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Galaxies
Volume 12
Issue 2
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Galaxies, Volume 12, Issue 2 (April 2024) – 11 articles

Cover Story (view full-size image): Active Galactic Nuclei (AGN) are events of growing supermassive black holes inside galaxies. The enormous energy released by AGN is believed to dramatically influence galaxy evolution. However, this is a complex process to measure with observations due to the orders of magnitude range in spatial scales (from the central black holes to galaxy haloes), temperature scales (from cold molecular gas to hot ionised gas), and time scales (from rapid growth events to long-term galaxy evolution). This review summarises the diverse range of observational approaches required to tackle this problem. It further demystifies some of the apparent contradictions in the literature and the challenges of comparing observations with theoretical ideas and simulations. The cover image is an artist’s impression of an AGN driven outflow (Credit: Image—NASA, ESA, Joseph Olmsted (STScI)). View this paper
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16 pages, 6667 KiB  
Review
The Seeding of Cosmic Ray Electrons by Cluster Radio Galaxies: A Review
by Franco Vazza and Andrea Botteon
Galaxies 2024, 12(2), 19; https://doi.org/10.3390/galaxies12020019 - 22 Apr 2024
Cited by 6 | Viewed by 1189
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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33 pages, 549 KiB  
Review
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
Viewed by 1346
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 [...] Read more.
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 ∼50% 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. Full article
(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)
30 pages, 1896 KiB  
Review
Observational Tests of Active Galactic Nuclei Feedback: An Overview of Approaches and Interpretation
by Chris M. Harrison and Cristina Ramos Almeida
Galaxies 2024, 12(2), 17; https://doi.org/10.3390/galaxies12020017 - 10 Apr 2024
Cited by 28 | Viewed by 2152
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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13 pages, 1202 KiB  
Review
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 - 8 Apr 2024
Viewed by 1249
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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16 pages, 2560 KiB  
Review
AGN Feedback Signatures in UV Emission
by K. Rubinur
Galaxies 2024, 12(2), 15; https://doi.org/10.3390/galaxies12020015 - 4 Apr 2024
Cited by 2 | Viewed by 1603
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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11 pages, 872 KiB  
Article
Teaming up Radio and Sub-mm/FIR Observations to Probe Dusty Star-Forming Galaxies
by 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
Viewed by 1229
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)
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19 pages, 4857 KiB  
Article
Significance of Fabry-Perot Cavities for Space Gravitational Wave Antenna DECIGO
by 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
Cited by 1 | Viewed by 1469
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 [...] Read more.
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 0.1 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. Full article
(This article belongs to the Special Issue Challenges for Third-Generation Gravitational Wave Detectors and Beyond)
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13 pages, 1478 KiB  
Article
Enhancing Gamma-Ray Burst Detection: Evaluation of Neural Network Background Estimator and Explainable AI Insights
by 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
Viewed by 1504
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Gamma-Ray Bursts in Multiwavelength: Theory, Observational Correlations and GRB Cosmology)
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16 pages, 2724 KiB  
Review
What Have We Learned about the Life Cycle of Radio Galaxies from New Radio Surveys
by Raffaella Morganti
Galaxies 2024, 12(2), 11; https://doi.org/10.3390/galaxies12020011 - 13 Mar 2024
Viewed by 1916
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)
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20 pages, 3011 KiB  
Article
Central Engine and Spectral Energy Distribution Properties of High Redshift Gamma Ray Blazars
by Anilkumar Tolamatti, Krishna Kumar Singh and Kuldeep Kumar Yadav
Galaxies 2024, 12(2), 10; https://doi.org/10.3390/galaxies12020010 - 11 Mar 2024
Viewed by 1456
Abstract
We report on the properties of central engines in the γ-ray blazars located at high redshifts beyond z > 0.4, where the extra-galactic background light (EBL) starts affecting their γ-ray spectra. The physical engine that provides power to the blazars of [...] Read more.
We report on the properties of central engines in the γ-ray blazars located at high redshifts beyond z > 0.4, where the extra-galactic background light (EBL) starts affecting their γ-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 γ-ray energy band. In this work, we investigate the physical properties of high-z γ-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. Full article
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20 pages, 479 KiB  
Review
Observing Dusty Star-Forming Galaxies at the Cosmic Noon through Gravitational Lensing: Perspectives from New-Generation Telescopes
by Marika Giulietti, Giovanni Gandolfi, Marcella Massardi, Meriem Behiri and Andrea Lapi
Galaxies 2024, 12(2), 9; https://doi.org/10.3390/galaxies12020009 - 8 Mar 2024
Viewed by 1600
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. [...] Read more.
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. Full article
(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)
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