Galaxies

Editorial

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7 pages, 207 KiB

Open AccessEditorial

New Window on the Radio Emission from Galaxies, Clusters and Cosmic Web—Conference Summary

by Francesca Loi and Tiziana Venturi

Galaxies 2022, 10(1), 29; https://doi.org/10.3390/galaxies10010029 - 7 Feb 2022

Viewed by 2325

Abstract

This manuscript summarizes the contributions presented and discussed during the conference “A new window on radio galaxies, clusters and cosmic web: current status and new challenges”. The meeting was held online in March 2021. The works presented during the conference have been published [...] Read more.

This manuscript summarizes the contributions presented and discussed during the conference “A new window on radio galaxies, clusters and cosmic web: current status and new challenges”. The meeting was held online in March 2021. The works presented during the conference have been published in this associated Special Issue. Here, we outline the scientific context of the published results. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

Research

Jump to: Editorial, Review

8 pages, 274 KiB

Open AccessArticle

On the Polarisation of Radio Relics

by Matthias Hoeft, Kamlesh Rajpurohit, Denis Wittor, Gabriella di Gennaro and Paola Domínguez-Fernández

Galaxies 2022, 10(1), 10; https://doi.org/10.3390/galaxies10010010 - 10 Jan 2022

Cited by 5 | Viewed by 2176

Abstract

Radio relics are extended radio emission features which trace shock waves in the periphery of galaxy clusters originating from cluster mergers. Some radio relics show a highly polarised emission, which make relics an excellent probe for the magnetisation of the intra-cluster medium. The [...] Read more.

Radio relics are extended radio emission features which trace shock waves in the periphery of galaxy clusters originating from cluster mergers. Some radio relics show a highly polarised emission, which make relics an excellent probe for the magnetisation of the intra-cluster medium. The origin of the relic polarisation is still debated. It could be a result of tangentially stretching the magnetic field at the shock surface. This scenario would naturally explain the alignment of the polarisation (E-vectors) with the shock normal. We have implemented a toy model for the relic polarisation according to this scenario. We find that the magnetic field strength itself crucially affects the fractional polarisation. Moreover, we find that the shock strength has surprisingly little effect on the overall polarisation fraction. Finally, we find that the fractional polarisation may decrease downstream depending on the magnetic field strength. Our results demonstrates that the shock compression scenario provides a very plausible explanation for the radio relic polarisation which specific features permitting to test the origin of radio relic polarisation. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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38 pages, 9576 KiB

Open AccessArticle

Finding Rare Quasars: VLA Snapshot Continuum Survey of FRI Quasar Candidates Selected from the LOFAR Two-Metre Sky Survey (LoTSS)

by Gülay Gürkan, Judith Croston, Martin J. Hardcastle, Vijay Mahatma, Beatriz Mingo and Wendy L. Williams

Galaxies 2022, 10(1), 2; https://doi.org/10.3390/galaxies10010002 - 22 Dec 2021

Cited by 4 | Viewed by 3978

Abstract

The radiative and jet power in active galactic nuclei is generated by accretion of material on to supermassive galactic-centre black holes. For quasars, where the radiative power is by definition very high, objects with high radio luminosities form 10 per cent of the [...] Read more.

The radiative and jet power in active galactic nuclei is generated by accretion of material on to supermassive galactic-centre black holes. For quasars, where the radiative power is by definition very high, objects with high radio luminosities form 10 per cent of the population, although it is not clear whether this is a stable phase. Traditionally, quasars with high radio luminosities have been thought to present jets with edge-brightened morphology (Fanaroff-Riley II—FR II) due to the limitations of previous radio surveys (i.e., FRIs were not observed as part of the quasar population). The LOw Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) with its unprecedented sensitivity and resolution covering wide sky areas has enabled the first systematic selection and investigation of quasars with core-brightened morphology (Fanaroff-Riley I—FR). We carried out a Very Large Array (VLA) snapshot survey to reveal inner structures of jets in selected quasar candidates; 15 (25 per cent) out of 60 sources show clear inner jet structures that are diagnostic of FRI jets and 13 quasars (∼22 per cent) show extended structures similar to those of FRI jets. Black hole masses and Eddington ratios do not show a clear difference between FRI and FRII quasars. FRII quasars tend to have higher jet powers than FRI quasars. Our results show that the occurrence of FRI jets in powerful radiatively efficient systems is not common, probably mainly due to two factors: galaxy environment and jet power. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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20 pages, 1022 KiB

Open AccessArticle

The Photometric and Spectroscopic Properties of Remnant and Restarted Radio Galaxies in the Lockman Hole Field

by Nika Jurlin, Raffaella Morganti, Natasha Maddox and Marisa Brienza

Galaxies 2021, 9(4), 122; https://doi.org/10.3390/galaxies9040122 - 17 Dec 2021

Cited by 3 | Viewed by 2835

Abstract

Radio galaxies are known to undergo phases of activity, where the stage after the jets have switched off is referred to as the remnant phase. This state can be followed by a restarted phase when the activity reignites. Remnant and restarted radio sources [...] Read more.

Radio galaxies are known to undergo phases of activity, where the stage after the jets have switched off is referred to as the remnant phase. This state can be followed by a restarted phase when the activity reignites. Remnant and restarted radio sources are important for testing models of the evolution of radio active galactic nuclei (AGN) and for understanding the impact the recurrent jet activity has on their host galaxies. Although we now have statistical samples of radio sources in various stages of their life cycle, how this intermittent radio activity is reflected in the optical properties in this sample has not yet been addressed, and is overall a much less studied aspect in the literature. In this work, we use the Wide-field Infrared Survey Explorer and the Sloan Digital Sky Survey (SDSS) photometry, and SDSS spectra to study these properties in a sample of the remnant, candidate restarted, and active radio galaxies selected using the LOw Frequency ARray at 150 MHz in the Lockman Hole extragalactic field. Within the range of radio luminosities and stellar masses studied in this work, we find no difference between the properties of the host galaxy and of the optical emission lines for objects in different phases of their radio life cycle. The vast majority of our radio sources (either remnant, candidate restarted, or comparison sample) are associated with radiatively inefficient optical AGN and red galaxies dominated by old stellar populations. Thus, the radio and emission-line AGN activity appears to be independent and regulated by different mechanisms. This suggests that, at least for the radio luminosities of our sample, the life cycle of the radio may depend on intrinsic reasons, such as the stability of the accretion disc, more than variation in the accretion rate and fuelling of the central black hole. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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21 pages, 3759 KiB

Open AccessArticle

Remnant Radio Galaxy Candidates of Small Angular Sizes

by Veeresh Singh, Sushant Dutta, Yogesh Wadadekar and C. H. Ishwara-Chandra

Galaxies 2021, 9(4), 121; https://doi.org/10.3390/galaxies9040121 - 16 Dec 2021

Cited by 5 | Viewed by 2531

Abstract

Remnant radio galaxies (RRGs), characterized by the cessation of AGN activity, represent a short-lived last phase of radio galaxy’s life-cycle. Hitherto, searches for RRGs, mainly based on the morphological criteria, have identified large angular size sources resulting into a bias towards the remnants [...] Read more.

Remnant radio galaxies (RRGs), characterized by the cessation of AGN activity, represent a short-lived last phase of radio galaxy’s life-cycle. Hitherto, searches for RRGs, mainly based on the morphological criteria, have identified large angular size sources resulting into a bias towards the remnants of powerful FR-II radio galaxies. In this study we make the first attempt to perform a systematic search for RRGs of small angular sizes (<30

^{″}

) in the XMM–LSS field. By using spectral curvature criterion we discover 48 remnant candidates exhibiting strong spectral curvature i.e.,

α_{150 MHz}^{325 MHz} - α_{325 MHz}^{1.4 GHz}

≥ 0.5. Spectral characteristics at higher frequency regime (>1.4 GHz) indicate that some of our remnant candidates can depict recurrent AGN activity with an active core. We place an upper limit on the remnant fraction (

f_{rem}

) to be 3.9%, which increases to 5.4% if flux cutoff limit of S

_{150 MHz}

≥ 10 mJy is considered. Our study unveils, hitherto unexplored, a new population of small-size (<200 kpc) remnant candidates that are often found to reside in less dense environments and at higher redshifts (z) > 1.0. We speculate that a relatively shorter active phase and/or low jet power can be plausible reasons for the small size of remnant candidates. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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16 pages, 6991 KiB

Open AccessArticle

A GMRT Narrowband vs. Wideband Analysis of the ACT−CL J0034.4+0225 Field Selected from the ACTPol Cluster Sample

by Sinenhlanhla P. Sikhosana, Kenda Knowles, C. H. Ishwara-Chandra, Matt Hilton, Kavilan Moodley and Neeraj Gupta

Galaxies 2021, 9(4), 117; https://doi.org/10.3390/galaxies9040117 - 12 Dec 2021

Cited by 2 | Viewed by 2386

Abstract

Low frequency radio observations of galaxy clusters are a useful probe of the non-thermal intracluster medium (ICM), through observations of diffuse radio emission such as radio halos and relics. Current formation theories cannot fully account for some of the observed properties of this [...] Read more.

Low frequency radio observations of galaxy clusters are a useful probe of the non-thermal intracluster medium (ICM), through observations of diffuse radio emission such as radio halos and relics. Current formation theories cannot fully account for some of the observed properties of this emission. In this study, we focus on the development of interferometric techniques for extracting extended, faint diffuse emissions in the presence of bright, compact sources in wide-field and broadband continuum imaging data. We aim to apply these techniques to the study of radio halos, relics and radio mini-halos using a uniformly selected and complete sample of galaxy clusters selected via the Sunyaev-Zel’dovich (SZ) effect by the Atacama Cosmology Telescope (ACT) project, and its polarimetric extension (ACTPol). We use the upgraded Giant Metrewave Radio Telescope (uGMRT) for targeted radio observations of a sample of 40 clusters. We present an overview of our sample, confirm the detection of a radio halo in ACT−CL J0034.4+0225, and compare the narrowband and wideband analysis results for this cluster. Due to the complexity of the ACT−CL J0034.4+0225 field, we use three pipelines to process the wideband data. We conclude that the experimental spam wideband pipeline produces the best results for this particular field. However, due to the severe artefacts in the field, further analysis is required to improve the image quality. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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13 pages, 650 KiB

Open AccessArticle

Multi-Wavelength Study of a Proto-BCG at z = 1.7

by Quirino D’Amato, Isabella Prandoni, Marisa Brienza, Roberto Gilli, Cristian Vignali, Rosita Paladino, Francesca Loi, Marcella Massardi, Marco Mignoli, Stefano Marchesi, Alessandro Peca and Preshanth Jagannathan

Galaxies 2021, 9(4), 115; https://doi.org/10.3390/galaxies9040115 - 7 Dec 2021

Cited by 3 | Viewed by 2258

Abstract

In this work we performed a spectral energy distribution (SED) analysis in the optical/infrared band of the host galaxy of a proto-brightest bluster galaxy (BCG, NVSS J103023 + 052426) in a proto-cluster at z = 1.7. We found that it features a vigorous [...] Read more.

In this work we performed a spectral energy distribution (SED) analysis in the optical/infrared band of the host galaxy of a proto-brightest bluster galaxy (BCG, NVSS J103023 + 052426) in a proto-cluster at z = 1.7. We found that it features a vigorous star formation rate (SFR) of ∼570

M_{⊙}

/yr and a stellar mass of

M_{*} \sim 3.7 \times 10^{11}

M_{⊙}

; the high corresponding specific SFR =

1.5 \pm 0.5

{Gyr}^{- 1}

classifies this object as a starburst galaxy that will deplete its molecular gas reservoir in ∼

3.5 \times 10^{8}

yr. Thus, this system represents a rare example of a proto-BCG caught during the short phase of its major stellar mass assembly. Moreover, we investigated the nature of the host galaxy emission at 3.3 mm. We found that it originates from the cold dust in the interstellar medium, even though a minor non-thermal AGN contribution cannot be completely ruled out. Finally, we studied the polarized emission of the lobes at 1.4 GHz. We unveiled a patchy structure where the polarization fraction increases in the regions in which the total intensity shows a bending morphology; in addition, the magnetic field orientation follows the direction of the bendings. We interpret these features as possible indications of an interaction with the intracluster medium. This strengthens the hypothesis of positive AGN feedback, as inferred in previous studies of this object on the basis of X-ray/mm/radio analysis. In this scenario, the proto-BCG heats the surrounding medium and possibly enhances the SFR in nearby galaxies. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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6 pages, 2275 KiB

Open AccessArticle

Spectral Index of the Filaments in the Abell 523 Radio Halo

by Valentina Vacca, Federica Govoni, Richard A. Perley, Matteo Murgia, Ettore Carretti, Francesca Loi, Luigina Feretti and Gabriele Giovannini

Galaxies 2021, 9(4), 112; https://doi.org/10.3390/galaxies9040112 - 4 Dec 2021

Cited by 4 | Viewed by 2128

Abstract

The galaxy cluster Abell 523 hosts a radio halo characterized by the presence of two filaments transversely located with respect to the cluster merger axis. In this paper, we present a spectral index image of these filaments between 1.410 and 1.782 GHz obtained [...] Read more.

The galaxy cluster Abell 523 hosts a radio halo characterized by the presence of two filaments transversely located with respect to the cluster merger axis. In this paper, we present a spectral index image of these filaments between 1.410 and 1.782 GHz obtained with Jansky Very Large Array observations. We find a steepening of the spectral index of the filaments at frequencies ≳1.4 GHz and an indication that bright patches are characterized by flat spectral indices. Our results are consistent with a scenario of highly-efficient turbulence induced by merger phenomena. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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11 pages, 1039 KiB

Open AccessArticle

Modelling the Energy Spectra of Radio Relics

by Denis Wittor, Matthias Hoeft and Marcus Brüggen

Galaxies 2021, 9(4), 111; https://doi.org/10.3390/galaxies9040111 - 1 Dec 2021

Cited by 8 | Viewed by 2481

Abstract

Radio relics are diffuse synchrotron sources that illuminate shock waves in the intracluster medium. In recent years, radio telescopes have provided detailed observations about relics. Consequently, cosmological simulations of radio relics need to provide a similar amount of detail. In this methodological work, [...] Read more.

Radio relics are diffuse synchrotron sources that illuminate shock waves in the intracluster medium. In recent years, radio telescopes have provided detailed observations about relics. Consequently, cosmological simulations of radio relics need to provide a similar amount of detail. In this methodological work, we include information on adiabatic compression and expansion, which have been neglected in the past in the modelling of relics. In a cosmological simulation of a merging galaxy cluster, we follow the energy spectra of shock accelerated cosmic-ray electrons using Lagrangian tracer particles. On board of each tracer particle, we compute the temporal evolution of the energy spectrum under the influence of synchrotron radiation, inverse Compton scattering, and adiabatic compression and expansion. Exploratory tests show that the total radio power and, hence, the integrated radio spectrum are not sensitive to the adiabatic processes. This is attributed to small changes in the compression ratio over time. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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24 pages, 12253 KiB

Open AccessArticle

Magnetogenesis and the Cosmic Web: A Joint Challenge for Radio Observations and Numerical Simulations

by Franco Vazza, Nicola Locatelli, Kamlesh Rajpurohit, Serena Banfi, Paola Domínguez-Fernández, Denis Wittor, Matteo Angelinelli, Giannandrea Inchingolo, Marisa Brienza, Stefan Hackstein, Daniele Dallacasa, Claudio Gheller, Marcus Brüggen, Gianfranco Brunetti, Annalisa Bonafede, Stefano Ettori, Chiara Stuardi, Daniela Paoletti and Fabio Finelli

Galaxies 2021, 9(4), 109; https://doi.org/10.3390/galaxies9040109 - 23 Nov 2021

Cited by 32 | Viewed by 3885

Abstract

The detection of the radio signal from filaments in the cosmic web is crucial to distinguish possible magnetogenesis scenarios. We review the status of the different attempts to detect the cosmic web at radio wavelengths. This is put into the context of the [...] Read more.

The detection of the radio signal from filaments in the cosmic web is crucial to distinguish possible magnetogenesis scenarios. We review the status of the different attempts to detect the cosmic web at radio wavelengths. This is put into the context of the advanced simulations of cosmic magnetism carried out in the last few years by our MAGCOW project. While first attempts of imaging the cosmic web with the MWA and LOFAR have been encouraging and could discard some magnetogenesis models, the complexity behind such observations makes a definitive answer still uncertain. A combination of total intensity and polarimetric data at low radio frequencies that the SKA and LOFAR2.0 will achieve is key to removing the existing uncertainties related to the contribution of many possible sources of signal along deep lines of sight. This will make it possible to isolate the contribution from filaments, and expose its deep physical connection with the origin of extragalactic magnetism. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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19 pages, 1120 KiB

Open AccessArticle

Radio and X-ray Observations of the Restarted Radio Galaxy in the Galaxy Cluster CL 0838+1948

by Simona Giacintucci, Tracy Clarke, Namir E. Kassim, Wendy Peters and Emil Polisensky

Galaxies 2021, 9(4), 108; https://doi.org/10.3390/galaxies9040108 - 21 Nov 2021

Cited by 4 | Viewed by 3038

Abstract

We present VLA Low-band Ionosphere and Transient Experiment (VLITE) 338 MHz observations of the galaxy cluster CL 0838+1948. We combine the VLITE data with Giant Metrewave Radio Telescope 610 MHz observations and survey data. The central galaxy hosts a 250 kpc source whose [...] Read more.

We present VLA Low-band Ionosphere and Transient Experiment (VLITE) 338 MHz observations of the galaxy cluster CL 0838+1948. We combine the VLITE data with Giant Metrewave Radio Telescope 610 MHz observations and survey data. The central galaxy hosts a 250 kpc source whose emission is dominated by two large lobes at low frequencies. At higher frequencies, a pair of smaller lobes (∼30 kpc) is detected within the galaxy optical envelope. The observed morphology is consistent with a restarted radio galaxy. The outer lobes have a spectral index

α_{out} = 1.6

, indicating that they are old, whereas the inner lobes have

α_{inn} = 0.6

, typical for an active source. Spectral modeling confirms that the outer emission is a dying source whose nuclear activity switched off not more than 110 Myr ago. Using archival Chandra X-ray data, we compare the radio and hot gas emission. We find that the active radio source is contained within the innermost and X-ray brightest region, possibly a galactic corona. Alternatively, it could be the remnant of a larger cool core whose outer layers have been heated by the former epoch of activity that has generated the outer lobes. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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15 pages, 1417 KiB

Open AccessArticle

How Are Red and Blue Quasars Different? The Radio Properties

by Victoria A. Fawcett, David M. Alexander, David J. Rosario and Lizelke Klindt

Galaxies 2021, 9(4), 107; https://doi.org/10.3390/galaxies9040107 - 19 Nov 2021

Cited by 5 | Viewed by 3049

Abstract

A non-negligible fraction of quasars are red at optical wavelengths, indicating (in the majority of cases) that the accretion disc is obscured by a column of dust which extinguishes the shorter-wavelength blue emission. In this paper, we summarize recent work by our group, [...] Read more.

A non-negligible fraction of quasars are red at optical wavelengths, indicating (in the majority of cases) that the accretion disc is obscured by a column of dust which extinguishes the shorter-wavelength blue emission. In this paper, we summarize recent work by our group, where we find fundamental differences in the radio properties of SDSS optically-selected red quasars. We also present new analyses, using a consistent color-selected quasar parent sample matched to four radio surveys (FIRST, VLA Stripe 82, VLA COSMOS 3 GHz, and LoTSS DR1) across a frequency range 144 MHz–3 GHz and four orders of magnitude in radio flux. We show that red quasars have enhanced small-scale radio emission (∼kpc) that peaks around the radio-quiet threshold (defined as the ratio of 1.4 GHz luminosity to 6

μ

m luminosity) across the four radio samples. Exploring the potential mechanisms behind this enhancement, we rule out star-formation and propose either small-scale synchrotron jets, frustrated jets, or dusty winds interacting with the interstellar medium; the latter two scenarios would provide a more direct connection between opacity (dust; gas) and the production of the radio emission. In our future study, using new multi-band uGMRT data, we aim to robustly distinguish between these scenarios. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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22 pages, 1626 KiB

Open AccessArticle

The eMERLIN and EVN View of FR 0 Radio Galaxies

by Ranieri D. Baldi, Gabriele Giovannini and Alessandro Capetti

Galaxies 2021, 9(4), 106; https://doi.org/10.3390/galaxies9040106 - 18 Nov 2021

Cited by 10 | Viewed by 3654

Abstract

We present the results from high-resolution observations carried out with the eMERLIN UK-array and the European VLBI network (EVN) for a sample of 15 FR 0s, i.e., compact core-dominated radio sources associated with nearby early-type galaxies (ETGs), which represent the bulk of [...] Read more.

We present the results from high-resolution observations carried out with the eMERLIN UK-array and the European VLBI network (EVN) for a sample of 15 FR 0s, i.e., compact core-dominated radio sources associated with nearby early-type galaxies (ETGs), which represent the bulk of the local radio galaxy population. The 5 GHz eMERLIN observations available for five objects exhibit sub-mJy core components and reveal pc-scale twin jets for four out of five FR 0s once the eMERLIN and JVLA archival visibilities data are combined. The 1.66 GHz EVN observations available for 10 FR 0s display one- and two-sided jetted morphologies and compact cores. The pc-scale core emission contributes, on average, to about one tenth of the total extended radio emission, although we noted an increasing core contribution for flat-/inverted-spectrum sources. We found an unprecedented linear correlation between the pc-scale core luminosity (∼10

^{21.3}

–10

^{23.6}

W Hz

^{- 1}

) and [O III] line luminosity, generally considered as proxy of the accretion power, for a large sample of LINER-type radio-loud low-luminosity active nuclei, all hosted in massive ETGs, which include FR 0s and FR Is. This result represents further evidence of a common jet–disc coupling in FR 0s and FR Is, despite then differing in kpc-scale radio structure. For our objects and for other FR 0 samples reported in the literature, we estimated the jet brightness sidedness ratios, which typically range between one and three. This parameter roughly gauges the jet bulk Lorentz factor

Γ

, which turns out to range from 1 to 2.5 for most of the sample. This corroborates the scenario that FR 0s are characterized by mildly relativistic jets, possibly as a result of lower-spinning black holes (BHs) than the highly spinning BHs of relativistic-jetted radio galaxies, FR Is. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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9 pages, 901 KiB

Open AccessArticle

Pilot Study and Early Results of the Cosmic Filaments and Magnetism Survey with Nenufar: The Coma Cluster Field

by Etienne Bonnassieux, Evangelia Tremou, Julien N. Girard, Alan Loh, Valentina Vacca, Stéphane Corbel, Baptiste Cecconi, Jean-Mathias Grießmeier, Léon V. E. Koopmans, Michel Tagger, Gilles Theureau and Philippe Zarka

Galaxies 2021, 9(4), 105; https://doi.org/10.3390/galaxies9040105 - 16 Nov 2021

Cited by 2 | Viewed by 2548

Abstract

NenuFAR, the New Extension in Nancay Upgrading LOFAR, is currently in its early science phase. It is in this context that the Cosmic Filaments and Magnetism Pilot Survey is observing sources with the array as it is still under construction—with 57 (56 core, [...] Read more.

NenuFAR, the New Extension in Nancay Upgrading LOFAR, is currently in its early science phase. It is in this context that the Cosmic Filaments and Magnetism Pilot Survey is observing sources with the array as it is still under construction—with 57 (56 core, 1 distant) out of a total planned 102 (96 core, 6 distant) mini-arrays online at the time of observation—to get a first look at the low-frequency sky with NenuFAR. One of its targets is the Coma galaxy cluster: a well-known object, host of the prototype radio halo. It also hosts other features of scientific import, including a radio relic, along with a bridge of emission connecting it with the halo. It is thus a well-studied object.In this paper, we show the first confirmed NenuFAR detection of the radio halo and radio relic of the Coma cluster at 34.4 MHz, with associated intrinsic flux density estimates: we find an integrated flux value of 106.3 ± 3.5 Jy for the radio halo, and 102.0 ± 7.4 Jy for the radio relic. These are upper bound values, as they do not include point-source subtraction. We also give an explanation of the technical difficulties encountered in reducing the data, along with steps taken to resolve them. This will be helpful for other scientific projects which will aim to make use of standalone NenuFAR imaging observations in the future. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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16 pages, 1897 KiB

Open AccessFeature PaperArticle

Discovery of Rare Dying Radio Galaxies Using MeerKAT

by Nadeem Oozeer, Lawrence Rudnick, Michael F. Bietenholz, Tiziana Venturi, Kenda Knowles, Konstantinos Kolokythas and Nceba Mhlahlo

Galaxies 2021, 9(4), 102; https://doi.org/10.3390/galaxies9040102 - 10 Nov 2021

Cited by 2 | Viewed by 2889

Abstract

Dying radio galaxies represent a stage of the evolution of active galactic nuclei (AGN), during which the accreting central black hole has switched off and/or falls to such a low level that the plasma outflow can no longer be sustained. When this happens, [...] Read more.

Dying radio galaxies represent a stage of the evolution of active galactic nuclei (AGN), during which the accreting central black hole has switched off and/or falls to such a low level that the plasma outflow can no longer be sustained. When this happens, the radio source undergoes a period of fading, the dying phase, before it disappears completely. We present the study of three potential dying radio sources using the MeerKAT radio telescope: MKT J072851.2-752743, MKT J001940.4-654722, and ACO 548B. The identification as dying radio sources came from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). We carry out a multi-wavelength analysis of the sources and derive their energetics. The ages of the sources are ∼30–70 Myr, they have magnetic fields of the order of a few

μ

G, and they have relatively low radio power. Their potential optical counterparts are associated with massive galaxies. We show that ACO 548B, previously classified as two peripheral relic radio sources, is a dying radio galaxy. With its good sensitivity and resolution, MeerKAT is an ideal instrument to detect potential dying radio sources, and contribute to the understanding of the evolution of AGN population. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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27 pages, 16172 KiB

Open AccessArticle

Discovery of 178 Giant Radio Galaxies in 1059 deg² of the Rapid ASKAP Continuum Survey at 888 MHz

by Heinz Andernach, Eric F. Jiménez-Andrade and Anthony G. Willis

Galaxies 2021, 9(4), 99; https://doi.org/10.3390/galaxies9040099 - 9 Nov 2021

Cited by 22 | Viewed by 3968

Abstract

We report the results of a visual inspection of images of the Rapid ASKAP Continuum Survey (RACS) in search of extended radio galaxies (ERG) that reach or exceed linear sizes on the order of one Megaparsec. We searched a contiguous area of 1059 [...] Read more.

We report the results of a visual inspection of images of the Rapid ASKAP Continuum Survey (RACS) in search of extended radio galaxies (ERG) that reach or exceed linear sizes on the order of one Megaparsec. We searched a contiguous area of 1059 deg

^{2}

from RA

_{J}

= 20

^{h}

20

^{m}

to 06

^{h}

20

^{m}

, and

- 50^{\circ} < {Dec}_{J} < - 40^{\circ}

, which is covered by deep multi-band optical images of the Dark Energy Survey (DES) and in which previously only three ERGs larger than 1 Mpc had been reported. For over 1800 radio galaxy candidates inspected, our search in optical and infrared images resulted in hosts for 1440 ERG, for which spectroscopic and photometric redshifts from various references were used to convert their largest angular size (LAS) to projected linear size (LLS). This resulted in 178 newly discovered giant radio sources (GRS) with LLS >1 Mpc, of which 18 exceed 2 Mpc and the largest one is 3.4 Mpc. Their redshifts range from 0.02 to ∼2.0, but only 10 of the 178 new GRS have spectroscopic redshifts. For the 146 host galaxies, the median r-band magnitude and redshift are 20.9 and 0.64, while for the 32 quasars or candidates these are 19.7 and 0.75. Merging the six most recent large compilations of GRS results in 458 GRS larger than 1 Mpc, so we were able to increase this number by ∼39% to 636. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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15 pages, 2554 KiB

Open AccessArticle

A Multiwavelength Dynamical State Analysis of ACT-CL J0019.6+0336

by Denisha S. Pillay, David J. Turner, Matt Hilton, Kenda Knowles, Kabelo C. Kesebonye, Kavilan Moodley, Tony Mroczkowski, Nadeem Oozeer, Christoph Pfrommer, Sinenhlanhla P. Sikhosana and Edward J. Wollack

Galaxies 2021, 9(4), 97; https://doi.org/10.3390/galaxies9040097 - 8 Nov 2021

Cited by 6 | Viewed by 3147

Abstract

In our study, we show a multiwavelength view of ACT-CL J0019.6+0336 (which hosts a radio halo), to investigate the cluster dynamics, morphology, and ICM. We use a combination of XMM-Newton images, Dark Energy Survey (DES) imaging and photometry, SDSS spectroscopic information, and 1.16 [...] Read more.

In our study, we show a multiwavelength view of ACT-CL J0019.6+0336 (which hosts a radio halo), to investigate the cluster dynamics, morphology, and ICM. We use a combination of XMM-Newton images, Dark Energy Survey (DES) imaging and photometry, SDSS spectroscopic information, and 1.16 GHz MeerKAT data to study the cluster properties. Various X-ray and optical morphology parameters are calculated to investigate the level of disturbance. We find disturbances in two X-ray parameters and the optical density map shows elongated and axisymmetric structures with the main cluster component southeast of the cluster centre and another component northwest of the cluster centre. We also find a BCG offset of ∼950 km/s from the mean velocity of the cluster, and a discrepancy between the SZ mass, X-ray mass, and dynamical mass (

M_{X, 500}

and

M_{S Z, 500}

lies

> 3 σ

away from

M_{dyn, 500}

), showing that J0019 is a merging cluster and probably in a post-merging phase. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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13 pages, 1534 KiB

Open AccessArticle

Bent It Like FRs: Extended Radio AGN in the COSMOS Field and Their Large-Scale Environment

by Eleni Vardoulaki, Franco Vazza, Eric F. Jiménez-Andrade, Ghassem Gozaliasl, Alexis Finoguenov and Denis Wittor

Galaxies 2021, 9(4), 93; https://doi.org/10.3390/galaxies9040093 - 5 Nov 2021

Cited by 6 | Viewed by 2057

Abstract

A fascinating topic in radio astronomy is how to associate the complexity of observed radio structures with their environment in order to understand their interplay and the reason for the plethora of radio structures found in surveys. In this project, we explore the [...] Read more.

A fascinating topic in radio astronomy is how to associate the complexity of observed radio structures with their environment in order to understand their interplay and the reason for the plethora of radio structures found in surveys. In this project, we explore the distortion of the radio structure of Fanaroff–Riley (FR)-type radio sources in the VLA-COSMOS Large Project at 3 GHz and relate it to their large-scale environment. We quantify the distortion by using the angle formed between the jets/lobes of two-sided FRs, namely bent angle (BA). Our sample includes 108 objects in the redshift range

0.08 < z < 3

, which we cross-correlate to a wide range of large-scale environments (X-ray galaxy groups, density fields, and cosmic web probes) in the COSMOS field. The median BA of FRs in COSMOS at

z_{med}

∼0.9 is 167.5

_{- 37.5}^{+ 11.5}

degrees. We do not find significant correlations between BA and large-scale environments within COSMOS covering scales from a few kpc to several hundred Mpc, nor between BA and host properties. Finally, we compare our observational data to magnetohydrodynamical (MHD) adaptive-mesh simulations ENZO-MHD of two FR sources at z = 0.5 and at z = 1. Although the scatter in BA of the observed data is large, we see an agreement between observations and simulations in the bent angles of FRs, following a mild redshift evolution with BA. We conclude that, for a given object, the dominant mechanism affecting the radio structures of FRs could be the evolution of the ambient medium, where higher densities of the intergalactic medium at lower redshifts as probed by our study allow more space for jet interactions. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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5 pages, 1053 KiB

Open AccessArticle

Polarization Tomography with Stokes Parameters

by Lawrence Rudnick, Debora Katz and Lerato Sebokolodi

Galaxies 2021, 9(4), 92; https://doi.org/10.3390/galaxies9040092 - 5 Nov 2021

Cited by 2 | Viewed by 1591

Abstract

We present a simple but powerful technique for the analysis of polarized emission from radio galaxies and other objects. It is based on the fact that images of Stokes parameters often contain considerably more information than is available in polarized intensity and angle [...] Read more.

We present a simple but powerful technique for the analysis of polarized emission from radio galaxies and other objects. It is based on the fact that images of Stokes parameters often contain considerably more information than is available in polarized intensity and angle maps. In general, however, the orientation of the Stokes parameters will not be matched to the position angles of structures in the source. Polarization tomography, the technique presented in this paper, consists of making a series of single linear Stokes parameter images,

S (ρ)

, where each image is rotated by an angle

ρ

from the initial orientation of Q and U. Examination of these images, in a series of still frames or a movie, reveals often hidden patterns of polarization angles, as well as structures that were obscured by the presence of overlapping polarized emission. We provide both cartoon examples and a quick look at the complex polarized structure in Cygnus A. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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9 pages, 3851 KiB

Open AccessArticle

Turning AGN Bubbles into Radio Relics with Sloshing: Modeling CR Transport with Realistic Physics

by John ZuHone, Kristian Ehlert, Rainer Weinberger and Christoph Pfrommer

Galaxies 2021, 9(4), 91; https://doi.org/10.3390/galaxies9040091 - 3 Nov 2021

Cited by 11 | Viewed by 1536

Abstract

Radio relics are arc-like synchrotron sources at the periphery of galaxy clusters, produced by cosmic-ray electrons in a

μ

G magnetic field, which are believed to have been (re-)accelerated by merger shock fronts. However, not all relics appear at the same location as [...] Read more.

Radio relics are arc-like synchrotron sources at the periphery of galaxy clusters, produced by cosmic-ray electrons in a

μ

G magnetic field, which are believed to have been (re-)accelerated by merger shock fronts. However, not all relics appear at the same location as shocks as seen in the X-ray. In a previous work, we suggested that the shape of some relics may result from the pre-existing spatial distribution of cosmic-ray electrons, and tested this hypothesis using simulations by launching AGN jets into a cluster atmosphere with sloshing gas motions generated by a previous merger event. We showed that these motions could transport the cosmic ray-enriched material of the AGN bubbles to large radii and stretch it in a tangential direction, producing a filamentary shape resembling a radio relic. In this work, we improve our physical description for the cosmic rays by modeling them as a separate fluid which undergoes diffusion and Alfvén losses. We find that, including this additional cosmic ray physics significantly diminishes the appearance of these filamentary features, showing that our original hypothesis is sensitive to the modeling of cosmic ray physics in the intracluster medium. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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11 pages, 20630 KiB

Open AccessArticle

Third-Generation Calibrations for MeerKAT Observation

by Viral Parekh, Robert Kincaid, Benjamin Hugo, Athanaseus Ramaila and Nadeem Oozeer

Galaxies 2021, 9(4), 90; https://doi.org/10.3390/galaxies9040090 - 3 Nov 2021

Cited by 4 | Viewed by 2011

Abstract

Superclusters and galaxy clusters offer a wide range of astrophysical science topics with regards to studying the evolution and distribution of galaxies, intra-cluster magnetization mediums, cosmic ray accelerations and large scale diffuse radio sources all in one observation. Recent developments in new radio [...] Read more.

Superclusters and galaxy clusters offer a wide range of astrophysical science topics with regards to studying the evolution and distribution of galaxies, intra-cluster magnetization mediums, cosmic ray accelerations and large scale diffuse radio sources all in one observation. Recent developments in new radio telescopes and advanced calibration software have completely changed data quality that was never possible with old generation telescopes. Hence, radio observations of superclusters are a very promising avenue to gather rich information of a large-scale structure (LSS) and their formation mechanisms. These newer wide-band and wide field-of-view (FOV) observations require state-of-the-art data analysis procedures, including calibration and imaging, in order to provide deep and high dynamic range (DR) images with which to study the diffuse and faint radio emissions in supercluster environments. Sometimes, strong point sources hamper the radio observations and limit the achievement of a high DR. In this paper, we have shown the DR improvements around strong radio sources in the MeerKAT observation of the Saraswati supercluster by applying newer third-generation calibration (3GC) techniques using CubiCal and killMS software. We have also calculated the statistical parameters to quantify the improvements around strong radio sources. This analysis advocates for the use of new calibration techniques to maximize the scientific returns from new-generation telescopes. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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10 pages, 394 KiB

Open AccessArticle

Searching for High-z Radio Galaxies with the MGCLS

by Kenda Knowles, Sinah Manaka, Michael F. Bietenholz, William D. Cotton, Matthew Hilton, Konstantinos Kolokythas, S. Ilani Loubser and Nadeem Oozeer

Galaxies 2021, 9(4), 89; https://doi.org/10.3390/galaxies9040089 - 2 Nov 2021

Cited by 2 | Viewed by 2186

Abstract

We present the results from a search for high-redshift radio galaxy (HzRG) candidates using 1.28 GHz data in the Abell 2751 field drawn from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). We used the HzRG criteria that a radio [...] Read more.

We present the results from a search for high-redshift radio galaxy (HzRG) candidates using 1.28 GHz data in the Abell 2751 field drawn from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). We used the HzRG criteria that a radio source is undetected in all-sky optical and infrared catalogues and that it has a very steep radio spectrum. We used the likelihood ratio method for cross-matching the radio catalogue against multi-wavelength galaxy catalogues from the Dark Energy Camera Legacy Survey (DECaLS) and the All-sky Wide Infrared Survey Explorer (AllWISE). For those radio sources with no multi-wavelength counterpart, we further implemented a radio spectral index criterium of

α < - 1

, using in-band spectral index measurements from the wide-band MeerKAT data. Using a 5

σ

signal-to-noise cut on the radio flux densities, we found a total of 274 HzRG candidates: 179 ultra-steep spectrum sources and 95 potential candidates, which could not be ruled out as they had no spectral information available. The spectral index assignments in this work were complete above a flux density of 0.3 mJy, which is at least an order of magnitude lower than existing studies in this frequency range or when extrapolating from lower frequency limits. Our faintest HzRG candidates with and without an in-band spectral index measurement had a 1.28 GHz flux density of 57 ± 8

μ

Jy and 68 ± 13

μ

Jy, respectively. Although our study is not complete down to these flux densities, our results indicate that the sensitivity and bandwidth of the MGCLS data make them a powerful radio resource to search for HzRG candidates in the Southern sky, with 20 of the MGCLS pointings having similar image quality as the Abell 2751 field and full coverage in both DECaLS and AllWISE. Data at additional radio frequencies will be needed for the faintest source populations, which could be provided in the near future by the MeerKAT UHF band (580–1015 MHz) at a similar resolution (∼8–10″). Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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17 pages, 3412 KiB

Open AccessArticle

Combining LOFAR and Apertif Data for Understanding the Life Cycle of Radio Galaxies

by Raffaella Morganti, Nika Jurlin, Tom Oosterloo, Marisa Brienza, Emanuela Orrú, Alexander Kutkin, Isabella Prandoni, Elizabeth A. K. Adams, Helga Dénes, Kelley M. Hess, Aleksandar Shulevski, Thijs van der Hulst and Jacob Ziemke

Galaxies 2021, 9(4), 88; https://doi.org/10.3390/galaxies9040088 - 2 Nov 2021

Cited by 16 | Viewed by 2611

Abstract

Active galactic nuclei (AGN) at the centres of galaxies can cycle between periods of activity and of quiescence. Characterising the duty-cycle of AGN is crucial for understanding their impact on the evolution of the host galaxy. For radio AGN, their evolutionary stage can [...] Read more.

Active galactic nuclei (AGN) at the centres of galaxies can cycle between periods of activity and of quiescence. Characterising the duty-cycle of AGN is crucial for understanding their impact on the evolution of the host galaxy. For radio AGN, their evolutionary stage can be identified from a combination of morphological and spectral properties. We summarise the results we have obtained in the last few years by studying radio galaxies in various crucial phases of their lives, such as remnant and restarted sources. We used morphological information derived from LOw Frequency ARray (LOFAR) images at 150 MHz, combined with resolved spectral indices maps, obtained using recently released images at 1400 MHz from the APERture Tile In Focus (Apertif) phased-array feed system installed on the Westerbork Synthesis Radio Telescope. Our study, limited so far to the Lockman Hole region, has identified radio galaxies in the dying and restarted phases. We found large varieties in their properties, relevant for understanding their evolutionary stage. We started by quantifying their occurrences, the duration of the ‘on’ (active) and ‘off’ (dying) phase, and we compared the results with models of the evolution of radio galaxies. In addition to these extreme phases, the resolved spectral index images can also reveal interesting secrets about the evolution of apparently normal radio galaxies. The spectral information can be connected with, and used to improve, the Fanaroff–Riley classification, and we present one example of this, illustrating what the combination of the LOFAR and Apertif surveys now allow us to do routinely. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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10 pages, 1729 KiB

Open AccessArticle

Viewing Classical Radio Galaxies with the Upgraded GMRT and MeerKAT—A Progress Report

by Dharam V. Lal, Portia Legodi, Bernie Fanaroff, Tiziana Venturi, Oleg M. Smirnov, Marco Bondi, Kshitij Thorat, Landman H. Bester, Gyula I. G. Józsa, Dane Kleiner, Francesca Loi, Sphesihle Makhathini and Sarah V. White

Galaxies 2021, 9(4), 87; https://doi.org/10.3390/galaxies9040087 - 29 Oct 2021

Cited by 4 | Viewed by 1976

Abstract

We present a progress report of a study of FR I and FR II radio galaxies. Several new morphological features in the radio emission are now revealed using the high (

μ

Jy) sensitivity reached in the range 550–1712 MHz, more than a [...] Read more.

We present a progress report of a study of FR I and FR II radio galaxies. Several new morphological features in the radio emission are now revealed using the high (

μ

Jy) sensitivity reached in the range 550–1712 MHz, more than a factor of three, at the high (∼

4^{″} - 7^{″}

) angular resolution with the upgraded Giant Metrewave Radio Telescope (uGMRT) and MeerKAT. Therefore, the aim of this study is to understand if we need to revise our current classification scheme for classical radio galaxies. In order to address our goals, we have carefully constructed a sample of 14 (6 FR I, 6 FR II and 2 FR 0) radio galaxies. The uGMRT and MeerKAT images of our four target sources revealed a wealth of morphological details, e.g., filamentary structure in the emission from the lobes, misalignments, radio emission beyond the hot-spots in three sources, etc.; see Fanaroff et al. (2021). Here, we present preliminary results for two more radio galaxies from our sample using uGMRT, in the light of the local environment. Finally, we are awaiting uGMRT and MeerKAT observations of remaining sample sources. Our results show that for the radio galaxies in this study, the morphological classification scheme for the classical FR I/FR II radio galaxies still holds, even with the improved imaging capabilities of the uGMRT and MeerKAT. Furthermore, we need to be cautious when using automated procedures for classification schemes, e.g., in surveys (with poorer sensitivities and angular resolutions) because of the rich morphological details that are shown in our uGMRT and MeerKAT images. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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16 pages, 727 KiB

Open AccessArticle

Exploring New Redshift Indicators for Radio-Powerful AGN

by Rodrigo Carvajal, Israel Matute, José Afonso, Stergios Amarantidis, Davi Barbosa, Pedro Cunha and Andrew Humphrey

Galaxies 2021, 9(4), 86; https://doi.org/10.3390/galaxies9040086 - 29 Oct 2021

Cited by 7 | Viewed by 2219

Abstract

Active Galactic Nuclei (AGN) are relevant sources of radiation that might have helped reionising the Universe during its early epochs. The super-massive black holes (SMBHs) they host helped accreting material and emitting large amounts of energy into the medium. Recent studies have shown [...] Read more.

Active Galactic Nuclei (AGN) are relevant sources of radiation that might have helped reionising the Universe during its early epochs. The super-massive black holes (SMBHs) they host helped accreting material and emitting large amounts of energy into the medium. Recent studies have shown that, for epochs earlier than

z \sim 5

, the number density of SMBHs is on the order of few hundreds per square degree. Latest observations place this value below 300 SMBHs at

z ≳ 6

for the full sky. To overcome this gap, it is necessary to detect large numbers of sources at the earliest epochs. Given the large areas needed to detect such quantities, using traditional redshift determination techniques—spectroscopic and photometric redshift—is no longer an efficient task. Machine Learning (ML) might help obtaining precise redshift for large samples in a fraction of the time used by other methods. We have developed and implemented an ML model which can predict redshift values for WISE-detected AGN in the HETDEX Spring Field. We obtained a median prediction error of

σ_{z}^{N} = 1.48 \times (z_{Predicted} - z_{True}) / (1 + z_{True}) = 0.1162

and an outlier fraction of

η = 11.58 %

at

(z_{Predicted} - z_{True}) / (1 + z_{True}) > 0.15

, in line with previous applications of ML to AGN. We also applied the model to data from the Stripe 82 area obtaining a prediction error of

σ_{z}^{N} = 0.2501

. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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6 pages, 583 KiB

Open AccessArticle

Radio Galaxy Classification: #Tags, Not Boxes

by Lawrence Rudnick

Galaxies 2021, 9(4), 85; https://doi.org/10.3390/galaxies9040085 - 29 Oct 2021

Cited by 11 | Viewed by 2061

Abstract

After six decades of studying radio galaxies, we are now delightfully overwhelmed by their exponentially expanding numbers and the complexity of their structures. Similarly, the methods we use to classify radio galaxies have exploded, often resulting in conflicting terminology, ambiguous classifications, and historical [...] Read more.

After six decades of studying radio galaxies, we are now delightfully overwhelmed by their exponentially expanding numbers and the complexity of their structures. Similarly, the methods we use to classify radio galaxies have exploded, often resulting in conflicting terminology, ambiguous classifications, and historical schemes that may or may not match our current physical understanding. After discussions with more than 100 radio astronomers over the last several years and listening to their ideas and aspirations, I propose that we reconceptualize the classification of radio galaxies. Instead of trying to place them into “boxes”, we should assign them #tags, a system that is easy to understand and apply, that is flexible and evolving, and that can accommodate conflicting ideas with respect to what is relevant and important. Here, I outline the basis of such a #tag system; the rest is up to the community. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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19 pages, 3075 KiB

Open AccessArticle

Somewhere in between: Tracing the Radio Emission from Galaxy Groups (or Why Does the Future of Observing Galaxy Groups with Radio Telescopes Look Promising?)

by Błażej Nikiel-Wroczyński

Galaxies 2021, 9(4), 84; https://doi.org/10.3390/galaxies9040084 - 28 Oct 2021

Cited by 1 | Viewed by 2128

Abstract

Galaxy groups constitute the most common class of galaxy systems in the known Universe, unique in terms of environmental properties. However, despite recent advances in optical and infrared observations as well as in theoretical research, little is known about magnetic fields and the [...] Read more.

Galaxy groups constitute the most common class of galaxy systems in the known Universe, unique in terms of environmental properties. However, despite recent advances in optical and infrared observations as well as in theoretical research, little is known about magnetic fields and the associated continuum radio emission. Studies on this issue have only been conducted in recent years, and many questions have yet to be resolved. This article aims to put the study of group magnetism in a broader context, to present recent advances in the field (mainly achieved with low-frequency radio interferometers), and to list the issues that need to be addressed in future observations. To make it easier for the Readers to get acquainted with the concepts presented in the manuscript, radio observations of two sample groups of galaxies are also presented. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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6 pages, 2481 KiB

Open AccessArticle

Odd Radio Circles and Their Environment

by Ray P. Norris, Evan Crawford and Peter Macgregor

Galaxies 2021, 9(4), 83; https://doi.org/10.3390/galaxies9040083 - 28 Oct 2021

Cited by 21 | Viewed by 2717

Abstract

Odd Radio Circles (ORCs) are unexpected faint circles of diffuse radio emission discovered in recent wide deep radio surveys. They are typically about one arcmin in diameter, and may be spherical shells of synchrotron emission about a million light years in diameter, surrounding [...] Read more.

Odd Radio Circles (ORCs) are unexpected faint circles of diffuse radio emission discovered in recent wide deep radio surveys. They are typically about one arcmin in diameter, and may be spherical shells of synchrotron emission about a million light years in diameter, surrounding galaxies at a redshift of ∼0.2–0.6. Here we study the properties and environment of the known ORCs. All three known single ORCs either lie in a significant overdensity or have a close companion. If the ORC is caused by an event in the host galaxy, then the fact that they tend to be in an overdensity, or have a close companion, may indicate that the environment is important in creating the ORC phenomenon, possibly because of an increased ambient density or magnetic field. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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12 pages, 3933 KiB

Open AccessArticle

One Source, Two Source(s): Ribs and Tethers

by Lawrence Rudnick, William Cotton, Kenda Knowles and Konstantinos Kolokythas

Galaxies 2021, 9(4), 81; https://doi.org/10.3390/galaxies9040081 - 23 Oct 2021

Cited by 12 | Viewed by 1927

Abstract

We present the unique and challenging case of a radio galaxy in Abell 3266 observed as part of the MeerKAT Galaxy Cluster Legacy Survey. It has quasi-periodic bright patches along the tail which connect to never-before-seen thin transverse extensions, which we call “ribs”, [...] Read more.

We present the unique and challenging case of a radio galaxy in Abell 3266 observed as part of the MeerKAT Galaxy Cluster Legacy Survey. It has quasi-periodic bright patches along the tail which connect to never-before-seen thin transverse extensions, which we call “ribs”, reaching up to ∼50 kpc from the central axis of the tail. At a distance of ∼400 kpc from the host (assuming the

z = 0.0594

redshift of Abell 3266) we found what appears to be a triple source with its own apparent host at a photometric redshift of 0.78. Mysteriously, the part of the tail far from the host and the triple are connected by a series of thin filaments, which we call “tethers”. The far tail, tethers and triple also have similar spectra and Faraday rotation measures, suggesting that there is only one—quite complicated—source, with a serendipitous background AGN in the triple. We look at possible causes for the “rib” and “tether” structures, and the emerging phenomena of intracluster medium filaments associated with radio galaxies. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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6 pages, 360 KiB

Open AccessArticle

Properties of Very Broad Line MgII Radio-Loud and Radio-Quiet Quasars

by Avinanda Chakraborty, Anirban Bhattacharjee and Suchetana Chatterjee

Galaxies 2021, 9(4), 74; https://doi.org/10.3390/galaxies9040074 - 7 Oct 2021

Cited by 3 | Viewed by 2083

Abstract

We perform an analysis of the properties of radio-loud (RL) and radio-quiet (RQ) quasars with MgII broad emission line (i-band magnitude

\leq 19.1

and z

\leq 1.9

), selected from the parent sample of SDSS DR7 catalogue. For sources with full-width half maxima [...] Read more.

We perform an analysis of the properties of radio-loud (RL) and radio-quiet (RQ) quasars with MgII broad emission line (i-band magnitude

\leq 19.1

and z

\leq 1.9

), selected from the parent sample of SDSS DR7 catalogue. For sources with full-width half maxima (FWHM) greater than 15,000 km s

^{- 1}

(very broad line sample; VBL) we find the radio loud fraction (RLF) to be about 40%. To further investigate this result we compare the bolometric luminosity, optical continuum luminosity, black hole (BH) mass and Eddington ratios of our VBL sample of RL and RQ quasars. Our analysis shows that in our VBL sample space, RL quasars have higher luminosities and BH mass than RQ quasars. The similarity in the distribution of their covering fraction (CF) shows that there is no difference in dust distribution between VBL RL and RQ quasars and hence dust is not affecting our results. We also find that there is no correlation of RL quasar properties with optical continuum luminosity and BH mass. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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20 pages, 3482 KiB

Open AccessArticle

Properties of Polarized Synchrotron Emission from Fluctuation Dynamo Action—II. Effects of Turbulence Driving in the ICM and Beam Smoothing

by Aritra Basu and Sharanya Sur

Galaxies 2021, 9(3), 62; https://doi.org/10.3390/galaxies9030062 - 5 Sep 2021

Cited by 6 | Viewed by 1936

Abstract

Polarized synchrotron emission from the radio halos of diffuse intracluster medium (ICM) in galaxy clusters are yet to be observed. To investigate the expected polarization in the ICM, we use high resolution (1 kpc) magnetohydrodynamic simulations of fluctuation dynamos, which produces intermittent magnetic [...] Read more.

Polarized synchrotron emission from the radio halos of diffuse intracluster medium (ICM) in galaxy clusters are yet to be observed. To investigate the expected polarization in the ICM, we use high resolution (1 kpc) magnetohydrodynamic simulations of fluctuation dynamos, which produces intermittent magnetic field structures, for varying scales of turbulent driving (

l_{f}

) to generate synthetic observations of the polarized emission. We focus on how the inferred diffuse polarized emission for different

l_{f}

is affected due to smoothing by a finite telescope resolution. The mean fractional polarization

⟨ p ⟩

vary as

⟨ p ⟩ \propto l_{f}^{1 / 2}

with

⟨ p ⟩ > 20 %

for

l_{f} ≳ 60

kpc, at frequencies

ν > 4 GHz

. Faraday depolarization at

ν < 3

GHz leads to deviation from this relation, and in combination with beam depolarization, filamentary polarized structures are completely erased, reducing

⟨ p ⟩

to below 5% level at

ν ≲ 1

GHz. Smoothing on scales up to 30 kpc reduces

⟨ p ⟩

above 4 GHz by at most a factor of 2 compared to that expected at 1 kpc resolution of the simulations, especially for

l_{f} ≳ 100

kpc, while at

ν < 3

GHz,

⟨ p ⟩

is reduced by a factor of more than 5 for

l_{f} ≳ 100

kpc, and by more than 10 for

l_{f} ≲ 100

kpc. Our results suggest that observational estimates of, or constrain on,

⟨ p ⟩

at

ν ≳ 4

GHz could be used as an indicator of the turbulent driving scale in the ICM. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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Review

Jump to: Editorial, Research

13 pages, 6495 KiB

Open AccessReview

Role of Magnetic Fields in Ram Pressure Stripped Galaxies

by Ancla Müller, Alessandro Ignesti, Bianca Poggianti, Alessia Moretti, Mpati Ramatsoku and Ralf-Jürgen Dettmar

Galaxies 2021, 9(4), 116; https://doi.org/10.3390/galaxies9040116 - 11 Dec 2021

Cited by 8 | Viewed by 2708

Abstract

Ram-pressure stripping is a crucial evolutionary driver for cluster galaxies and jellyfish galaxies characterized by very extended tails of stripped gas, and they are the most striking examples of it in action. Recently, those extended tails are found to show ongoing star formation, [...] Read more.

Ram-pressure stripping is a crucial evolutionary driver for cluster galaxies and jellyfish galaxies characterized by very extended tails of stripped gas, and they are the most striking examples of it in action. Recently, those extended tails are found to show ongoing star formation, raising the question of how the stripped, cold gas can survive long enough to form new stars outside the stellar disk. In this study, we summarize the most recent results achieved within the GASP collaboration to provide a holistic explanation for this phenomenon. We focus on two textbook examples of jellyfish galaxies, JO206 and JW100, for which, via multi-wavelength observations from radio to X-ray and numerical simulations, we have explored the different gas phases (neutral, molecular, diffuse-ionized, and hot). Based on additional multi-phase gas studies, we now propose a scenario of stripped tail evolution including all phases that are driven by a magnetic draping sheath, where the intracluster turbulent magnetized plasma condenses onto the galaxy disk and tail and produces a magnetized interface that protects the stripped galaxy tail gas from evaporation. In such a scenario, the accreted environmental plasma can cool down and eventually join the tail gas, hence providing additional gas to form stars. The implications of our findings can shed light on the more general scenario of draping, condensation, and cooling of hot gas surrounding cold clouds that is fundamental in many astrophysical phenomena. Full article

(This article belongs to the Special Issue A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives)

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