered during Mars and Venus entries. The review emphasizes on VUV and IR radiation, since recent experimental efforts has been devoted to these wavelength [...] Read more.

This paper focuses on a survey of experimental data related to radiation into CO₂ plasma flows, which are encountered during Mars and Venus entries. The review emphasizes on VUV and IR radiation, since recent experimental efforts has been devoted to these wavelength ranges since they contribute mostly to CO₂ plasma radiation. The main objective of the study is to identify the most attractive datasets for future crosscheck comparisons with the results obtained during future test campaigns with ESTHER shock-tube. The survey accounts for the results obtained in shock-tubes, expansion tube and plasma arc-jets for Mars and Venus test campaigns. The experimental results obtained for propulsion related studies have also been considered. Full article

► Show Figures

Figure 1

16 pages, 3439 KiB

Open AccessArticle

Dark Supernova Remnants Revealed by CO-Line Bubbles in the W43 Molecular Complex along the 4-kpc Galactic Arm

by Yoshiaki Sofue

Galaxies 2021, 9(1), 13; https://doi.org/10.3390/galaxies9010013 - 4 Feb 2021

Cited by 6 | Viewed by 2562

Abstract

Fine structure of the density distribution in giant molecular clouds (GMCs) around W43 (G31+00+90 km s

^{- 1}

at ∼5.5 kpc) was analyzed using the FUGIN

^{*}

CO-line survey at high-angular (20”∼0.5 pc) and velocity (1.3 km s

^{- 1}

) resolutions ( [...] Read more.

Fine structure of the density distribution in giant molecular clouds (GMCs) around W43 (G31+00+90 km s

^{- 1}

at ∼5.5 kpc) was analyzed using the FUGIN

^{*}

CO-line survey at high-angular (20”∼0.5 pc) and velocity (1.3 km s

^{- 1}

) resolutions (

^{*}

Four-receiver-system Unbiased Galactic Imaging survey with the Nobeyama 45-m telescope). The GMCs show highly turbulent structures, and the eddies are found to exhibit spherical bubble morphology appearing in narrow ranges of velocity channels. The bubbles are dark in radio continuum emission, unlike usual supernova remnants (SNR) or HII regions, and in infrared dust emission, unlike molecular bubbles around young stellar objects. The CO bubbles are interpreted as due to fully evolved buried SNRs in molecular clouds after rapid exhaustion of the released energy in dense molecular clouds. Then, the CO bubbles may be a direct evidence for exciting and maintaining the turbulence in GMCs by SN origin. Search for CO bubbles as “dark SNRs” (dSNR) will have implication to estimate the supernova rate more accurately, and hence the star formation activity in the Milky Way. Full article

► Show Figures

Figure 1

14 pages, 382 KiB

Open AccessEditor’s ChoiceReview

Radio Constraints of Dark Matter: A Review and Some Future Perspectives

by Man Ho Chan

Galaxies 2021, 9(1), 11; https://doi.org/10.3390/galaxies9010011 - 28 Jan 2021

Cited by 7 | Viewed by 3311

Abstract

In the past few decades, many studies have analyzed the data of gamma-rays, X-rays, radio waves, electrons, positrons, anti-protons, and neutrinos to search for the signal of dark matter annihilation. In particular, analyzing radio data has been one of the most important and [...] Read more.

In the past few decades, many studies have analyzed the data of gamma-rays, X-rays, radio waves, electrons, positrons, anti-protons, and neutrinos to search for the signal of dark matter annihilation. In particular, analyzing radio data has been one of the most important and effective ways to constrain dark matter. In this article, we review the physics and the theoretical framework of using radio data to constrain annihilating dark matter. We also review some important radio constraints of annihilating dark matter and discuss the future perspectives of using radio detection to reveal the nature of dark matter. Full article

► Show Figures

Figure 1

16 pages, 579 KiB

Open AccessArticle

Gravitational Measurements in Higher Dimensions

by Davood Mahdavian Yekta, Seyed Aliasghar Alavi and Majid Karimabadi

Galaxies 2021, 9(1), 4; https://doi.org/10.3390/galaxies9010004 - 11 Jan 2021

Cited by 5 | Viewed by 3472

Abstract

We attempt to study three significant tests of general relativity in higher dimensions, both in commutative and non-commutative spaces. In the context of non-commutative geometry, we will consider a solution of Einstein’s equation in higher dimensions, with a source given by a static, [...] Read more.

We attempt to study three significant tests of general relativity in higher dimensions, both in commutative and non-commutative spaces. In the context of non-commutative geometry, we will consider a solution of Einstein’s equation in higher dimensions, with a source given by a static, spherically symmetric Gaussian distribution of mass. The resulting metric would describe a regular or curvature singularity free black hole in higher dimensions. The metric should smoothly interpolate between Schwarzschild geometry at large distance, and de-Sitter spacetime at short distance. We will consider gravitational redshift, lensing, and time delay in each sector. It will be shown that, compared to the four-dimensional spacetime, there can be significant modifications due to the presence of extra dimensions and the non-commutative corrected black holes. Finally, we shall attempt to obtain a lower bound on the size of the extra dimensions and on the mass needed to form a black hole in different dimensions. Full article

► Show Figures

Figure 1

2020

Jump to: 2024, 2023, 2022, 2021

16 pages, 3065 KiB

Open AccessReview

Messengers of the Universe-Cosmic Rays Exploring Supermassive Black Holes

by Anna Uryson

Galaxies 2021, 9(1), 2; https://doi.org/10.3390/galaxies9010002 - 29 Dec 2020

Viewed by 2676

Abstract

Cosmic rays were discovered over one hundred years ago but there are still unsolved problems. One of the hot problems is the origin of cosmic rays of the highest energies. Sources are still unclear and it is neither clear how particles gain ultra-high [...] Read more.

Cosmic rays were discovered over one hundred years ago but there are still unsolved problems. One of the hot problems is the origin of cosmic rays of the highest energies. Sources are still unclear and it is neither clear how particles gain ultra-high energies. Possible sources of cosmic rays at the highest energies are supermassive black holes. From this perspective we discuss in a popular form some recent developments in cosmic ray studies along with author’s recent results. The paper also offers materials for further reading. Full article

► Show Figures

Figure 1

17 pages, 381 KiB

Open AccessArticle

Anomalous Sun Flyby of 1I/2017 U1 (`Oumuamua)

by Klaus Wilhelm and Bhola N. Dwivedi

Galaxies 2020, 8(4), 83; https://doi.org/10.3390/galaxies8040083 - 7 Dec 2020

Cited by 2 | Viewed by 3536

Abstract

The findings of Micheli et al. (Nature2018, 559, 223–226) that 1I/2017 U1 (`Oumuamua) showed anomalous orbital accelerations have motivated us to apply an impact model of gravity in search for an explanation. A small deviation from the [...] Read more.

The findings of Micheli et al. (Nature2018, 559, 223–226) that 1I/2017 U1 (`Oumuamua) showed anomalous orbital accelerations have motivated us to apply an impact model of gravity in search for an explanation. A small deviation from the

1 / r

potential, where r is the heliocentric distance, is expected for the gravitational interaction of extended bodies as a consequence of this model. This modification of the potential results from an offset of the effective gravitational centre from the geometric centre of a spherically symmetric body. Applied to anomalous Earth flybys, the model accounts for energy gains relative to an exact Kepler orbit and an increased speed of several spacecraft. In addition, the flat rotation profiles of eight disk galaxies could be explained, as well as the anomalous perihelion advances of the inner planets and the asteroid Icarus. The solution in the case of `Oumuamua is also based on the proposal that the offset leads to an approach and flyby trajectory different from a Kepler orbit without postulating cometary activity. As a consequence, an adjustment of the potential and centrifugal orbital energies can be envisaged outside the narrow uncertainty ranges of the published post-perihelion data without a need to re-analyse the original data. The observed anomalous acceleration has been modelled with respect to the orbit solutions JPL 16 and “Pseudo-MPEC” for 1I/`Oumuamua. Full article

► Show Figures

Figure 1

Journal Menu

Journal Browser

A Trip across the Universe: Our Present Knowledge and Future Perspectives

Share This Topical Collection

Editor

Topical Collection Information

Keywords

Published Papers (29 papers)

2024

Jump to: 2023, 2022, 2021, 2020

2023

Jump to: 2024, 2022, 2021, 2020

2022

Jump to: 2024, 2023, 2021, 2020

2021

Jump to: 2024, 2023, 2022, 2020

2020

Jump to: 2024, 2023, 2022, 2021

Further Information

Guidelines

MDPI Initiatives

Follow MDPI

Best Paper Award 2020 (800 CHF)

Winner announcement date: 31 January 2022
download description