Journal Description
Galaxies
Galaxies
is a peer-reviewed, open access journal of astronomy, astrophysics, and cosmology published bimonthly online by MDPI. We urge all authors to post the papers on the arXiv.
- Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
- High Visibility: indexed within Scopus, ESCI (Web of Science), Astrophysics Data System, INSPIRE, Inspec, and many other databases.
- Journal Rank: CiteScore - Q2 (Astronomy and Astrophysics)
- Rapid Publication: manuscripts are peer-reviewed and a first decision provided to authors approximately 20.3 days after submission; acceptance to publication is undertaken in 3.9 days (median values for papers published in this journal in the second half of 2021).
- Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.
Latest Articles
Direct Imaging of the Cosmic Battery in M87*? Not Yet
Galaxies 2022, 10(4), 80; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10040080 - 29 Jun 2022
Abstract
One of the fundamental conclusions of the Cosmic Battery (a means for producing poloidal magnetic flux in the vicinity of a black hole via the Poynting-Robertson effect on the accretion disk) is that it determines the direction of the axial magnetic field: near
[...] Read more.
One of the fundamental conclusions of the Cosmic Battery (a means for producing poloidal magnetic flux in the vicinity of a black hole via the Poynting-Robertson effect on the accretion disk) is that it determines the direction of the axial magnetic field: near the black hole it is parallel to the angular velocity of the surrounding disk, while further away it is anti-parallel to . The EHT polarization observations of M87 where the innermost accretion disk is observed almost face-on thus offer an ideal opportunity to study the action of the Cosmic Battery, by deciding whether the field geometry is consistent with its premises. Unfortunately, such a determination is difficult at the moment due to the lack of reliable Faraday Rotation Measure (RM) maps of M87* at event horizon scales. Furthermore, in agreement with recent General Relativistic Magnetohydrodynamic (GRMHD) numerical simulations, if the inner accretion disk is highly turbulent, one would expect the RM to flip sign on dynamical time scales. While such RM observations are paramount for the determination of the field geometry in confirmation or refutation of the Cosmic Battery, this may have to wait for long term monitoring at event horizon scales and perhaps the synergy of lower resolution RM observations.
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?)
►
Show Figures
Open AccessArticle
Temperature Effects on Core g-Modes of Neutron Stars
Galaxies 2022, 10(4), 79; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10040079 - 28 Jun 2022
Abstract
Neutron stars provide a unique physical laboratory in which to study the properties of matter at high density and temperature. We study a diagnostic of the composition of high-density matter, namely, g-mode oscillations, which are driven by buoyancy forces. These oscillations can be
[...] Read more.
Neutron stars provide a unique physical laboratory in which to study the properties of matter at high density and temperature. We study a diagnostic of the composition of high-density matter, namely, g-mode oscillations, which are driven by buoyancy forces. These oscillations can be excited by tidal forces and couple to gravitational waves. We extend prior results for the g-mode spectrum of cold neutron star matter to high temperatures that are expected to be achieved in neutron star mergers using a parameterization for finite-temperature effects on equations of state recently proposed by Raithel, Özel and Psaltis. We find that the g-modes of canonical mass neutron stars (≈1.4 ) are suppressed at high temperatures, and core g-modes are supported only in the most massive (≥2 ) of hot neutron stars.
Full article
(This article belongs to the Special Issue Neutron Stars and Hadrons in the Era of Gravitational Wave Astrophysics)
►▼
Show Figures

Figure 1
Open AccessArticle
Two Classes of Gamma-ray Bursts Distinguished within the First Second of Their Prompt Emission
Galaxies 2022, 10(4), 78; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10040078 - 26 Jun 2022
Abstract
Studies of Gamma-Ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes, beyond the short/hard and long/soft prototypes, using model-dependent methods. In this paper, a model-independent approach was used to analyse the gamma-ray light curves of large
[...] Read more.
Studies of Gamma-Ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes, beyond the short/hard and long/soft prototypes, using model-dependent methods. In this paper, a model-independent approach was used to analyse the gamma-ray light curves of large samples of GRBs detected by BATSE, Swift/BAT and Fermi/GBM. All the features were extracted from the GRB time profiles in four energy bands using the Stationary Wavelet Transform and Principal Component Analysis. t-distributed Stochastic Neighbourhood Embedding (t-SNE) visualisation of the features revealed two distinct groups of Swift/BAT bursts using the T interval with 64 ms resolution data. When the same analysis was applied to 4 ms resolution data, two groups were seen to emerge within the first second (T ) post-trigger. These two groups primarily consisted of short/hard (Group 1) and long/soft (Group 2) bursts, and were 95% consistent with the groups identified using the T 64 ms resolution data. Kilonova candidates, arising from compact object mergers, were found to belong to Group 1, while those events with associated supernovae fell into Group 2. Differences in cumulative counts between the two groups in the first second, and in the minimum variability timescale, identifiable only with the 4 ms resolution data, may account for this result. Short GRBs have particular significance for multi-messenger science as a distinctive EM signature of a binary merger, which may be discovered by its gravitational wave emissions. Incorporating the T interval into classification algorithms may support the rapid classification of GRBs, allowing for an improved prioritisation of targets for follow-up observations.
Full article
(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)
Open AccessArticle
Two Dimensional Clustering of Swift/BAT and Fermi/GBM Gamma-ray Bursts
Galaxies 2022, 10(4), 77; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10040077 - 25 Jun 2022
Abstract
Studies of Gamma-ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes beyond the short-hard (merger) and long-soft (collapsar) prototypes. Several clustering analyses of the duration-hardness plane identified a third, intermediate duration, class. In this work, Gaussian
[...] Read more.
Studies of Gamma-ray Burst (GRB) properties, such as duration and spectral hardness, have found evidence for additional classes beyond the short-hard (merger) and long-soft (collapsar) prototypes. Several clustering analyses of the duration-hardness plane identified a third, intermediate duration, class. In this work, Gaussian Mixture Model-based (GMM) clustering is applied to the Swift/BAT and Fermi/GBM samples of GRBs. The results obtained by the hierarchical combination of Gaussian components (or clusters) based on an entropy criterion are presented. This method counteracts possible overfitting arising from the application of Gaussian models to non-Gaussian underlying data. While the initial GMM clustering of the hardness-duration plane identifies three components (short/intermediate/long) for the Swift/BAT and Fermi/GBM samples, only two components (short/long) remain once the entropy criterion is applied. The analysis presented here suggests that the intermediate duration class may be the result of overfitting, rather than evidence of a distinct underlying population.
Full article
(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)
Open AccessReview
Compact Binary Coalescences: Astrophysical Processes and Lessons Learned
Galaxies 2022, 10(4), 76; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10040076 - 25 Jun 2022
Abstract
On 11 February 2016, the LIGO and Virgo scientific collaborations announced the first direct detection of gravitational waves, a signal caught by the LIGO interferometers on 14 September 2015, and produced by the coalescence of two stellar-mass black holes. The discovery represented the
[...] Read more.
On 11 February 2016, the LIGO and Virgo scientific collaborations announced the first direct detection of gravitational waves, a signal caught by the LIGO interferometers on 14 September 2015, and produced by the coalescence of two stellar-mass black holes. The discovery represented the beginning of an entirely new way to investigate the Universe. The latest gravitational-wave catalog by LIGO, Virgo and KAGRA brings the total number of gravitational-wave events to 90, and the count is expected to significantly increase in the next years, when additional ground-based and space-born interferometers will be operational. From the theoretical point of view, we have only fuzzy ideas about where the detected events came from, and the answers to most of the five Ws and How for the astrophysics of compact binary coalescences are still unknown. In this work, we review our current knowledge and uncertainties on the astrophysical processes behind merging compact-object binaries. Furthermore, we discuss the astrophysical lessons learned through the latest gravitational-wave detections, paying specific attention to the theoretical challenges coming from exceptional events (e.g., GW190521 and GW190814).
Full article
(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)
►▼
Show Figures

Figure 1
Open AccessReview
Determination of the Cosmic-Ray Chemical Composition: Open Issues and Prospects
Galaxies 2022, 10(3), 75; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030075 - 17 Jun 2022
Abstract
Cosmic rays are relativistic particles that come to the Earth from outer space. Despite a great effort made in both experimental and theoretical research, their origin is still unknown. One of the main keys to understand their nature is the determination of its
[...] Read more.
Cosmic rays are relativistic particles that come to the Earth from outer space. Despite a great effort made in both experimental and theoretical research, their origin is still unknown. One of the main keys to understand their nature is the determination of its chemical composition as a function of primary energy. In this paper, we review the measurements of the mass composition above eV. We first summarize the main aspects of air shower physics that are relevant in composition analyses. We discuss the composition measurements made by using optical, radio, and surface detectors and the limitations imposed by current high-energy hadronic interaction models that are used to interpret the experimental data. We also review the photons and neutrinos searches conducted in different experiments, which, in addition to being important to understand the nature of cosmic rays, can provide relevant information related to the abundance of heavy or light elements in the flux at the highest energies. Finally, we summarize the future composition measurements that are currently being planned or under development.
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?)
►▼
Show Figures

Figure 1
Open AccessReview
Gamma-Ray Bursts at TeV Energies: Theoretical Considerations
by
and
Galaxies 2022, 10(3), 74; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030074 - 30 May 2022
Abstract
Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are powered by ultra-relativistic jets. Their prompt -ray emission briefly outshines the rest of the -ray sky, making them detectable from cosmological distances. A burst is followed by, and
[...] Read more.
Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are powered by ultra-relativistic jets. Their prompt -ray emission briefly outshines the rest of the -ray sky, making them detectable from cosmological distances. A burst is followed by, and sometimes partially overlaps with, a similarly energetic but very broadband and longer-lasting afterglow emission. While most GRBs are detected below a few MeV, over 100 have been detected at high (≳0.1 GeV) energies, and several have now been observed up to tens of GeV with the Fermi Large Area Telescope (LAT). A new electromagnetic window in the very-high-energy (VHE) domain (≳0.1 TeV) was recently opened with the detection of an afterglow emission in the – TeV energy band by ground-based imaging atmospheric Cherenkov telescopes. The emission mechanism for the VHE spectral component is not fully understood, and its detection offers important constraints for GRB physics. This review provides a brief overview of the different leptonic and hadronic mechanisms capable of producing a VHE emission in GRBs. The same mechanisms possibly give rise to the high-energy spectral component seen during the prompt emission of many Fermi-LAT GRBs. Possible origins of its delayed onset and long duration well into the afterglow phase, with implications for the emission region and relativistic collisionless shock physics, are discussed. Key results for using GRBs as ideal probes for constraining models of extra-galactic background light and intergalactic magnetic fields, as well as for testing Lorentz invariance violation, are presented.
Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
►▼
Show Figures

Figure 1
Open AccessArticle
Accretion of Galaxies around Supermassive Black Holes and a Theoretical Model of the Tully-Fisher and M-Sigma Relations
Galaxies 2022, 10(3), 73; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030073 - 27 May 2022
Abstract
The observed Tully-Fisher and Faber-Jackson laws between the baryonic mass of galaxies and the velocity of motion of stars at the edge of galaxies are explained within the framework of the model of accretion of galaxies around supermassive black holes (SMBH). The accretion
[...] Read more.
The observed Tully-Fisher and Faber-Jackson laws between the baryonic mass of galaxies and the velocity of motion of stars at the edge of galaxies are explained within the framework of the model of accretion of galaxies around supermassive black holes (SMBH). The accretion model can also explain the M-sigma relation between the mass of a supermassive black hole and the velocity of stars in the bulge. The difference in the mechanisms of origin of elliptical galaxies with low angular momentum and disk galaxies with high angular momentum can be associated with 3D and 2D accretion.
Full article
(This article belongs to the Special Issue Particles and Fields in Black Hole Environment)
Open AccessReview
Status and Perspectives of Continuous Gravitational Wave Searches
Galaxies 2022, 10(3), 72; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030072 - 25 May 2022
Cited by 1
Abstract
The birth of gravitational wave astronomy was triggered by the first detection of a signal produced by the merger of two compact objects (also known as a compact binary coalescence event). The following detections made by the Earth-based network of advanced interferometers had
[...] Read more.
The birth of gravitational wave astronomy was triggered by the first detection of a signal produced by the merger of two compact objects (also known as a compact binary coalescence event). The following detections made by the Earth-based network of advanced interferometers had a significant impact in many fields of science: astrophysics, cosmology, nuclear physics and fundamental physics. However, compact binary coalescence signals are not the only type of gravitational waves potentially detectable by LIGO, Virgo, and KAGRA. An interesting family of still undetected signals, and the ones that are considered in this review, are the so-called continuous waves, paradigmatically exemplified by the gravitational radiation emitted by galactic, fast-spinning isolated neutron stars with a certain degree of asymmetry in their mass distribution. In this work, I will review the status and the latest results from the analyses of advanced detector data.
Full article
(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)
►▼
Show Figures

Figure 1
Open AccessArticle
Anthropic Principle and the Hubble-Lemaître Constant
by
and
Galaxies 2022, 10(3), 71; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030071 - 24 May 2022
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
(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)
►▼
Show Figures

Figure 1
Open AccessArticle
Gravitational Waves from a Core-Collapse Supernova: Perspectives with Detectors in the Late 2020s and Early 2030s
Galaxies 2022, 10(3), 70; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030070 - 23 May 2022
Abstract
We studied the detectability and reconstruction of gravitational waves from core-collapse supernova multidimensional models using simulated data from detectors predicted to operate in the late 2020s and early 2030s. We found that the detection range will improve by a factor of around two
[...] Read more.
We studied the detectability and reconstruction of gravitational waves from core-collapse supernova multidimensional models using simulated data from detectors predicted to operate in the late 2020s and early 2030s. We found that the detection range will improve by a factor of around two with respect to the second-generation gravitational-wave detectors, and the sky localization will significantly improve. We analyzed the reconstruction accuracy for the lower frequency and higher frequency portion of supernova signals with a 250 Hz cutoff. Since the waveform’s peak frequencies are usually at high frequencies, the gravitational-wave signals in this frequency band were reconstructed more accurately.
Full article
(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)
►▼
Show Figures

Figure 1
Open AccessArticle
A Mass Dependent Density Profile from Dwarfs to Clusters
Galaxies 2022, 10(3), 69; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030069 - 18 May 2022
Abstract
In this paper, we extend the work of Freundlich et al. 2020 who showed how to obtain a Dekel–Zhao density profile with mass dependent shape parameters in the case of galaxies. In the case of Freundlich et al. 2020, the baryonic dependence was
[...] Read more.
In this paper, we extend the work of Freundlich et al. 2020 who showed how to obtain a Dekel–Zhao density profile with mass dependent shape parameters in the case of galaxies. In the case of Freundlich et al. 2020, the baryonic dependence was obtained using the NIHAO set of simulations. In our case, we used simulations based on a model of ours. Following Freundlich et al. 2020, we obtained the dependence from baryon physics of the two shape parameters, obtaining in this way a mass dependent Dekel–Zhao profile describing the dark matter profiles from galaxies to clusters of galaxies. The extension to the Dekel–Zhao mass dependent profile to clusters of galaxies is the main result of the paper. In the paper, we show how the Dekel–Zhao mass dependent profile gives a good description of the density profiles of galaxies, already shown by Freundlich et al. 2020, but also to a set of clusters of galaxies.
Full article
(This article belongs to the Special Issue Dark Cosmology: Shedding Light on Our Current Universe)
►▼
Show Figures

Figure 1
Open AccessArticle
Avoiding the Great Filter: Predicting the Timeline for Humanity to Reach Kardashev Type I Civilization
by
, , , , , , and
Galaxies 2022, 10(3), 68; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030068 - 12 May 2022
Abstract
►▼
Show Figures
The level of technological development of any civilization can be gauged in large part by the amount of energy it produces for its use, but also encompasses that civilization’s stewardship of its home world. Following the Kardashev definition, a Type I civilization is
[...] Read more.
The level of technological development of any civilization can be gauged in large part by the amount of energy it produces for its use, but also encompasses that civilization’s stewardship of its home world. Following the Kardashev definition, a Type I civilization is able to store and use all the energy available on its planet. In this study, we develop a model based on Carl Sagan’s K formula, and use this model to analyze the consumption and energy supply of the three most important energy sources: fossil fuels (e.g., coal, oil, natural gas, crude, NGL, and feedstocks), nuclear energy, and renewable energy. We also consider environmental limitations suggested by the United Nations Framework Convention on Climate Change, the International Energy Agency, and those specific to our calculations, to predict when humanity will reach the level of a Kardashev Scale Type I civilization. Our findings suggest that the best estimate for our civilization to attain Type I status is within the common calendar year range of 2333 to 2404.
Full article

Figure 1
Open AccessReview
The Detection of GRBs at VHE: A Challenge Lasting for More than Two Decades, What Is Next?
by
and
Galaxies 2022, 10(3), 67; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030067 - 10 May 2022
Abstract
Unveiling the mystery of gamma-ray bursts (GRBs) has been the target of many multi-waveband observational and theoretical efforts during the last decades. The results collected by current and past space-based instruments have provided important insights into the mechanisms at the origin of their
[...] Read more.
Unveiling the mystery of gamma-ray bursts (GRBs) has been the target of many multi-waveband observational and theoretical efforts during the last decades. The results collected by current and past space-based instruments have provided important insights into the mechanisms at the origin of their prompt and afterglow phases. On the other hand, many questions, such as the the origin of the multi-GeV signal observed in a large number of events, remained unanswered. Within this framework, the first firm detections of a very-high-energy (VHE, GeV) emission component by MAGIC and H.E.S.S. collaborations represented an important, long-awaited result for the VHE astrophysics community. However, while such discoveries opened a new era in the study of GRBs, they also provided an unexpected complexity due to the differences between the phenomenology of the observed events. This revealed that we still have an incomplete comprehension of GRB physics. In the nearby future, observations by the Cherenkov Telescope Array Observatory (CTAO), with unprecedented sensitivity in the VHE band, will have a key role in the study of these enigmatic objects and their interactions with the surrounding environment. In this review we will cover the recent GRB history, highlighting the efforts of follow-up campaigns by the VHE community that led to the first VHE GRB detection, and outlining what we can expect from future facilities in the next decades.
Full article
(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)
►▼
Show Figures

Figure 1
Open AccessArticle
Gamma-Ray Bursts Afterglow Physics and the VHE Domain
by
and
Galaxies 2022, 10(3), 66; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030066 - 05 May 2022
Abstract
Afterglow radiation in gamma-ray bursts (GRB), extending from the radio band to GeV energies, is produced as a result of the interaction between the relativistic jet and the ambient medium. Although in general the origin of the emission is robustly identified as synchrotron
[...] Read more.
Afterglow radiation in gamma-ray bursts (GRB), extending from the radio band to GeV energies, is produced as a result of the interaction between the relativistic jet and the ambient medium. Although in general the origin of the emission is robustly identified as synchrotron radiation from the shock-accelerated electrons, many aspects remain poorly constrained, such as the role of inverse Compton emission, the particle acceleration mechanism, the properties of the environment and of the GRB jet itself. The extension of the afterglow emission into the TeV band has been discussed and theorized for years, but has eluded for a long time the observations. Recently, the Cherenkov telescopes, MAGIC and H.E.S.S., have unequivocally proven that afterglow radiation is also produced above 100 GeV, up to at least a few TeV. The accessibility of the TeV spectral window will largely improve with the upcoming facility CTA (the Cherenkov Telescope Array). In this review article, we first revise the current model for afterglow emission in GRBs, its limitations and open issues. Then, we describe the recent detections of very high energy emission from GRBs and the origin of this radiation. Implications on the understanding of afterglow radiation and constraints on the physics of the involved processes will be deeply investigated, demonstrating how future observations, especially by the CTA Observatory, are expected to give a key contribution in improving our comprehension of such elusive sources.
Full article
(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)
►▼
Show Figures

Figure 1
Open AccessArticle
Research Facilities for Europe’s Next Generation Gravitational-Wave Detector Einstein Telescope
by
, , , , , , , , , , , , and
Galaxies 2022, 10(3), 65; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030065 - 28 Apr 2022
Abstract
The Einstein Telescope is Europe’s next generation gravitational-wave detector. To develop all necessary technology, four research facilities have emerged across Europe: The Amaldi Research Center (ARC) in Rome (Italy), ETpathfinder in Maastricht (The Netherlands), SarGrav in the Sos Enattos mines on Sardinia (Italy)
[...] Read more.
The Einstein Telescope is Europe’s next generation gravitational-wave detector. To develop all necessary technology, four research facilities have emerged across Europe: The Amaldi Research Center (ARC) in Rome (Italy), ETpathfinder in Maastricht (The Netherlands), SarGrav in the Sos Enattos mines on Sardinia (Italy) and E-TEST in Liége (Belgium) and its surroundings. The ARC pursues the investigation of a large cryostat, equipped with dedicated low-vibration cooling lines, to test full-scale cryogenic payloads. The installation will be gradual and interlaced with the payload development. ETpathfinder aims to provide a low-noise facility that allows the testing of full interferometer configurations and the interplay of their subsystems in an ET-like environment. ETpathfinder will focus amongst others on cryogenic technologies, silicon mirrors, lasers and optics at 1550 and 2090 nm and advanced quantum noise reduction schemes. The SarGrav laboratory has a surface lab and an underground operation. On the surface, the Archimedes experiment investigates the interaction of vacuum fluctuations with gravity and is developing (tilt) sensor technology for the Einstein Telescope. In an underground laboratory, seismic characterisation campaigns are undertaken for the Sardinian site characterisation. Lastly, the Einstein Telecope Euregio meuse-rhine Site & Technology (E-TEST) is a single cryogenic suspension of an ET-sized silicon mirror. Additionally, E-TEST investigates the Belgian–Dutch–German border region that is the other candidate site for Einstein Telescope using boreholes and seismic arrays and hydrogeological characterisation. In this article, we describe the Einstein Telescope, the low-frequency part of its science case and the four research facilities.
Full article
(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)
►▼
Show Figures

Figure 1
Open AccessEditorial
Global Understanding of Accretion and Ejection around Black Holes
Galaxies 2022, 10(3), 64; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030064 - 27 Apr 2022
Abstract
Accretion and ejection around compact objects, mainly around black holes, both in low mass, and supermassive, is rich and has been studied exhaustively [...]
Full article
(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)
Open AccessArticle
The Current Status and Future Prospects of KAGRA, the Large-Scale Cryogenic Gravitational Wave Telescope Built in the Kamioka Underground
by
, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , and add
Show full author list
remove
Hide full author list
Galaxies 2022, 10(3), 63; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030063 - 26 Apr 2022
Cited by 1
Abstract
KAGRA is a gravitational-wave (GW) detector constructed in Japan with two unique key features: It was constructed underground, and the test-mass mirrors are cooled to cryogenic temperatures. These features are not included in other kilometer-scale detectors but will be adopted in future detectors
[...] Read more.
KAGRA is a gravitational-wave (GW) detector constructed in Japan with two unique key features: It was constructed underground, and the test-mass mirrors are cooled to cryogenic temperatures. These features are not included in other kilometer-scale detectors but will be adopted in future detectors such as the Einstein Telescope. KAGRA performed its first joint observation run with GEO600 in 2020. In this observation, the sensitivity of KAGRA to GWs was inferior to that of other kilometer-scale detectors such as LIGO and Virgo. However, further upgrades to the detector are ongoing to reach the sensitivity for detecting GWs in the next observation run, which is scheduled for 2022. In this article, the current situation, sensitivity, and future perspectives are reviewed.
Full article
(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)
►▼
Show Figures

Figure 1
Open AccessArticle
Understanding High-Energy (UV and X-ray) Emission from AGB Stars—Episodic Accretion in Binary Systems
by
, , , and
Galaxies 2022, 10(3), 62; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030062 - 25 Apr 2022
Abstract
X-ray surveys of UV-emitting AGB stars show that ∼40% of objects with FUV emission and GALEX FUV/NUV flux ratio 0.2 (fuvAGB stars) have variable X-ray emission characterized by very high temperatures (Tx∼35–160
[...] Read more.
X-ray surveys of UV-emitting AGB stars show that ∼40% of objects with FUV emission and GALEX FUV/NUV flux ratio 0.2 (fuvAGB stars) have variable X-ray emission characterized by very high temperatures (Tx∼35–160 MK) and luminosities (Lx∼0.002–0.2 ), indicating the presence of accretion associated with a close binary companion. However, the UV-emitting AGB star population is dominated by objects with ≲ 0.06 (nuvAGB stars), and we do not know whether the UV emission from these is intrinsic to the AGB star or extrinsic (i.e., due to binarity). In order to help distinguish between intrinsic and extrinsic models of the puzzling high-energy emission of cool AGB stars, we report results from two studies—(i) XMM-Newton X-observations of two nuvAGB stars, and (ii) simple chromosphere modeling. In study (i), we detect the one which has the lower FUV/NUV ratio, with a total Lx = 0.00027 , and a spectrum best fitted with a dominant component at Tx∼10 MK, most likely coronal emission from a main-sequence companion. Therefore, a significant fraction of nuvAGB stars may also be binaries with active, but weak accretion. Study (ii) shows that chromospheres with temperatures of ∼10,000 K can produce ; higher ratios require hotter gas, implying active accretion.
Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
►▼
Show Figures

Figure 1
Open AccessReview
Radio Galaxies at TeV Energies
Galaxies 2022, 10(3), 61; https://0-doi-org.brum.beds.ac.uk/10.3390/galaxies10030061 - 22 Apr 2022
Abstract
Unlike blazars, radio galaxies have jets that are misaligned relative to our line-of-sight. This misaligned geometry provides us with a unique view of both the jet and super massive black hole. To date, four radio galaxies have been detected at TeV energies with
[...] Read more.
Unlike blazars, radio galaxies have jets that are misaligned relative to our line-of-sight. This misaligned geometry provides us with a unique view of both the jet and super massive black hole. To date, four radio galaxies have been detected at TeV energies with an additional two active galactic nuclei shown to exhibit both radio galaxy and BL Lac-type properties. TeV observations of radio galaxies have revealed these objects to be fascinating, displaying ultra-fast variability and often relatively hard spectral energy distributions. This work aims to provide a review of the current state of radio galaxy observations within the context of very-high-energy -ray astronomy, while also highlighting that radio galaxies are excellent targets for multi-wavelength observations. A number of motivations for the continued study of radio galaxies are provided, and these are discussed with a focus on the key observational results, including implications for future observations with next-generation instruments soon to be operational.
Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
►▼
Show Figures

Figure 1
Highly Accessed Articles
Latest Books
E-Mail Alert
News
Topics

Conferences
Special Issues
Special Issue in
Galaxies
Astrophysics of Eclipsing Binaries in the Era of Space-Borne Telescopes
Guest Editor: Kosmas GazeasDeadline: 1 July 2022
Special Issue in
Galaxies
From Vision to Instrument: Creating a Next-Generation Event Horizon Telescope for a New Era of Black Hole Science
Guest Editors: Michael D. Johnson, Shep Doeleman, Jose L. GómezDeadline: 29 July 2022
Special Issue in
Galaxies
Relativistic Cosmology, Numerical Analysis, General Relativity and Modified Gravity Theories
Guest Editors: Zeeshan Yousaf, Kazuharu Bamba, M. Z. BhattiDeadline: 31 July 2022
Special Issue in
Galaxies
Challenges in Understanding Black Hole Powered Jets with VLBI
Guest Editors: Motoki Kino, Yosuke Mizuno, Taehyun JungDeadline: 31 August 2022
Topical Collections
Topical Collection in
Galaxies
A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives
Collection Editors: Francesca Loi, Tiziana Venturi
Topical Collection in
Galaxies
A Trip across the Universe: Our Present Knowledge and Future Perspectives
Collection Editor: Emilio Elizalde