Galaxies

15 pages, 782 KiB

Open AccessArticle

Radial Oscillations in Neutron Stars from Unified Hadronic and Quarkyonic Equation of States

by Souhardya Sen, Shubham Kumar, Athul Kunjipurayil, Pinku Routaray, Sayantan Ghosh, Probit J. Kalita, Tianqi Zhao and Bharat Kumar

Galaxies 2023, 11(2), 60; https://doi.org/10.3390/galaxies11020060 - 19 Apr 2023

Cited by 12 | Viewed by 2494

Abstract

We study radial oscillations in non-rotating neutron stars by considering the unified equation of states (EoSs), which support the 2 M_⊙ star criterion. We solve the Sturm–Liouville problem to compute the 20 lowest radial oscillation modes and their eigenfunctions for a neutron [...] Read more.

We study radial oscillations in non-rotating neutron stars by considering the unified equation of states (EoSs), which support the 2 M_⊙ star criterion. We solve the Sturm–Liouville problem to compute the 20 lowest radial oscillation modes and their eigenfunctions for a neutron star modeled with eight selected unified EoSs from distinct Skyrme–Hartree–Fock, relativistic mean field and quarkyonic models. We compare the behavior of the computed eigenfrequency for an NS modeled with hadronic to one with quarkyonic EoSs while varying the central densities. The lowest-order f-mode frequency varies substantially between the two classes of the EoS at 1.4 M_⊙ but vanishes at their respective maximum masses, consistent with the stability criterion

\partial M / \partial ρ_{c} > 0

. Moreover, we also compute large frequency separation and discover that higher-order mode frequencies are significantly reduced by incorporating a crust in the EoS. Full article

(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)

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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 4723

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 [...] Read more.

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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11 pages, 296 KiB

Open AccessArticle

On the Periodic Orbits of the Perturbed Two- and Three-Body Problems

by Elbaz I. Abouelmagd, Juan Luis García Guirao and Jaume Llibre

Galaxies 2023, 11(2), 58; https://doi.org/10.3390/galaxies11020058 - 18 Apr 2023

Cited by 8 | Viewed by 2087

Abstract

In this work, a perturbed system of the restricted three-body problem is derived when the perturbation forces are conservative alongside the corresponding mean motion of two primaries bodies. Thus, we have proved that the first and second types of periodic orbits of the [...] Read more.

In this work, a perturbed system of the restricted three-body problem is derived when the perturbation forces are conservative alongside the corresponding mean motion of two primaries bodies. Thus, we have proved that the first and second types of periodic orbits of the rotating Kepler problem can persist for all perturbed two-body and circular restricted three-body problems when the perturbation forces are conservative or the perturbed motion has its own extended Jacobian integral. Full article

(This article belongs to the Special Issue Relativistic Cosmology, Numerical Analysis, General Relativity and Modified Gravity Theories)

17 pages, 538 KiB

Open AccessArticle

The Simplest Parametrization of the Equation of State Parameter in the Scalar Field Universe

by Preeti Shrivastava, Abdul Junaid Khan, Mukesh Kumar, Gopikant Goswami, Jainendra Kumar Singh and Anil Kumar Yadav

Galaxies 2023, 11(2), 57; https://doi.org/10.3390/galaxies11020057 - 17 Apr 2023

Cited by 15 | Viewed by 2091

Abstract

In this paper, we investigate a scalar field cosmological model of accelerating Universe with the simplest parametrization of the equation of state parameter of the scalar field. We use

H (z)

data, pantheon compilation of SN Ia data and BAO data [...] Read more.

In this paper, we investigate a scalar field cosmological model of accelerating Universe with the simplest parametrization of the equation of state parameter of the scalar field. We use

H (z)

data, pantheon compilation of SN Ia data and BAO data to constrain the model parameters using the

χ^{2}

minimization technique. We obtain the present values of Hubble constant

H_{0}

as

66 . 2_{- 1.34}^{+ 1.42}

,

70 . 7_{- 0.31}^{+ 0.32}

and

67 . 74_{- 1.04}^{+ 1.24}

for

H (z)

,

H (z)

+ Pantheon and

H (z)

+ BAO respectively. In addition, we estimate the present age of the Universe in a derived model

t_{0} = 14 . 38_{- 0.64}^{+ 0.63}

for joint

H (z)

and pantheon compilation of SN Ia data which has only

0.88 σ

tension with its empirical value obtained in Plank collaboration. Moreover, the present values of the deceleration parameter

q_{0}

come out to be

- 0 . 55_{- 0.038}^{+ 0.031}

,

- 0 . 61_{- 0.021}^{+ 0.030}

and

- 0 . 627_{- 0.025}^{+ 0.022}

by bounding the Universe in the derived model with

H (z)

,

H (z)

+ Pantheon compilation of SN Ia and

H (z)

+ BAO data sets, respectively. We also have performed the state-finder diagnostics to discover the nature of dark energy. Full article

(This article belongs to the Special Issue Relativistic Cosmology, Numerical Analysis, General Relativity and Modified Gravity Theories)

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45 pages, 9124 KiB

Open AccessReview

Convective Boundary Mixing in Main-Sequence Stars: Theory and Empirical Constraints

by Evan H. Anders and May G. Pedersen

Galaxies 2023, 11(2), 56; https://doi.org/10.3390/galaxies11020056 - 14 Apr 2023

Cited by 23 | Viewed by 3247

Abstract

The convective envelopes of solar-type stars and the convective cores of intermediate- and high-mass stars share boundaries with stable radiative zones. Through a host of processes we collectively refer to as “convective boundary mixing” (CBM), convection can drive efficient mixing in these nominally [...] Read more.

The convective envelopes of solar-type stars and the convective cores of intermediate- and high-mass stars share boundaries with stable radiative zones. Through a host of processes we collectively refer to as “convective boundary mixing” (CBM), convection can drive efficient mixing in these nominally stable regions. In this review, we discuss the current state of CBM research in the context of main-sequence stars through three lenses. (1) We examine the most frequently implemented 1D prescriptions of CBM—exponential overshoot, step overshoot, and convective penetration—and we include a discussion of implementation degeneracies and how to convert between various prescriptions. (2) Next, we examine the literature of CBM from a fluid dynamical perspective, with a focus on three distinct processes: convective overshoot, entrainment, and convective penetration. (3) Finally, we discuss observational inferences regarding how much mixing should occur in the cores of intermediate- and high-mass stars as well as the implied constraints that these observations place on 1D CBM implementations. We conclude with a discussion of pathways forward for future studies to place better constraints on this difficult challenge in stellar evolution modeling. Full article

(This article belongs to the Special Issue The Structure and Evolution of Stars)

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15 pages, 2491 KiB

Open AccessArticle

Revealing the Binarity of HD 36030—One of the Hottest Flare Stars

by Olga Maryeva, Péter Németh and Sergey Karpov

Galaxies 2023, 11(2), 55; https://doi.org/10.3390/galaxies11020055 - 12 Apr 2023

Cited by 1 | Viewed by 1735

Abstract

The Kepler and TESS space missions significantly expanded our knowledge of what types of stars display flaring activity by recording a vast amount of super-flares from solar-like stars, as well as detecting flares from hotter stars of A-F spectral types. Currently, we know [...] Read more.

The Kepler and TESS space missions significantly expanded our knowledge of what types of stars display flaring activity by recording a vast amount of super-flares from solar-like stars, as well as detecting flares from hotter stars of A-F spectral types. Currently, we know that flaring occurs in the stars as hot as B-type ones. However, the structures of atmospheres of hot B-A stars crucially differ from the ones of late types, and thus the occurrence of flaring in B-A type stars requires some extension of our theoretical views of flare formation and therefore a detailed study of individual objects. Here we present the results of our spectral and photometric study of HD 36030, which is a B9 V star with flares detected by the TESS satellite. The spectra we acquired suggest that the star is in a binary system with a low-mass secondary component, but the light curve lacks any signs of periodic variability related to orbital motion or surface magnetic fields. Because of that, we argue that the flares originate due to magnetic interaction between the components of the system. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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103 pages, 3133 KiB

Open AccessReview

BCD Spectrophotometry and Rotation of Active B-Type Stars: Theory and Observations

by Juan Zorec

Galaxies 2023, 11(2), 54; https://doi.org/10.3390/galaxies11020054 - 10 Apr 2023

Cited by 6 | Viewed by 3641

Abstract

This review has two parts. The first one is devoted to the Barbier–Chalonge–Divan (BCD) spectrophotometric system, also known as the Paris spectral classification system. Although the BCD system has been applied and is still used for all stellar objects from O to F [...] Read more.

This review has two parts. The first one is devoted to the Barbier–Chalonge–Divan (BCD) spectrophotometric system, also known as the Paris spectral classification system. Although the BCD system has been applied and is still used for all stellar objects from O to F spectral types, the present account mainly concerns normal and ‘active’ B-type stars. The second part treats topics related to stellar rotation, considered one of the key phenomena determining the structure and evolution of stars. The first part is eminently observational. In contrast, the second part deals with observational aspects related to stellar rotation but also recalls some supporting or basic theoretical concepts that may help better understand the gains and shortcomings of today’s existent interpretation of stellar data. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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14 pages, 906 KiB

Open AccessArticle

Intermediate-Mass Black Holes: The Essential Population to Explore the Unified Model for Accretion and Ejection Processes

by Xiaolong Yang and Jun Yang

Galaxies 2023, 11(2), 53; https://doi.org/10.3390/galaxies11020053 - 6 Apr 2023

Cited by 1 | Viewed by 2116

Abstract

We study radio and X-ray emissions from intermediate-mass black holes (IMBHs) and explore the unified model for accretion and ejection processes. The radio band survey of IMBH (candidate) hosted galaxies indicates that only a small fraction (∼0.6%) of them are radio-band active. In [...] Read more.

We study radio and X-ray emissions from intermediate-mass black holes (IMBHs) and explore the unified model for accretion and ejection processes. The radio band survey of IMBH (candidate) hosted galaxies indicates that only a small fraction (∼0.6%) of them are radio-band active. In addition, very long baseline interferometry observations reveal parsec-scale radio emission of IMBHs, further resulting in a lower fraction of actively ejecting objects (radio emission is produced by IMBHs other than hosts), which is consistent with a long quiescent state in the evolution cycle of IMBHs. Most (75%, i.e., 3 out of 4 samples according to a recent mini-survey) of the radio-emitting IMBHs are associated with radio relics and there is also evidence of dual radio blobs from episodic ejecting phases. Taking the radio emission and the corresponding core X-ray emission of IMBH, we confirm a universal fundamental plane relation (FMP) of black hole activity. Furthermore, state transitions can be inferred by comparing a few cases in XRBs and IMBHs in FMP, i.e., both radio luminosity and emission regions evolve along these state transitions. These signatures and evidence suggest an analogy among all kinds of accretion systems which span from stellar mass to supermassive black holes, hinting at unified accretion and ejection physics. To validate the unified model, we explore the correlation between the scale of outflows (corresponding to ejection powers) and the masses of central engines; it shows that the largest scale of outflows

{\hat{L S}}_{out}

follows a power-law correlation with the masses of accretors

M_{core}

, i.e.,

log {\hat{L S}}_{out} = (0.73 \pm 0.01) log M_{core} - (3.34 \pm 0.10)

. In conclusion, this work provides evidence to support the claim that the ejection (and accretion) process behaves as scale-invariant and their power is regulated by the masses of accretors. Full article

(This article belongs to the Special Issue The Symbiosis between Radio Source and Galaxy Evolution)

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13 pages, 756 KiB

Open AccessArticle

Metal Content in Relativistically Jetted and Radio-Quiet Quasars in the Main Sequence Context

by Paola Marziani, Swayamtrupta Panda, Alice Deconto Machado and Ascension Del Olmo

Galaxies 2023, 11(2), 52; https://doi.org/10.3390/galaxies11020052 - 30 Mar 2023

Cited by 7 | Viewed by 1550

Abstract

The optical and UV properties of radio-quiet (RQ) and radio-loud (RL, relativistically “jetted”) active galactic nuclei (AGN) are known to differ markedly; however, it is still unclear what is due to a sample selection and what is associated with intrinsic differences in the [...] Read more.

The optical and UV properties of radio-quiet (RQ) and radio-loud (RL, relativistically “jetted”) active galactic nuclei (AGN) are known to differ markedly; however, it is still unclear what is due to a sample selection and what is associated with intrinsic differences in the inner workings of their emitting regions. Chemical composition is an important parameter related to the trends of the quasar main sequence. Recent works suggest that in addition to physical properties such as density, column density, and ionization level, strong Feii emitters require very high metal content. Little is known, however, about the chemical composition of jetted radio-loud sources. In this short note, we present a pilot analysis of the chemical composition of low-z radio-loud and radio-quiet quasars. Optical and UV spectra from ground and space were combined to allow for precise measurements of metallicity-sensitive diagnostic ratios. The comparison between radio-quiet and radio-loud was carried out for sources in the same domain of the Eigenvector 1/main sequence parameter space. Arrays of dedicated photo-ionization simulations with the input of appropriate spectral energy distributions indicate that metallicity is sub-solar for RL AGN, and slightly sub-solar or around solar for RQ AGN. The metal content of the broad line-emitting region likely reflects a similar enrichment story for both classes of AGN not involving recent circum-nuclear or nuclear starbursts. Full article

(This article belongs to the Special Issue Jet Physics of Accreting Super Massive Black Holes)

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15 pages, 5062 KiB

Open AccessFeature PaperArticle

Hydrodynamic Simulations of a Relativistic Jet Interacting with the Intracluster Medium: Application to Cygnus A

by John A. ZuHone, Paul E. J. Nulsen, Po-Hsun Tseng, Hsi-Yu Schive and Tom W. Jones

Galaxies 2023, 11(2), 51; https://doi.org/10.3390/galaxies11020051 - 23 Mar 2023

Viewed by 1861

Abstract

The Fanaroff–Riley Class II radio galaxy Cygnus A hosts jets that produce radio emission, X-ray cavities, cocoon shocks, and X-ray hotspots, where the jet interacts with the ICM. Surrounding one hotspot is a peculiar “hole” feature, which appears as a deficit in X-ray [...] Read more.

The Fanaroff–Riley Class II radio galaxy Cygnus A hosts jets that produce radio emission, X-ray cavities, cocoon shocks, and X-ray hotspots, where the jet interacts with the ICM. Surrounding one hotspot is a peculiar “hole” feature, which appears as a deficit in X-ray emission. We used relativistic hydrodynamic simulations of a collimated jet interacting with an inclined interface between lobe and cluster plasma to model the basic processes that may lead to such a feature. We found that the jet reflects off of the interface into a broad, turbulent flow back out into the lobe, which is dominated by gas stripped from the interface at first and from the intracluster medium itself at later times. We produced simple models of X-ray emission from the ICM, the hotspot, and the reflected jet to show that a hole of emission surrounding the hotspot as seen in Cygnus A may be produced by Doppler de-boosting of the emission from the reflected jet, as seen by an observer with a sight line nearly along the axis of the outgoing material. Full article

(This article belongs to the Special Issue New Perspectives on Radio Galaxy 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 1675

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 [...] Read more.

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, 2742 KiB

Open AccessReview

The Past and Future of East Asia to Italy: Nearly Global VLBI

by Gabriele Giovannini, Yuzhu Cui, Kazuhiro Hada, Kunwoo Yi, Hyunwook Ro, Bong Won Sohn, Mieko Takamura, Salvatore Buttaccio, Filippo D’Ammando, Marcello Giroletti, Yoshiaki Hagiwara, Motoki Kino, Evgeniya Kravchenko, Giuseppe Maccaferri, Alexey Melnikov, Kotaro Niinuma, Monica Orienti, Kiyoaki Wajima, Kazunori Akiyama, Akihiro Doi, Do-Young Byun, Tomoya Hirota, Mareki Honma, Taehyun Jung, Hideyuki Kobayashi, Shoko Koyama, Andrea Melis, Carlo Migoni, Yasuhiro Murata, Hiroshi Nagai, Satoko Sawada-Satoh and Matteo Stagni Show full author list Hide full author list

Galaxies 2023, 11(2), 49; https://doi.org/10.3390/galaxies11020049 - 22 Mar 2023

Cited by 3 | Viewed by 3167

Abstract

We present here the East Asia to Italy Nearly Global VLBI (EATING VLBI) project. How this project started and the evolution of the international collaboration between Korean, Japanese, and Italian researchers to study compact sources with VLBI observations is reported. Problems related to [...] Read more.

We present here the East Asia to Italy Nearly Global VLBI (EATING VLBI) project. How this project started and the evolution of the international collaboration between Korean, Japanese, and Italian researchers to study compact sources with VLBI observations is reported. Problems related to the synchronization of the very different arrays and technical details of the telescopes involved are presented and discussed. The relatively high observation frequency (22 and 43 GHz) and the long baselines between Italy and East Asia produced high-resolution images. We present example images to demonstrate the typical performance of the EATING VLBI array. The results attracted international researchers and the collaboration is growing, now including Chinese and Russian stations. New in progress projects are discussed and future possibilities with a larger number of telescopes and a better frequency coverage are briefly discussed herein. Full article

(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)

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2 pages, 172 KiB

Open AccessEditorial

Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics, and Multi-Messenger Observations: Quo Vadis?

by Jaziel G. Coelho and Rita. C. Dos Anjos

Galaxies 2023, 11(2), 48; https://doi.org/10.3390/galaxies11020048 - 22 Mar 2023

Viewed by 1257

Abstract

The era of multi-messenger astronomy has recently be inaugurated with pioneering experiments, theoretical interpretation, and models [...] Full article

(This article belongs to the Special Issue Challenges of This Century in High-Density Compact Objects, High-Energy Astrophysics, and Multi-Messenger Observations. Quo Vadis?)

21 pages, 16327 KiB

Open AccessReview

The Interplay between Radio AGN Activity and Their Host Galaxies

by Guilherme S. Couto and Thaisa Storchi-Bergmann

Galaxies 2023, 11(2), 47; https://doi.org/10.3390/galaxies11020047 - 21 Mar 2023

Cited by 6 | Viewed by 2194

Abstract

Radio activity in AGN (Active Galactic Nuclei) produce feedback on the host galaxy via the impact of the relativistic jets on the circumnuclear gas. Although radio jets can reach up to several times the optical radius of the host galaxy, in this review [...] Read more.

Radio activity in AGN (Active Galactic Nuclei) produce feedback on the host galaxy via the impact of the relativistic jets on the circumnuclear gas. Although radio jets can reach up to several times the optical radius of the host galaxy, in this review we focus on the observation of the feedback deposited locally in the central region of the host galaxies, in the form of outflows due to the jet-gas interaction. We begin by discussing how galaxy mergers and interactions are the most favored scenario for triggering radio AGN after gas accretion to the nuclear supermassive black hole and star formation enhancement in the nuclear region, observed in particular in the most luminous sources. We then discuss observational signatures of the process of jet-gas coupling, in particular the resulting outflows and their effects on the host galaxy. These include the presence of shock signatures and the detection of outflows not only along the radio jet but perpendicular to it in many sources. Although most of the studies are done via the observation of ionized gas, molecular gas is also being increasingly observed in outflow, contributing to the bulk of the mass outflow rate. Even though most radio sources present outflow kinetic powers that do not reach

1 % L_{b o l}

, and thus do not seem to provide an immediate impact on the host galaxy, they act to heat the ISM gas, preventing star formation, slowing the galaxy mass build-up process and limiting the stellar mass growth, in a “maintenance mode” feedback. Full article

(This article belongs to the Special Issue The Symbiosis between Radio Source and Galaxy Evolution)

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15 pages, 5043 KiB

Open AccessArticle

Monitoring the Size and Flux Density of Sgr A* during the Active State in 2019 with East Asian VLBI Network

by Xiaopeng Cheng, Ilje Cho, Tomohisa Kawashima, Motoki Kino, Guang-Yao Zhao, Juan-Carlos Algaba, Yutaro Kofuji, Sang-Sung Lee, Jee-Won Lee, Whee Yeon Cheong, Wu Jiang and Junghwan Oh

Galaxies 2023, 11(2), 46; https://doi.org/10.3390/galaxies11020046 - 20 Mar 2023

Cited by 3 | Viewed by 2293

Abstract

In this work, we studied the Galactic Center supermassive black hole (SMBH), Sagittarius A* (Sgr A

^{*}

), with the KVN and VERA Array (KaVA)/East Asian VLBI Network (EAVN) monitoring observations. Especially in 13 May 2019, Sgr A

^{*}

experienced an unprecedented bright [...] Read more.

In this work, we studied the Galactic Center supermassive black hole (SMBH), Sagittarius A* (Sgr A

^{*}

), with the KVN and VERA Array (KaVA)/East Asian VLBI Network (EAVN) monitoring observations. Especially in 13 May 2019, Sgr A

^{*}

experienced an unprecedented bright near infra-red (NIR) flare; so, we find a possible counterpart at 43 GHz (7 mm). As a result, a large temporal variation of the flux density at the level ∼15.4%, with the highest flux density of 2.04 Jy, is found on 11 May 2019. Interestingly, the intrinsic sizes are also variable, and the area and major-axis size show marginal correlation with flux density with ≳2

σ

. Thus, we interpret that the emission region at 43 GHz follows the larger-when-brighter relation in 2019. The possible origins are discussed with an emergence of a weak jet/outflow component and the position angle change of the rotation axis of the accretion disk in time. Full article

(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)

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20 pages, 6922 KiB

Open AccessArticle

Simulating the Enrichment of Fossil Radio Electrons by Multiple Radio Galaxies

by Franco Vazza, Denis Wittor, Marcus Brüggen and Gianfranco Brunetti

Galaxies 2023, 11(2), 45; https://doi.org/10.3390/galaxies11020045 - 13 Mar 2023

Cited by 7 | Viewed by 1781

Abstract

We simulate the evolution of relativistic electrons injected into the intracluster medium by five radio galaxies. We study the spatial transport and the emission properties of the injected radio plasma over a ∼5 Gyr period, and the sequence of cooling and re-acceleration events [...] Read more.

We simulate the evolution of relativistic electrons injected into the intracluster medium by five radio galaxies. We study the spatial transport and the emission properties of the injected radio plasma over a ∼5 Gyr period, and the sequence of cooling and re-acceleration events experienced by electrons, using a Lagrangian approach joined with a numerical method to model the evolution of momentum spectra of relativistic electrons. When compared with electrons injected by shock waves, electrons injected by radio galaxies (here limited to a single injection event) in our tests are unable to fuel large ∼Mpc-sized radio relics with fossil electrons, as required by current theoretical models, while electrons previously seeded by other shocks can do this. On the other hand, the combination of seeding from radio galaxies and of re-acceleration events from plasma perturbation can produce detectable, small-scale, and filamentary emissions in the proximity (≤100–200 kpc) of radio galaxies. Full article

(This article belongs to the Special Issue New Perspectives on Radio Galaxy Dynamics)

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8 pages, 851 KiB

Open AccessCommunication

Characterizing the Regular Orbits of Binary Pulsars: An Initial Prospection Study

by Ali Taani

Galaxies 2023, 11(2), 44; https://doi.org/10.3390/galaxies11020044 - 8 Mar 2023

Cited by 1 | Viewed by 1457

Abstract

The dynamical interaction between binary systems is crucial in understanding the nature of orbital motion under the influence of gravitational potential. In our study, we focused on investigating the effects of dynamical forces on the regularity of binary pulsar orbits, which represent a [...] Read more.

The dynamical interaction between binary systems is crucial in understanding the nature of orbital motion under the influence of gravitational potential. In our study, we focused on investigating the effects of dynamical forces on the regularity of binary pulsar orbits, which represent a pure two-body system. To incorporate the necessary time dependence and have a regular 3-D axisymmetric potential, we utilized the Rebound package as a numerical integrator. This package integrates the motion of particles under the influence of gravity, allowing for changing orbital parameters at a given instant, and providing a variety of integrators to be used. By analyzing the regularity properties of binaries and their sensitivity to initial conditions, we gained insight into the importance of considering even small perturbations to the system, as they can lead to significant changes in its dynamics. Full article

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12 pages, 366 KiB

Open AccessArticle

Explaining the Hardening Structures of Helium Spectrum and Boron to Carbon Ratio through Different Propagation Models

by Qing Luo, Jie Feng and Pak-Hin Thomas Tam

Galaxies 2023, 11(2), 43; https://doi.org/10.3390/galaxies11020043 - 8 Mar 2023

Cited by 1 | Viewed by 1746

Abstract

Recently, a series of high-precision measurements by various experiments show that cosmic ray nuclei spectra begin to harden at ∼200 GV and the boron-to-carbon (B/C) ratio has a similar trend around the same energy. These anomalous structures possibly result from the journey of [...] Read more.

Recently, a series of high-precision measurements by various experiments show that cosmic ray nuclei spectra begin to harden at ∼200 GV and the boron-to-carbon (B/C) ratio has a similar trend around the same energy. These anomalous structures possibly result from the journey of cosmic rays (CRs) from their sources to our solar system, which has important implications for our understanding of the origin and propagation of Galactic cosmic rays (GCRs). In this work, we investigate several propagation models and attempt to explain these anomalous observations. We have verified that an extension of the traditional propagation model taking into account spatially dependent propagation and secondary particle acceleration provides a more accurate description of the latest B/C ratio and the Helium flux data measured by DAMPE, CALET, and AMS-02. Full article

(This article belongs to the Special Issue The Large-Scale Structure of the Universe: Dark and Luminous Matters)

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12 pages, 1787 KiB

Open AccessFeature PaperArticle

High-Frequency and High-Resolution VLBI Observations of GHz Peaked Spectrum Objects

by Xiaopeng Cheng, Tao An, Ailing Wang and Sumit Jaiswal

Galaxies 2023, 11(2), 42; https://doi.org/10.3390/galaxies11020042 - 7 Mar 2023

Cited by 3 | Viewed by 2057

Abstract

Observational studies of GHz peaked spectrum (GPS) sources contribute to the understanding of the radiative properties and interstellar environment of host galaxies. We present the results from the multi-frequency high-resolution VLBI observations of a sample of nine GPS sources at 8, 15, and [...] Read more.

Observational studies of GHz peaked spectrum (GPS) sources contribute to the understanding of the radiative properties and interstellar environment of host galaxies. We present the results from the multi-frequency high-resolution VLBI observations of a sample of nine GPS sources at 8, 15, and 43 GHz. All sources show core-jet structure. Four sources show relativistic jets with Doppler boosting factors ranging from 2.0 to 5.0 and a jet viewing angle between 10° and 30°. The core brightness temperatures of the other five sources are below the equipartition brightness temperature limit with their jet viewing angles in the range of 13.6° to 71.9°, which are systematically larger than those of relativistic jets in this sample. The sources show diverse variability properties, with variability levels ranging from 0.11 to 0.56. The measured turnover frequency in the radio spectrum ranges from 6.2 and 31.8 GHz (in the source’s rest frame). We estimate the equipartition magnetic field strength to be between 9 and 48 mG. These results strongly support the notion that these GPS sources are young radio sources in the very early stage of their evolution. Full article

(This article belongs to the Special Issue The Symbiosis between Radio Source and Galaxy Evolution)

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15 pages, 7341 KiB

Open AccessArticle

Gravitational Instability of Gas–Dust Circumnuclear Disks in Galaxies

by Roman Tkachenko, Vladimir Korchagin and Boris Jmailov

Galaxies 2023, 11(2), 41; https://doi.org/10.3390/galaxies11020041 - 7 Mar 2023

Viewed by 1811

Abstract

We numerically study the origin of the multi-armed spiral structure observed in the circumnuclear gaseous mini-disks of nearby galaxies. We show that the presence of dust in such disks and its interaction with the gravitationally stable gaseous component leads to the development of [...] Read more.

We numerically study the origin of the multi-armed spiral structure observed in the circumnuclear gaseous mini-disks of nearby galaxies. We show that the presence of dust in such disks and its interaction with the gravitationally stable gaseous component leads to the development of a multi-armed spiral structure. As a particular example, we study the formation of the multi-armed spiral pattern in the mini-disk of the galaxy NGC 4736, for which the observational data for the rotation and the density distribution are available. We find that the multi-armed spiral structure grows in the stable gaseous mini-disk of NGC 4736 if the gas-to-dust ratio is about 5–20 percent. We also demonstrate that together with the dust concentration, the important factor for the development of instability is the size of the dust grains. A nonlinear multi-armed spiral pattern develops in the stable gaseous disk with sizes of grains larger than one micron. If future observations confirm the presence of a large amount of dust in the mini-disks of galaxies, this will pinpoint the mechanism of the formation of the multi-armed spiral structure in them. Full article

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26 pages, 5012 KiB

Open AccessReview

Magnetism in High-Mass Stars

by Zsolt Keszthelyi

Galaxies 2023, 11(2), 40; https://doi.org/10.3390/galaxies11020040 - 5 Mar 2023

Cited by 11 | Viewed by 3319

Abstract

Magnetism is a ubiquitous property of astrophysical plasmas, yet stellar magnetism still remains far from being completely understood. In this review, we describe recent observational and modelling efforts and progress to expand our knowledge of the magnetic properties of high-mass stars. Several mechanisms [...] Read more.

Magnetism is a ubiquitous property of astrophysical plasmas, yet stellar magnetism still remains far from being completely understood. In this review, we describe recent observational and modelling efforts and progress to expand our knowledge of the magnetic properties of high-mass stars. Several mechanisms (magneto-convection, mass-loss quenching, internal angular momentum transport, and magnetic braking) have significant implications for stellar evolution, populations, and end-products. Consequently, it remains an urgent issue to address and resolve open questions related to magnetism in high-mass stars. Full article

(This article belongs to the Special Issue The Structure and Evolution of Stars)

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9 pages, 788 KiB

Open AccessArticle

Super-Resolved Image of M87 Observed with East Asian VLBI Network

by Fumie Tazaki, Yuzhu Cui, Kazuhiro Hada, Motoki Kino, Ilje Cho, Guang-Yao Zhao, Kazunori Akiyama, Yosuke Mizuno, Hyunwook Ro, Mareki Honma, Ru-Sen Lu, Zhi-Qiang Shen, Lang Cui and Yoshinori Yonekura

Galaxies 2023, 11(2), 39; https://doi.org/10.3390/galaxies11020039 - 28 Feb 2023

Viewed by 1777

Abstract

Obtaining high-resolution images at centimeter-or-longer wavelengths is vital for understanding the physics of jets. We reconstructed images from the M87 22 GHz data observed with the East Asian VLBI Network (EAVN) by using the regularized maximum likelihood (RML) method, which is different from [...] Read more.

Obtaining high-resolution images at centimeter-or-longer wavelengths is vital for understanding the physics of jets. We reconstructed images from the M87 22 GHz data observed with the East Asian VLBI Network (EAVN) by using the regularized maximum likelihood (RML) method, which is different from the conventional imaging method CLEAN. Consequently, a bright core and jet extending about 30 mas to the northwest were detected with a higher resolution than in the CLEAN image. The width of the jet was 0.5 mas at 0.3 mas from the core, consistent with the width measured in the 86 GHz image in the previous study. In addition, three ridges were able to be detected at around 8 mas from the core, even though the peak-to-peak separation was only 1.0 mas. This indicates that the RML image’s spatial resolution is at least 30% higher than that of the CLEAN image. This study is an important step for future multi-frequency and high-cadence observations of the EAVN to discuss the more detailed structure of the jet and its time variability. Full article

(This article belongs to the Special Issue Challenges in Understanding Black Hole Powered Jets with VLBI)

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23 pages, 2492 KiB

Open AccessArticle

Accretion Flow Morphology in Numerical Simulations of Black Holes from the ngEHT Model Library: The Impact of Radiation Physics

by Koushik Chatterjee, Andrew Chael, Paul Tiede, Yosuke Mizuno, Razieh Emami, Christian Fromm, Angelo Ricarte, Lindy Blackburn, Freek Roelofs, Michael D. Johnson, Sheperd S. Doeleman, Philipp Arras, Antonio Fuentes, Jakob Knollmüller, Nikita Kosogorov, Greg Lindahl, Hendrik Müller, Nimesh Patel, Alexander Raymond, Efthalia Traianou and Justin Vega Show full author list Hide full author list

Galaxies 2023, 11(2), 38; https://doi.org/10.3390/galaxies11020038 - 23 Feb 2023

Cited by 16 | Viewed by 2738

Abstract

In the past few years, the Event Horizon Telescope (EHT) has provided the first-ever event horizon-scale images of the supermassive black holes (BHs) M87

^{*}

and Sagittarius A

^{*}

(Sgr A

^{*}

). The next-generation EHT project is an extension of the EHT [...] Read more.

In the past few years, the Event Horizon Telescope (EHT) has provided the first-ever event horizon-scale images of the supermassive black holes (BHs) M87

^{*}

and Sagittarius A

^{*}

(Sgr A

^{*}

). The next-generation EHT project is an extension of the EHT array that promises larger angular resolution and higher sensitivity to the dim, extended flux around the central ring-like structure, possibly connecting the accretion flow and the jet. The ngEHT Analysis Challenges aim to understand the science extractability from synthetic images and movies to inform the ngEHT array design and analysis algorithm development. In this work, we compare the accretion flow structure and dynamics in numerical fluid simulations that specifically target M87

^{*}

and Sgr A

^{*}

, and were used to construct the source models in the challenge set. We consider (1) a steady-state axisymmetric radiatively inefficient accretion flow model with a time-dependent shearing hotspot, (2) two time-dependent single fluid general relativistic magnetohydrodynamic (GRMHD) simulations from the H-AMR code, (3) a two-temperature GRMHD simulation from the BHAC code, and (4) a two-temperature radiative GRMHD simulation from the KORAL code. We find that the different models exhibit remarkably similar temporal and spatial properties, except for the electron temperature, since radiative losses substantially cool down electrons near the BH and the jet sheath, signaling the importance of radiative cooling even for slowly accreting BHs such as M87

^{*}

. We restrict ourselves to standard torus accretion flows, and leave larger explorations of alternate accretion models to future work. Full article

(This article belongs to the Special Issue From Vision to Instrument: Creating a Next-Generation Event Horizon Telescope for a New Era of Black Hole Science)

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Galaxies, Volume 11, Issue 2 (April 2023) – 23 articles

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