Submit to Universe Review for Universe Propose a Special Issue

Journal Menu

Journal Browser

► Journal Browser

Multi-wavelength Properties of Active Galactic Nuclei

Editor
Special Issue Information
Keywords
Benefits of Publishing in a Special Issue
Published Papers

A Special Issue of Universe (ISSN 2218-1997) belonging to the section "Galaxies and Clusters".

Deadline for manuscript submissions: closed (31 March 2026) | Viewed by 7138

Share This Special Issue

Editor

Prof. Stefano Vercellone

E-Mail Website
Guest Editor

INAF—Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
Interests: high-energy astrophysics; jetted sources; multi-wavelength astrophysics

Special Issue Information

Dear Colleagues,

The multi-wavelength study of active galactic nuclei (AGNs) dates back to the 1970s, when the need to obtain observations taken "at the same time and at different wavelengths" started to emerge. At the very beginning, radio and optical data were the main windows through which to investigate these extragalactic sources. The advent of both space- and ground-based facilities allowed us to investigate almost the entire electromagnetic spectrum, spanning about 20 orders of magnitude in energy.

Multi-wavelength studies of AGNs allow us to probe different physical structures of these sources, from the boundaries of central black holes to kiloparsec scales typical of emerging jets.

The present Special Issue aims to discuss some of the currently open questions on AGNs from a multi-wavelength point of view, including both observational and theoretical results. Potential topics include but are not limited to the following:

Is there a consensus on what a jetted source looks like?
How do we investigate the central black hole? What if it is a binary structure?
How does a jet form and propagate, and what does it look like at different wavelengths?
How do we investigate AGN winds?
How can different wavelengths probe different AGN structures?
What can we infer from theoretical modelling from the spectral energy distribution?
How can AGNs probe fundamental physics?
How do we exploit AGN multi-wavelength catalogues?
How will new facilities allow us to shed light on AGN controversies?

Prof. Stefano Vercellone
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-anonymized peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

active galactic nuclei
black-hole physics
accretion
winds
jets
emission mechanism
theoretical modelling
observing facilities
software and databases

Benefits of Publishing in a Special Issue

Ease of navigation: Grouping papers by topic helps scholars navigate broad scope journals more efficiently.
Greater discoverability: Special Issues support the reach and impact of scientific research. Articles in Special Issues are more discoverable and cited more frequently.
Expansion of research network: Special Issues facilitate connections among authors, fostering scientific collaborations.
External promotion: Articles in Special Issues are often promoted through the journal's social media, increasing their visibility.
Reprint: MDPI Books provides the opportunity to republish successful Special Issues in book format, both online and in print.

Further information on MDPI's Special Issue policies can be found here.

Published Papers (7 papers)

Download All Papers

Order results

Result details

Show export options Show export options

Select all

Export citation of selected articles as:

Research

Jump to: Review

25 pages, 1319 KB

Open AccessArticle

Testing [O ii] λ3727 as a Star Formation Rate Tracer in Quasar Host Galaxies

by Xiaotong Feng, Xue-Bing Wu, Yuming Fu, Yuxuan Pang, Rui Zhu and Huimei Wang

Universe 2026, 12(7), 214; https://doi.org/10.3390/universe12070214 - 16 Jul 2026

Viewed by 587

Abstract

The [O ii]

λ 3727

The [O ii]

λ 3727

emission line is a widely used star formation rate (SFR) tracer. However, its application to type I quasars is not straightforward, because the line can be affected by dust extinction, metallicity and contamination from the AGN narrow-line region (NLR). We test the reliability of [O ii] SFRs using a sample of 202 SDSS and PG quasars, by comparing [O ii] SFRs and reference far-infrared (FIR) SFRs derived from multiwavelength SED decomposition. We measure [O ii], [O iii], and narrow Balmer emission lines by spectral fitting. Then, we calculate [O ii] SFRs after correcting dust extinction and metallicity. We then compare these SFRs with the FIR SFRs, both with and without subtracting the AGN contribution estimated from [O iii]. After this correction, the median offset between [O ii] and FIR SFRs is

- 0.20 \pm 0.72

dex for the full analysis sample and

- 0.17 \pm 0.69

dex for sources with S/N

> 5

in both [O ii] and [O iii]. Without subtracting the AGN contribution, the corresponding offsets are

0.00 \pm 0.69

and

0.12 \pm 0.66

dex. We conclude that [O ii] is useful as a statistical SFR tracer for quasar host galaxies, but individual objects still require careful treatment of AGN contamination, extinction, metallicity, aperture effects, and redshift-dependent systematics. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

► Show Figures

Figure 1

42 pages, 3818 KB

Open AccessArticle

Far-Infrared Star Formation Rates of Quasar Host Galaxies from Multiwavelength Spectral Energy Distribution Decomposition

by Xiaotong Feng, Xue-Bing Wu, Yuming Fu, Yuxuan Pang, Rui Zhu and Huimei Wang

Universe 2026, 12(7), 213; https://doi.org/10.3390/universe12070213 - 16 Jul 2026

Viewed by 367

Abstract

Reliable star formation rates (SFRs) are essential for studying the connection between black hole growth and quasar host galaxies. We study the far-infrared (FIR) SFRs and the host galaxy properties of 202 SDSS and PG quasars at

0.02 < z ≲ 0.8

, [...] Read more.

0.02 < z ≲ 0.8

, spanning

log ({SFR}_{FIR} / M_{⊙} {yr}^{- 1}) ≃ - 0.45

–

2.76

, using multiwavelength spectral energy distribution (SED) decomposition. The photometry covers wavelengths from the optical to the FIR and is supplemented by JCMT/SCUBA-2 observations at 450 and 850

μ

m. We model the SEDs with CIGALE and AGNfitter and adopt multiple cold dust templates to quantify systematic uncertainties. The median model-dependent scatter among the five FIR SFR estimates is

0.14

dex, and AGNfitter gives FIR SFRs lower than the mean CIGALE estimate by a median of

0.09

dex. For the 58 quasars with SCUBA-2 coverage, including SCUBA-2 data changes the adopted FIR SFR by only ∼0.01 dex on average but can affect individual sources with limited Herschel coverage or radio-loud emission. Within our FIR-constrained sample, many quasar hosts lie on or above the star-forming main sequence, but the redshift-dependent FIR selection of the SDSS subsample limits conclusions about the full quasar-host population. We find no clear correlation between the main-sequence (MS) offset and the direct Eddington ratio, while the offset is positively related to the infrared-based

L_{tor} / L_{Edd}

proxy. The minimum radiation field intensity in the dust model,

U_{\min}

, increases with bolometric luminosity and dust temperature. WISE W2 (4.6

μ

m) and W3 (12

μ

m) combined with Herschel bands can also provide useful empirical indicators of

f_{AGN}

. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

► Show Figures

Figure 1

14 pages, 2918 KB

Open AccessArticle

A New Phase of Optical Activity of BL Lacertae in the Fall of 2024: Intra-Night Flux and Polarization Variations

by Rumen Bachev, Milen Minev, Anton Strigachev and Alexander Kurtenkov

Universe 2025, 11(11), 372; https://doi.org/10.3390/universe11110372 - 9 Nov 2025

Viewed by 593

Abstract

BL Lacertae is not only archetypical of an entire class of jet-dominated active galactic nuclei, blazars, but also one of the most active and rapidly changing objects in this class. In the fall of 2024 (September–November), BL Lacertae underwent another episode of strong optical activity, reaching an R-band magnitude of about 12 and showing extremely rapid and large-amplitude inter- and intra-night flux and polarization variations. During this period, the object was monitored over 40 nights using telescopes with an aperture of up to 2 m at three observatories: Rozhen and Belogradchik in Bulgaria and Skinakas in Greece. The results from this study include some of the most spectacular intra-night variability episodes detected in a blazar. These rapid variations, combined with high photometric accuracy and high time resolution, allowed for confirmation of consistency between different optical bands with zero time delays, down to a minute scale. Unlike previous activity reports, polarization was relatively stable on these short time-scales. Possible connections between polarization, flux, and intra-night variability were explored in order to better model or constrain the physical processes and emission mechanisms in the relativistic jets. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

► Show Figures

Figure 1

19 pages, 649 KB

Open AccessArticle

Study on the Gamma-Ray Radiation Properties of High-Redshift Blazars at z > 2.5

by Fan Wu, Li Zhang and Benzhong Dai

Universe 2025, 11(9), 320; https://doi.org/10.3390/universe11090320 - 18 Sep 2025

Viewed by 826

Abstract

The high-redshift blazars are important cosmological probes for exploring the early universe and unraveling the fundamental emission processes and the structure of the active galactic nuclei. The high-energy GeV gamma-ray emissions of 38 high-redshift blazars (z > 2.5) observed by Fermi-LAT were analyzed. Along with the Archive multiwavelength data, we employ one-zone leptonic external Compton (EC) models to reproduce the spectral energy distributions (SEDs) of 38 sources. Both the external photons from the molecular torus (MT) and the broad-line region (BLR) are considered. We obtained the best-fitting parameters for describing the characteristics of the jets and accretion disks. The results indicate that high-redshift blazars exhibit higher

γ

-ray luminosities, energy densities, jet powers, kinetic powers, accretion disk luminosities, black hole (BH) masses, radiation efficiencies, and mass accretion rates compared to low-redshift blazars. For high-redshift blazars, the influence of the accretion rate on jet power appears to weaken, and in most cases, the jet power exceeds the total accretion power. We speculate that for high-redshift blazars, rapid accretion may lead to magnetic field saturation, thereby reducing the effectiveness of the Blandford–Payne (BP) process. Consequently, the Blandford–Znajek (BZ) process is likely to play a more dominant role in powering jets in high-redshift blazars compared to low-redshift blazars. Naturally, we acknowledge that selection effects cannot be fully eliminated. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

► Show Figures

Figure 1

17 pages, 858 KB

Open AccessArticle

Optical Photometric Monitoring of the Blazar OT 355 and Local Standard Stars’ Calibration

by R. Bachev, Tushar Tripathi, Alok C. Gupta, A. Kurtenkov, Y. Nikolov, A. Strigachev, S. Boeva, G. Latev, B. Spassov, M. Minev, E. Ovcharov, W.-X. Yang, Yi Liu and J.-H. Fan

Universe 2025, 11(6), 171; https://doi.org/10.3390/universe11060171 - 27 May 2025

Cited by 1 | Viewed by 1254

Abstract

OT 355 (4FGL J1734.3 + 3858) is a relatively rarely studied but highly variable, moderate-redshift (z = 0.975) flat-spectrum radio quasar (blazar). With this work, we aim to study its optical variability on different timescales, which can help us to better understand the physical processes in relativistic jets operating in blazar-type active galactic nuclei. OT 355 was observed in four colors (BVRI) during 41 nights between 2017 and 2023 using three 1 and 2 m class telescopes. The object was also monitored on intra-night timescales, for about 100 h in total. In addition, secondary standard stars in the field of OT 355 were calibrated in order to facilitate future photometric studies. We detected significant intra-night and night-to-night variations of up to 0.5 mag. Variability characteristics, color changes, and a possible “rms-flux” relation were studied and discussed. Using simple arguments, we show that a negative “rms-flux” relation should be expected if many independent processes/regions drive the short-term variability via Doppler factor changes, which is not observed in this and other cases. This finding raises arguments for the idea that more complex multiplicative processes are responsible for blazar variability. Studying blazar variability, especially on the shortest possible timescales, can help to estimate the strength and geometry of their magnetic fields, the linear sizes of the emitting regions, and other aspects, which may be of importance for constraining and modeling blazars’ emitting mechanisms. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

► Show Figures

Figure 1

16 pages, 4105 KB

Open AccessArticle

Structure Formation Through Magnetohydrodynamical Instabilities in Primordial Disks

by Koichi Noguchi, Toshiki Tajima and Wendell Horton

Universe 2024, 10(11), 411; https://doi.org/10.3390/universe10110411 - 31 Oct 2024

Viewed by 1469

Abstract

The shear flow instabilities under the presence of magnetic fields in the primordial disk can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the gravitational Jeans instability. We evaluate the effects of the Parker, magnetorotational and kinematic dynamo instabilities by comparing the properties of these instabilities. We calculate the mass spectra of coagulated density structures by the above mechanism in the radial direction for an axisymmetric magnetohydrodynamic (MHD) torus equilibrium and power density profile models. Our local three-dimensional MHD simulation indicates that the coupling of the Parker and magnetorotational instability creates spiral arms and gas blobs in an accretion disk, reinforcing the theory and model. Such a mechanism for the early structure formation may be tested in a laboratory. The recent progress in experiments involving shear flows in rotating tokamak, field reversed configuration (FRC) and laser plasmas may become a key element to advance in nonlinear studies. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

► Show Figures

Figure 1

Review

Jump to: Research

18 pages, 1611 KB

Open AccessReview

Blazars as Probes for Fundamental Physics

by Giorgio Galanti

Universe 2025, 11(10), 327; https://doi.org/10.3390/universe11100327 - 27 Sep 2025

Cited by 1 | Viewed by 808

Abstract

Blazars are a class of active galactic nuclei characterized by having one of their relativistic jets oriented close to our line of sight. Their broad emission spectrum makes them exceptional laboratories for probing fundamental physics. In this review, we explore the potential impact on blazar observations of three scenarios beyond the standard paradigm: (i) the hadron beam model, (ii) the interaction of photons with axion-like particles (ALPs), and (iii) Lorentz invariance violation. We focus on the very-high-energy spectral features these scenarios induce in the blazars Markarian 501 and 1ES 0229+200, making them ideal targets for testing such effects. Additionally, we examine ALP-induced effects on the polarization of UV-X-ray and high-energy photons from the blazar OJ 287. The unique signatures produced by these models are accessible to current and upcoming instruments—such as the ASTRI Mini Array, CTAO, LHAASO, IXPE, COSI, and AMEGO—offering new opportunities to probe and constrain fundamental physics through blazar observations. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

► Show Figures

Journal Menu

Journal Browser

Multi-wavelength Properties of Active Galactic Nuclei

Share This Special Issue

Editor

Special Issue Information

Keywords

Benefits of Publishing in a Special Issue

Published Papers (7 papers)

Research

Review

Further Information

Guidelines

MDPI Initiatives

Follow MDPI