Journal Description
Galaxies
Galaxies
is an international, peer-reviewed, open access journal on astronomy, astrophysics, and cosmology published bimonthly online by MDPI.
- 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 other databases.
- Journal Rank: CiteScore - Q2 (Astronomy and Astrophysics)
- Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 21 days after submission; acceptance to publication is undertaken in 4.9 days (median values for papers published in this journal in the second half of 2023).
- 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.
Impact Factor:
2.5 (2022);
5-Year Impact Factor:
1.9 (2022)
Latest Articles
Flattened Galaxy Rotation Curves in the Exochronous Metric
Galaxies 2024, 12(3), 21; https://doi.org/10.3390/galaxies12030021 - 24 Apr 2024
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We examine some of the consequences of the Exochronous (timeless) metric and the associated GR cosmological model for the formation of galaxies, and, in particular, their characteristic rotation curves. We show how the cumulative curvature from the multiple spatial hypersurfaces in this
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We examine some of the consequences of the Exochronous (timeless) metric and the associated GR cosmological model for the formation of galaxies, and, in particular, their characteristic rotation curves. We show how the cumulative curvature from the multiple spatial hypersurfaces in this model leads to a modified version of the Poisson equation, in which the gravitational potential is computed over 4D space. Using this new form of the Poisson equation, we derive an analytic expression for gravitational potential as a function of radial distance for a uniform gas cloud undergoing gravitational collapse. We show that this results in a radial velocity profile that provides an excellent fit with commonly observed galaxy rotation curves, and hence fully accounts for the effects previously ascribed to dark matter. An expression can be derived for the equivalent matter density profile corresponding to the GR gravitational potential, from which it is evident that this is very similar in form to the well-known Navarro–Frenk–White profile. As a further illustration of the consequences of adopting the Exochronous metric, we show how the principle can readily be incorporated into particle-mesh N-body simulations of large-scale structure evolution, using a relaxation solver for the solution to the Poisson equation and the evolution of the gravitational potential. Examples of the use of this simulation model are shown for the following cases: (a) the initial evolution of a large-scale structure, and (b) galaxy formation from a gravitationally collapsing gas cloud. In both cases, it is possible to directly visualise the build-up of the gravitational potential in 3D space as the simulation evolves and note how this corresponds to what is currently assumed to be dark matter.
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Investigating the Properties of the Relativistic Jet and Hot Corona in AGN with X-ray Polarimetry
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Dawoon E. Kim, Laura Di Gesu, Frédéric Marin, Alan P. Marscher, Giorgio Matt, Paolo Soffitta, Francesco Tombesi, Enrico Costa and Immacolata Donnarumma
Galaxies 2024, 12(3), 20; https://doi.org/10.3390/galaxies12030020 - 23 Apr 2024
Abstract
X-ray polarimetry has been suggested as a prominent tool for investigating the geometrical and physical properties of the emissions from active galactic nuclei (AGN). The successful launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 9 December 2021 has expanded the previously restricted
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X-ray polarimetry has been suggested as a prominent tool for investigating the geometrical and physical properties of the emissions from active galactic nuclei (AGN). The successful launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 9 December 2021 has expanded the previously restricted scope of polarimetry into the X-ray domain, enabling X-ray polarimetric studies of AGN. Over a span of two years, IXPE has observed various AGN populations, including blazars and radio-quiet AGN. In this paper, we summarize the remarkable discoveries achieved thanks to the opening of the new window of X-ray polarimetry of AGN through IXPE observations. We will delve into two primary areas of interest: first, the magnetic field geometry and particle acceleration mechanisms in the jets of radio-loud AGN, such as blazars, where the relativistic acceleration process dominates the spectral energy distribution; and second, the geometry of the hot corona in radio-quiet AGN. Thus far, the IXPE results from blazars favor the energy-stratified shock acceleration model, and they provide evidence of helical magnetic fields inside the jet. Concerning the corona geometry, the IXPE results are consistent with a disk-originated slab-like or wedge-like shape, as could result from Comptonization around the accretion disk.
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(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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The Seeding of Cosmic Ray Electrons by Cluster Radio Galaxies: A Review
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Franco Vazza and Andrea Botteon
Galaxies 2024, 12(2), 19; https://doi.org/10.3390/galaxies12020019 - 22 Apr 2024
Abstract
Radio galaxies in clusters of galaxies are a prominent reservoir of magnetic fields and of non-thermal particles, which become mixed with the intracluster medium. We review the observational and theoretical knowledge of the role of these crucial ingredients for the formation of diffuse
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Radio galaxies in clusters of galaxies are a prominent reservoir of magnetic fields and of non-thermal particles, which become mixed with the intracluster medium. We review the observational and theoretical knowledge of the role of these crucial ingredients for the formation of diffuse radio emission in clusters (radio halos, relics, mini halos) and outline the open questions in this field.
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(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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Astrochemistry of the Molecular Gas in Dusty Star-Forming Galaxies at the Cosmic Noon
by
Francesca Perrotta, Martina Torsello, Marika Giulietti and Andrea Lapi
Galaxies 2024, 12(2), 18; https://doi.org/10.3390/galaxies12020018 - 22 Apr 2024
Abstract
Far-infrared and submillimeter observations have established the fundamental role of dust-obscured star formation in the assembly of stellar mass over the past ∼12 billion years. At z = 2–4, the so-called “cosmic noon”, the bulk of star formation is enshrouded in dust, and
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Far-infrared and submillimeter observations have established the fundamental role of dust-obscured star formation in the assembly of stellar mass over the past ∼12 billion years. At z = 2–4, the so-called “cosmic noon”, the bulk of star formation is enshrouded in dust, and dusty star-forming galaxies (DSFGs) contain ∼ of the total stellar mass density. Star formation occurs in dense molecular clouds, and is regulated by a complex interplay between all the ISM components that contribute to the energy budget of a galaxy: gas, dust, cosmic rays, interstellar electromagnetic fields, gravitational field, and dark matter. Molecular gas is the actual link between star-forming gas and its complex environment: much of what we know about star formation comes from observations of molecular line emissions. They provide by far the richest information about the star formation process. However, their interpretation requires complex modeling of the astrochemical networks which regulate molecular formation and establish molecular abundances in a cloud, and a modeling of the physical conditions of the gas in which molecular energy levels become populated. This paper critically reviews the main astrochemical parameters needed to obtain predictions about molecular signals in DSFGs. Molecular lines can be very bright compared to the continuum emission, but radiative transfer models are required to properly interpret the observed brightness. We review the current knowledge and the open questions about the interstellar medium of DSFGs, outlining the key role of molecular gas as a tracer and shaper of the star formation process.
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(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)
Open AccessReview
Observational Tests of Active Galactic Nuclei Feedback: An Overview of Approaches and Interpretation
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Chris M. Harrison and Cristina Ramos Almeida
Galaxies 2024, 12(2), 17; https://doi.org/10.3390/galaxies12020017 - 10 Apr 2024
Abstract
Growing supermassive black holes (Active Galactic Nuclei; AGN) release energy with the potential to alter their host galaxies and larger-scale environment; a process named “AGN feedback”. Feedback is a required component of galaxy formation models and simulations to explain the observed properties of
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Growing supermassive black holes (Active Galactic Nuclei; AGN) release energy with the potential to alter their host galaxies and larger-scale environment; a process named “AGN feedback”. Feedback is a required component of galaxy formation models and simulations to explain the observed properties of galaxy populations. We provide a broad overview of observational approaches that are designed to establish the physical processes that couple AGN energy to the multi-phase gas, or to find evidence that AGN impact upon galaxy evolution. The orders-of-magnitude range in spatial, temporal, and temperature scales, requires a diverse set of observational studies. For example, studying individual targets in detail sheds light on coupling mechanisms; however, evidence for the long-term impact of AGN is better established within galaxy populations that are not necessarily currently active. We emphasise how modern surveys have revealed the importance of radio emission for identifying and characterising feedback mechanisms. At the achieved sensitivities, the detected radio emission can trace a range of processes, including a shocked interstellar medium caused by AGN outflows (driven by various mechanisms including radiation pressure, accretion disc winds, and jets). We also describe how interpreting observations in the context of theoretical work can be challenging, in part, due to some of the adopted terminology.
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(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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Open AccessReview
Impact of Active Galactic Nuclei Feedback on the Dynamics of Gas: A Review across Diverse Environments
by
Mojtaba Raouf, Mohammad Hossein Purabbas and Fatemeh Fazel Hesar
Galaxies 2024, 12(2), 16; https://doi.org/10.3390/galaxies12020016 - 08 Apr 2024
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This review examines the relationship between black hole activity and kinematic gas–star misalignment in brightest group galaxies (BGGs) with different merger rates. The formation history of galaxy groups is assessed through “age-dating” as an indicator of distinct major mergers involving the BGGs. BGGs
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This review examines the relationship between black hole activity and kinematic gas–star misalignment in brightest group galaxies (BGGs) with different merger rates. The formation history of galaxy groups is assessed through “age-dating” as an indicator of distinct major mergers involving the BGGs. BGGs within groups characterized by a higher frequency of major mergers are more likely to host active SMBHs. A consistent correlation is identified between the level of black hole activity, as indicated by the 1.4 GHz and 325 MHz radio emissions, and the degree of kinematic misalignment between the gas and stellar components in BGGs. In dynamically fossil groups, where black hole accretion rate is relatively (∼1 dex) lower due to the lack of recent (≤1 Gyr) major mergers, there is reduced (∼30%) misalignment between the gas and stellar components of BGGs compared to non-fossil groups. Additionally, this study reveals that BGGs in non-fossil groups show higher levels of star formation rate and increased occurrence of mergers, contributing to observed color differences. Exploring the properties and dynamics of the gas disk influenced by mechanical AGN feedback through hydrodynamic simulations suggests that AGN wind-induced effects further lead to the persistent gas misalignment in the disk around the supermassive black hole.
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(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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AGN Feedback Signatures in UV Emission
by
K. Rubinur
Galaxies 2024, 12(2), 15; https://doi.org/10.3390/galaxies12020015 - 04 Apr 2024
Abstract
Supermassive black holes (SMBH) are believed to influence galaxy evolution through AGN (active galactic nuclei) feedback. Galaxy mergers are key processes of galaxy formation that lead to AGN activity and star formation. The relative contribution of AGN feedback and mergers to star formation
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Supermassive black holes (SMBH) are believed to influence galaxy evolution through AGN (active galactic nuclei) feedback. Galaxy mergers are key processes of galaxy formation that lead to AGN activity and star formation. The relative contribution of AGN feedback and mergers to star formation is not yet well understood. In radio-loud objects, AGN outflows are dominated by large jets. However, in radio-quiet objects, outflows are more complex and involve jet, wind, and radiation. In this review, we discuss the signatures of AGN feedback through the alignment of radio and UV emissions. Current research on AGN feedback is discussed, along with a few examples of studies such as the galaxy merger system MRK 212, the radio-quiet AGN NGC 2639, and the radio-loud system Centaurus A. Multi-frequency observations of MRK 212 indicate the presence of dual AGN, as well as feedback-induced star-forming UV clumps. The fourth episode of AGN activity was detected in radio observations of the Seyfert galaxy NGC 2639, which also showed a central cavity of 6 kpc radius in CO and UV maps. This indicates that multi-epoch jets of radio-quiet AGN can blow out cold molecular gas, which can further reduce star formation in the center of the galaxies. Recent UV observations of Cen A have revealed two sets of stellar population in the northern star-forming region, which may have two different origins. Recent studies have shown that there is evidence that both positive and negative feedback can be present in galaxies at different scales and times. High-resolution, multi-band observations of large samples of different types of AGN and their host galaxies are important for understanding the two types of AGN feedback and their effect on the host galaxies. Future instruments like INSIST and UVEX will be able to help achieve some of these goals.
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(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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Open AccessArticle
Teaming up Radio and Sub-mm/FIR Observations to Probe Dusty Star-Forming Galaxies
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Meriem Behiri, Marika Giulietti, Vincenzo Galluzzi, Andrea Lapi, Elisabetta Liuzzo and Marcella Massardi
Galaxies 2024, 12(2), 14; https://doi.org/10.3390/galaxies12020014 - 29 Mar 2024
Abstract
In this paper, we investigate the benefits of teaming up data from the radio to the far-infrared (FIR) regime for the characterization of dusty star-forming galaxies (DSFGs). These galaxies are thought to be the star-forming progenitors of local massive quiescent galaxies and to
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In this paper, we investigate the benefits of teaming up data from the radio to the far-infrared (FIR) regime for the characterization of dusty star-forming galaxies (DSFGs). These galaxies are thought to be the star-forming progenitors of local massive quiescent galaxies and to play a pivotal role in the reconstruction of the cosmic star formation rate density up to high redshift. Due to their dust-enshrouded nature, DSFGs are often invisible in the near-infrared/optical/UV bands. Therefore, they necessitate observations at longer wavelengths, primarily the FIR band, where dust emission occurs, and the radio band, which is not affected by dust absorption. Combining data from these two spectral windows makes it possible to characterize even the dustiest objects, enabling the retrieval of information about their age, dust temperature, and star-formation status, and facilitates the differentiation between various galaxy populations that evolve throughout cosmic history. Despite the detection of faint radio sources being a challenging task, this study demonstrates that an effective strategy to build statistically relevant samples of DSFGs would be reaching deep sensitivities in the radio band, even restricted to smaller areas, and then combining these radio observations with FIR/submm data. Additionally, this paper quantifies the improvement in the spectral energy distribution (SED) reconstruction of DSFGs by incorporating ALMA band measurements, in particular, in its upgraded status thanks to the anticipated Wideband Sensitivity Upgrade.
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(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)
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Open AccessArticle
Significance of Fabry-Perot Cavities for Space Gravitational Wave Antenna DECIGO
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Kenji Tsuji, Tomohiro Ishikawa, Kurumi Umemura, Yuki Kawasaki, Shoki Iwaguchi, Ryuma Shimizu, Masaki Ando and Seiji Kawamura
Galaxies 2024, 12(2), 13; https://doi.org/10.3390/galaxies12020013 - 15 Mar 2024
Abstract
DECIGO is a future Japanese project for the detection of gravitational waves in space. To conduct various scientific missions, including the verification of cosmic inflation through the detection of primordial gravitational waves as the main objective, DECIGO is designed to have high sensitivity
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DECIGO is a future Japanese project for the detection of gravitational waves in space. To conduct various scientific missions, including the verification of cosmic inflation through the detection of primordial gravitational waves as the main objective, DECIGO is designed to have high sensitivity in the frequency band from to 10 Hz, with arms of length 1000 km. Furthermore, the use of the Fabry-Perotcavity in these arms has been established for the DECIGO project. In this paper, we scrutinize the significance of the Fabry-Perot cavity for promoting this project, with a focus on the possibility of observing gravitational waves from cosmic inflation and binary compact star systems as indicators. The results show that using the Fabry-Perot cavity is extremely beneficial for detecting them, and it is anticipated to enable the opening of a new window in gravitational wave astronomy.
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(This article belongs to the Special Issue Third-Generation Gravitational Wave Detectors and Beyond)
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Enhancing Gamma-Ray Burst Detection: Evaluation of Neural Network Background Estimator and Explainable AI Insights
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Riccardo Crupi, Giuseppe Dilillo, Giovanni Della Casa, Fabrizio Fiore and Andrea Vacchi
Galaxies 2024, 12(2), 12; https://doi.org/10.3390/galaxies12020012 - 14 Mar 2024
Abstract
The detection of Gamma-Ray Bursts (GRBs) using spaceborne X/gamma-ray photon detectors depends on a reliable background count rate estimate. This study focuses on evaluating a data-driven background estimator based on a neural network designed to adapt to various X/gamma-ray space telescopes. Three trials
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The detection of Gamma-Ray Bursts (GRBs) using spaceborne X/gamma-ray photon detectors depends on a reliable background count rate estimate. This study focuses on evaluating a data-driven background estimator based on a neural network designed to adapt to various X/gamma-ray space telescopes. Three trials were conducted to assess the effectiveness and limitations of the proposed estimator. Firstly, quantile regression was employed to obtain an estimation with a confidence range prediction. Secondly, we assessed the performance of the neural network, emphasizing that a dataset of four months is sufficient for training. We tested its adaptability across various temporal contexts, identified its limitations and recommended re-training for each specific period. Thirdly, utilizing Explainable Artificial Intelligence (XAI) techniques, we delved into the neural network output, determining distinctions between a network trained during solar maxima and one trained during solar minima. This entails conducting a thorough analysis of the neural network behavior under varying solar conditions.
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(This article belongs to the Special Issue Gamma-Ray Bursts in Multiwavelength: Theory, Observational Correlations and GRB Cosmology)
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Open AccessReview
What Have We Learned about the Life Cycle of Radio Galaxies from New Radio Surveys
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Raffaella Morganti
Galaxies 2024, 12(2), 11; https://doi.org/10.3390/galaxies12020011 - 13 Mar 2024
Abstract
The recurrent activity of radio AGN, with phases of activity alternating with periods of quiescence, has been known since the early studies of these objects. The full relevance of this cycle is emphasised by the requirement, from the AGN feedback scenario, of a
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The recurrent activity of radio AGN, with phases of activity alternating with periods of quiescence, has been known since the early studies of these objects. The full relevance of this cycle is emphasised by the requirement, from the AGN feedback scenario, of a recurrent impact of the energy released by the SMBH during the lifetime of the host galaxy: only in this way can AGN feedback influence galaxy evolution. Radio AGN in different evolutionary phases can be identified by their properties, like morphology and spectral indices. Dying/remnant and restarted sources have been the most elusive to select and characterise, but they are crucial to quantify the full life cycle. Thanks to the availability of new, large radio surveys (particularly at low frequencies), it is finally possible to make a more complete census of these rare sources and start building larger samples. This paper gives an overview of the recent work conducted using a variety of radio telescopes and surveys, highlighting some of the new results characterising the properties of dying/remnant and restarted radio sources and what has been learned about the life cycle of radio AGN. The comparison with the predictions from numerical simulations is also discussed. The results so far show that remnant and restarted radio AGN have a variety of properties which make these objects more complex than previously thought.
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(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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Open AccessArticle
Central Engine and Spectral Energy Distribution Properties of High Redshift Gamma Ray Blazars
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Anilkumar Tolamatti, Krishna Kumar Singh and Kuldeep Kumar Yadav
Galaxies 2024, 12(2), 10; https://doi.org/10.3390/galaxies12020010 - 11 Mar 2024
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We report on the properties of central engines in the -ray blazars located at high redshifts beyond z > 0.4, where the extra-galactic background light (EBL) starts affecting their -ray spectra. The physical engine that provides power to the blazars of
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We report on the properties of central engines in the -ray blazars located at high redshifts beyond z > 0.4, where the extra-galactic background light (EBL) starts affecting their -ray spectra. The physical engine that provides power to the blazars of very high bolometric luminosity is assumed to be a highly collimated jet of matter moving relativistically away from the supermassive black hole (SMBH), located in the central region of the host galaxy, in a direction aligned toward the Earth. Due to their peculiar geometry and special physical conditions, blazars at redshifts beyond z > 0.4 are bright enough to be detected in the -ray energy band. In this work, we investigate the physical properties of high-z -ray blazars detected by the Large Area Telescope (LAT) on board the Fermi satellite. We also study the properties of their emission regions and the central engines and discuss cosmological and astrophysical implications.
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Open AccessReview
Observing Dusty Star-Forming Galaxies at the Cosmic Noon through Gravitational Lensing: Perspectives from New-Generation Telescopes
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Marika Giulietti, Giovanni Gandolfi, Marcella Massardi, Meriem Behiri and Andrea Lapi
Galaxies 2024, 12(2), 9; https://doi.org/10.3390/galaxies12020009 - 08 Mar 2024
Abstract
Gravitational lensing, a compelling physical phenomenon, offers a unique avenue to investigate the morphology and physical properties of distant and faint celestial objects. This paper seeks to provide a comprehensive overview of the current state of observations concerning strongly lensed Dusty Star-Forming Galaxies.
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Gravitational lensing, a compelling physical phenomenon, offers a unique avenue to investigate the morphology and physical properties of distant and faint celestial objects. This paper seeks to provide a comprehensive overview of the current state of observations concerning strongly lensed Dusty Star-Forming Galaxies. Emphasis is placed on the pivotal role played by cutting-edge facilities like the James Webb Space Telescope and the Square Kilometer Array Observatory. These advanced instruments operating at the two opposite ends of the electromagnetic spectrum, in conjunction with the amplifying effect of gravitational lensing, promise significant steps in our understanding of these sources. The synergy between these observatories is poised to unlock crucial insights into the evolutionary path of high-redshift, dust-obscured systems and unravel the intricate interplay between Active Galactic Nuclei and their host galaxies.
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(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)
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Open AccessArticle
Superluminal Motion and Jet Parameters in the Gamma-ray-Emitting Narrow-Line Seyfert 1 Galaxy TXS 1206+549
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Bettina Kozák, Sándor Frey and Krisztina Éva Gabányi
Galaxies 2024, 12(1), 8; https://doi.org/10.3390/galaxies12010008 - 17 Feb 2024
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Narrow-line Seyfert 1 (NLS1) galaxies are a peculiar subclass of active galactic nuclei (AGN). Among them, TXS 1206+549 belongs to a small group of radio-loud and -ray-emitting NLS1 galaxies. We focus on the radio properties of this galaxy by analysing archival, high-resolution,
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Narrow-line Seyfert 1 (NLS1) galaxies are a peculiar subclass of active galactic nuclei (AGN). Among them, TXS 1206+549 belongs to a small group of radio-loud and -ray-emitting NLS1 galaxies. We focus on the radio properties of this galaxy by analysing archival, high-resolution, very long baseline interferometry (VLBI) imaging observations taken at 8 GHz frequency in six epochs between 1994 and 2018. Using the milliarcsecond-scale radio structure, we can resolve a core and a jet component whose angular separation increases by mas yr−1. This corresponds to an apparent superluminal jet component motion of . From the core brightness temperature and the jet component proper motion, we determine the characteristic Doppler-boosting factor, the bulk Lorentz factor, and the jet viewing angle. We find no compelling evidence for a very closely aligned blazar-type jet. The parameters for TXS 1206+549 resemble those of radio-loud quasar jets with a moderate Lorentz factor ( ) and inclination to the line of sight.
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Open AccessReview
Spin Evolution of Neutron Stars
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Pavel Abolmasov, Anton Biryukov and Sergei B. Popov
Galaxies 2024, 12(1), 7; https://doi.org/10.3390/galaxies12010007 - 10 Feb 2024
Abstract
In this paper we review the basics of magneto-rotational properties of neutron stars focusing on spin-up/spin-down behavior at different evolutionary stages. The main goal is to provide equations for the spin frequency changes in various regimes (radio pulsar, propeller, accretor, etc.). Since presently
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In this paper we review the basics of magneto-rotational properties of neutron stars focusing on spin-up/spin-down behavior at different evolutionary stages. The main goal is to provide equations for the spin frequency changes in various regimes (radio pulsar, propeller, accretor, etc.). Since presently the spin behavior of neutron stars at all stages remains a subject of many uncertainties, we review different suggestions made over the years in the literature.
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(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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X-ray Polarization from Magnetar Sources
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Roberto Taverna and Roberto Turolla
Galaxies 2024, 12(1), 6; https://doi.org/10.3390/galaxies12010006 - 10 Feb 2024
Cited by 1
Abstract
The launch of the IXPE telescope in late 2021 finally made polarization measurements in the 2– band a reality, more than 40 years after the pioneering observations of the OSO-8 satellite. In the first two years of operations, IXPE targeted more
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The launch of the IXPE telescope in late 2021 finally made polarization measurements in the 2– band a reality, more than 40 years after the pioneering observations of the OSO-8 satellite. In the first two years of operations, IXPE targeted more than 60 sources, including four magnetars, neutron stars with magnetic fields in the petaGauss range. In this paper we summarize the IXPE main findings and discuss their implications for the physics of ultra-magnetized neutron stars. Polarimetric observations confirmed theoretical predictions, according to which X-ray radiation from magnetar sources is highly polarized, up to ≈ , the highest value detected so far. This provides an independent confirmation that magnetars are indeed endowed with a super-strong magnetic field and that the twisted magnetosphere scenario is the most likely explanation for their soft X-ray emission. Polarization measurements allowed us to probe the physical conditions of the star’s outermost layers, showing that the cooler surface regions are in a condensed state, with no atmosphere on top. Although no smoking-gun of vacuum QED effects was found, the phase-dependent behavior of the polarization angle strongly hints that vacuum birefringence is indeed at work in magnetar magnetospheres.
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(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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Open AccessArticle
Parameter Inference of a State-of-the-Art Physical Afterglow Model for GRB 190114C
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Enrico Rinaldi, Nissim Fraija and Maria Giovanna Dainotti
Galaxies 2024, 12(1), 5; https://doi.org/10.3390/galaxies12010005 - 07 Feb 2024
Abstract
A state-of-the-art semi-analytic gamma-ray burst (GRB) afterglow model with synchrotron self-Compton (SSC) emission has been applied for the first time for parameter inference using real GRB data. We analyzed the famous GRB 190114C as a case study. GRB 190114C, characterized by its long
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A state-of-the-art semi-analytic gamma-ray burst (GRB) afterglow model with synchrotron self-Compton (SSC) emission has been applied for the first time for parameter inference using real GRB data. We analyzed the famous GRB 190114C as a case study. GRB 190114C, characterized by its long duration and high luminosity, was observed by many ground-based and orbiting telescopes spanning a wide range of electromagnetic wavelengths, from radio to GeV gamma rays. We used two advanced algorithms for inference: a nested sampling algorithm called UltraNest and an MCMC algorithm emcee. Evoking the standard afterglow model, the inference result and the best-fit values lead to an initial bulk Lorentz factor (a rough estimate of ), which aligns with the values often seen in GRBs identified by the Fermi-LAT instrument. Similarly to the best-fit values of other studies in the literature, the derived values of microphysical parameters, the circumburst density, and the kinetic efficiency are consistent with those found after modeling the multi-wavelength observations in GRB 190114C. We show that the SSC from the forward-shock region can only describe the highest-energy photons above a few GeVs.
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(This article belongs to the Special Issue Gamma-Ray Bursts in Multiwavelength: Theory, Observational Correlations and GRB Cosmology)
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Open AccessArticle
The Scavenger Hunt for Quasar Samples to Be Used as Cosmological Tools
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Maria Giovanna Dainotti, Giada Bargiacchi, Aleksander Łukasz Lenart and Salvatore Capozziello
Galaxies 2024, 12(1), 4; https://doi.org/10.3390/galaxies12010004 - 23 Jan 2024
Cited by 2
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Although the Cold Dark Matter model is the most accredited cosmological model, information at high redshifts (z) between type Ia supernovae ( ) and the Cosmic Microwave Background ( ) is crucial to validate
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Although the Cold Dark Matter model is the most accredited cosmological model, information at high redshifts (z) between type Ia supernovae ( ) and the Cosmic Microwave Background ( ) is crucial to validate this model further. To this end, we have discovered a sample of 1132 quasars up to exhibiting a reduced intrinsic dispersion of the relation between ultraviolet and X-ray fluxes, vs. ( less), than the original sample. This gold sample, once we correct the luminosities for selection biases and redshift evolution, enables us to determine the matter density parameter with a precision of 0.09. Unprecedentedly, this quasar sample is the only one that, as a standalone cosmological probe, yields such tight constraints on while being drawn from the same parent population of the initial sample.
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Open AccessEditorial
The Revised Third Cambridge Catalogue at 60: To Jet or Not to Jet …
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Peter Barthel and Paolo Padovani
Galaxies 2024, 12(1), 3; https://doi.org/10.3390/galaxies12010003 - 11 Jan 2024
Abstract
The Revised Third Cambridge Catalogue of Radio Sources (in the northern sky), or 3CR, published sixty years ago by Bennett (1962) [...]
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(This article belongs to the Special Issue The Symbiosis between Radio Source and Galaxy Evolution)
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A Lepton–Hadron Model for the Multi-Wavelength Emission from Extreme High-Frequency Peaked BL Lacertae 1ES 1218+304
by
Wenjing Dong, Qian Dong and Yonggang Zheng
Galaxies 2024, 12(1), 2; https://doi.org/10.3390/galaxies12010002 - 29 Dec 2023
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We develop a lepton–hadron model for the possible origin of hard very high energy (VHE) spectra from a distant blazar. The model includes synchrotron self-Compton (SSC) and hadronic components. The lepton components include synchrotron radiation and inverse Compton scattering of relativistic electrons. For
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We develop a lepton–hadron model for the possible origin of hard very high energy (VHE) spectra from a distant blazar. The model includes synchrotron self-Compton (SSC) and hadronic components. The lepton components include synchrotron radiation and inverse Compton scattering of relativistic electrons. For the hadronic components, we consider proton synchrotron radiation and investigate the interaction of protons with the synchrotron emission soft photons or cosmic microwave background (CMB) photons. Upon adopting the parametrization of the observed spectrum of 1ES 1218+304, we obtain the following results: (1) the model is able to match the spectral energy distribution of 1ES 1218+304; (2) we find that in , the -ray process contributes the majority of the secondary photons; and (3) the interaction of protons with the low-energy photons may occur in or outside the jet.
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