Evolution Of Active Galactic Nuclei Research Articles

Probing the Heart of Active Narrow-line Seyfert 1 Galaxies with VERA Wideband Polarimetry

We explored the parsec-scale nuclear regions of a sample of radio-loud narrow-line Seyfert 1 galaxies (NLSy1s) using the VLBI Exploration of Radio Astrometry wideband (at a recording rate of 16 Gbps) polarimetry at 22 and 43 GHz. Our targets include 1H 0323+342, SBS 0846+513, PMN J0948+0022, 1219+044, PKS 1502+036, and TXS 2116-077, which are all known to exhibit γ-ray emission indicative of possessing highly beamed jets similar to blazars. For the first time, we unambiguously detected Faraday rotation toward the parsec-scale radio core of NLSy1s, with a median observed core rotation measure (RM) of 2.7 × 103 rad m−2 (or 6.3 × 103 rad m−2 for redshift-corrected). This level of RM magnitude is significantly larger than those seen in the core of BL Lacertae objects (BLOs; a dominant subclass of blazars), suggesting that the nuclear environment of NLSy1s is more gas-rich than that in BLOs. Interestingly, the observed parsec-scale polarimetric properties of NLSy1s (low core fractional polarization, large core RM and jet–EVPA misalignment) are rather similar to those of flat-spectrum radio quasars (FSRQs). Our results are in accordance with the scenario that NLSy1s are in an early stage of active galactic nucleus evolution with their central black hole masses being smaller than those of more evolved FSRQs.

Nucleosynthesis Contribution of Neutrino-dominated Accretion Flows to the Chemical Evolution of Active Galactic Nuclei

Recent observations of quasars show high line-flux ratios in their broad emission lines and the ratios appear to be independent of redshift up to z ≳ 6, which indicates that the broad-line regions of these early quasars are surprisingly metal-rich. Here, we revisit the chemical evolution of high-redshift quasars by adding a new ingredient, i.e., the neutrino-dominated accretion flows (NDAFs) with outflows, on top of the conventional core-collapse supernovae (CCSNe). In the presence of the chemical contribution from NDAFs with outflows, the total metal mass (i.e., the summation of the conventional CCSN and NDAFs with outflows) per CCSN depends weakly upon the mass of the progenitor star if the mass is in the range of ∼25–55 M ⊙. We model the chemical evolution by adopting a improved open-box model with three typical initial mass functions (IMFs). We find that, with the additional chemical contribution from NDAFs with outflows, the quasar metallicity can be enriched more rapidly in the very early universe (z ∼ 10) and reaches a higher saturation than the no-NDAF case at z ∼ 8, after which they evolve slowly with redshift. The quasar metallicity can reach ∼20 Z ⊙ (Z ⊙ denotes the metallicity of the Sun, ∼20% of which is produced by NDAF outflows) at z ∼ 8 for the “top-heavy” IMF model in Toyouchi et al., which readily explains the quasar observations on the supersolar metal abundance and redshift-independent evolution.

Discovery of Rare Dying Radio Galaxies Using MeerKAT

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 μG, 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.

Do Current X-Ray Observations Capture Most of the Black-hole Accretion at High Redshifts?

Abstract The cosmic black hole accretion density (BHAD) is critical for our understanding of the formation and evolution of supermassive black holes (BHs). However, at high redshifts (z > 3), X-ray observations report BHADs significantly (∼10 times) lower than those predicted by cosmological simulations. It is therefore paramount to constrain the high-z BHAD using independent methods other than direct X-ray detections. The recently established relation between star formation rate and BH accretion rate among bulge-dominated galaxies provides such a chance, as it enables an estimate of the BHAD from the star formation histories (SFHs) of lower-redshift objects. Using the CANDELS Lyα Emission At Reionization (CLEAR) survey, we model the SFHs for a sample of 108 bulge-dominated galaxies at z = 0.7–1.5, and further estimate the BHAD contributed by their high-z progenitors. The predicted BHAD at z ≈ 4–5 is consistent with the simulation-predicted values, but higher than the X-ray measurements (by ≈3–10 times at z = 4–5). Our result suggests that the current X-ray surveys could be missing many heavily obscured Compton-thick active galactic nuclei (AGNs) at high redshifts. However, this BHAD estimation assumes that the high-z progenitors of our z = 0.7–1.5 sample remain bulge-dominated where star formation is correlated with BH cold-gas accretion. Alternatively, our prediction could signify a stark decline in the fraction of bulges in high-z galaxies (with an associated drop in BH accretion). JWST and Origins will resolve the discrepancy between our predicted BHAD and the X-ray results by constraining Compton-thick AGN and bulge evolution at high redshifts.

Hunting for the nature of the enigmatic narrow-line Seyfert 1 galaxy PKS 2004-447

Narrow-line Seyfert 1 (NLS1) galaxies are a class of active galactic nuclei (AGN) that, in some cases, can harbor powerful relativistic jets. One of them, PKS 2004-447, showsγ-ray emission, and underwent its first recorded multifrequency flare in 2019. However, past studies revealed that in radio this source can be classified as a compact steep-spectrum source (CSS), suggesting that, unlike otherγ-ray sources, the relativistic jets of PKS 2004-447 have a large inclination with respect to the line of sight. We present here a set of spectroscopic observations of this object, aimed at carefully measuring its black hole mass and Eddington ratio, determining the properties of its emission lines, and characterizing its long term variability. We find that the black hole mass is (1.5 ± 0.2)×107 M⊙, and the Eddington ratio is 0.08. Both values are within the typical range of NLS1s. The spectra also suggest that the 2019 flare was caused mainly by the relativistic jet, while the accretion disk played a minor role during the event. In conclusion, we confirm that PKS 2004-447 is one of the rare examples ofγ-ray emitting CSS/NLS1s hybrid, and that these two classes of objects are likely connected in the framework of AGN evolution.

Building robust active galactic nuclei mock catalogs to unveil black hole evolution and for survey planning

The statistical distributions of active galactic nuclei (AGNs), i.e., accreting supermassive black holes (BHs), in mass, space, and time are controlled by a series of key properties, namely, the BH–galaxy scaling relations, Eddington ratio distributions, and fraction of active BH (duty cycle). Shedding light on these properties yields strong constraints on the AGN triggering mechanisms while providing a clear baseline to create useful mock catalogs for the planning of large galaxy surveys. Here we delineate a robust methodology to create mock AGN catalogs built on top of large N-body dark matter simulations via state-of-the-art semiempirical models. We show that by using as independent tests the AGN clustering at fixed X-ray luminosity, galaxy stellar mass, and BH mass, along with the fraction of AGNs in groups and clusters, it is possible to significantly narrow down the choice in the relation between BH mass and host galaxy stellar mass, the duty cycle, and the average Eddington ratio distribution, delivering well-suited constraints to guide cosmological models for the coevolution of BHs and galaxies. Avoiding such a step-by-step methodology inevitably leads to strong degeneracies in the final mock catalogs, severely limiting their usefulness in understanding AGN evolution and in survey planning and testing.

Building Robust Active Galactic Nuclei Mock Catalogs to Unveil Black Hole Evolution and for Survey Planning

Abstract The statistical distributions of active galactic nuclei (AGNs), i.e., accreting supermassive black holes (BHs), in mass, space, and time are controlled by a series of key properties, namely, the BH–galaxy scaling relations, Eddington ratio distributions, and fraction of active BH (duty cycle). Shedding light on these properties yields strong constraints on the AGN triggering mechanisms while providing a clear baseline to create useful mock catalogs for the planning of large galaxy surveys. Here we delineate a robust methodology to create mock AGN catalogs built on top of large N-body dark matter simulations via state-of-the-art semiempirical models. We show that by using as independent tests the AGN clustering at fixed X-ray luminosity, galaxy stellar mass, and BH mass, along with the fraction of AGNs in groups and clusters, it is possible to significantly narrow down the choice in the relation between BH mass and host galaxy stellar mass, the duty cycle, and the average Eddington ratio distribution, delivering well-suited constraints to guide cosmological models for the coevolution of BHs and galaxies. Avoiding such a step-by-step methodology inevitably leads to strong degeneracies in the final mock catalogs, severely limiting their usefulness in understanding AGN evolution and in survey planning and testing.

The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1

We present the source associations, cross-identifications, and multi-wavelength properties of the faint radio source population detected in the deep tier of the LOFAR Two Metre Sky Survey (LoTSS): the LoTSS Deep Fields. The first LoTSS Deep Fields data release consists of deep radio imaging at 150 MHz of the ELAIS-N1, Lockman Hole, and Boötes fields, down to RMS sensitives of around 20, 22, and 32 μJy beam−1, respectively. These fields are some of the best studied extra-galactic fields in the northern sky, with existing deep, wide-area panchromatic photometry from X-ray to infrared wavelengths, covering a total of ≈26 deg2. We first generated improved multi-wavelength catalogues in ELAIS-N1 and Lockman Hole; combined with the existing catalogue for Boötes, we present forced, matched aperture photometry for over 7.2 million sources across the three fields. We identified multi-wavelength counterparts to the radio detected sources, using a combination of the Likelihood Ratio method and visual classification, which greatly enhances the scientific potential of radio surveys and allows for the characterisation of the photometric redshifts and the physical properties of the host galaxies. The final radio-optical cross-matched catalogue consists of 81 951 radio-detected sources, with counterparts identified and multi-wavelength properties presented for 79 820 (>97%) sources. We also examine the properties of the host galaxies, and through stacking analysis find that the radio population with no identified counterpart is likely dominated by active galactic nuclei (AGN) atz~ 3−4. This dataset contains one of the largest samples of radio-selected star-forming galaxies and AGN at these depths, making it ideal for studying the history of star-formation, and the evolution of galaxies and AGN across cosmic time.

Active Galactic Nucleus Ghosts: A Systematic Search for Faded Nuclei

Abstract Physical processes such as reignition, enhancement, and fading of active galactic nuclei (AGN) are not entirely understood because the timeline of these events is expected to last many years. However, it is well known that the differences in the energy budget between AGN components, like the optical ionizing region and the mid-infrared (MIR) dust echoes, can be interpreted as a hint of AGN evolution. Here we present a catalog of 88 AGN candidates showing hints of the fading and rising of their activity in the nearby universe. We use AGN scaling relations to select them from an initial sample of 877 candidates using publicly available optical, X-ray, and MIR luminosities. We then use the multiwavelength information to discard sources contaminated with extranuclear emission and those with an X-ray luminosity not well corrected for absorption. We find that 96% of our candidates are fading sources. This result suggests a scenario where the universe had its peak of AGN activity somewhere in the past and is dominated by a fading phase at the present time. Alternatively, the fading phase is longer than the rising phase, which is consistent with galaxy merger simulations. Around 50% of these fading candidates are associated with merging or interacting systems. Finally, we also find the existence of jets in ∼30% of these candidates and that the preferred AGN dust geometry is torus-like instead of wind-like. Our results are compatible with the fading of nuclear activity, expected if they are in an inefficient state.

New constraints on the 1.4 GHz source number counts and luminosity functions in the Lockman Hole field

ABSTRACT We present a study of the 1173 sources brighter than $S_{1.4\, \rm GHz}= 120\, \mu$Jy detected over an area of $\simeq 1.4\, \hbox{deg}^{2}$ in the Lockman Hole field. Exploiting the multiband information available in this field for ∼79 per cent of the sample, sources have been classified into radio loud (RL) active galactic nuclei (AGNs), star-forming galaxies (SFGs), and radio quiet (RQ) AGNs, using a variety of diagnostics available in the literature. Exploiting the observed tight anticorrelations between IRAC band 1 or band 2 and the source redshift we could assign a redshift to 177 sources missing a spectroscopic measurement or a reliable photometric estimate. A Monte Carlo approach was used to take into account the spread around the mean relation. The derived differential number counts and luminosity functions at several redshifts of each population show a good consistency with models and with earlier estimates made using data from different surveys and applying different approaches. Our results confirm that below $\sim 300\, \mu$Jy SFGs+RQ AGNs overtake RL AGNs that dominate at brighter flux densities. We also confirm earlier indications of a similar evolution of RQ AGNs and SFGs. Finally, we discuss the angular correlation function of our sources and highlight its sensitivity to the criteria used for the classification.

Mock catalogs for the extragalactic X-ray sky: Simulating AGN surveys with ATHENA and with the AXIS probe

We present a series of new, publicly available mock catalogs of X-ray selected active galactic nuclei (AGNs), nonactive galaxies, and clusters of galaxies. These mocks are based on up-to-date observational results on the demographic of extragalactic X-ray sources and their extrapolations. They reach fluxes below 10−20 erg cm−2 s−1 in the 0.5–2 keV band, that is, more than an order of magnitude below the predicted limits of future deep fields, and they therefore represent an important tool for simulating extragalactic X-ray surveys with both current and future telescopes. We used our mocks to perform a set of end-to-end simulations of X-ray surveys with the forthcoming ATHENA mission and with the AXIS probe, a subarcsecond resolution X-ray mission concept proposed to the Astro 2020 Decadal Survey. We find that these proposed, next generation surveys may transform our knowledge of the deep X-ray Universe. As an example, in a total observing time of 15 Ms, AXIS would detect ∼225 000 AGNs and ∼50 000 nonactive galaxies, reaching a flux limit of f0.5−2 ∼ 5 × 10−19 erg cm−2 s−1 in the 0.5–2 keV band, with an improvement of over an order of magnitude with respect to surveys with current X-ray facilities. Consequently, 90% of these sources would be detected for the first time in the X-rays. Furthermore, we show that deep and wide X-ray surveys with instruments such as AXIS and ATHENA are expected to detect ∼20 000 z > 3 AGNs and ∼250 sources at redshift z > 6, thus opening a new window of knowledge on the evolution of AGNs over cosmic time and putting strong constraints on the predictions of theoretical models of black hole seed accretion in the early universe.

The XXL Survey

We study the space density evolution of active galactic nuclei (AGN) using the 610 MHz radio survey of the XXL-North field, performed with the Giant Metrewave Radio Telescope. The survey covers an area of 30.4 deg2, with a beamsize of 6.5 arcsec. The survey is divided into two parts, one covering an area of 11.9 deg2 with 1σ rms noise of 200 μJy beam−1 and the other spanning 18.5 deg2 with rms noise of 45 μJy beam−1. We extracted the catalog of radio components above 7σ. The catalog was cross-matched with a multi-wavelength catalog of the XXL-North field (covering about 80% of the radio XXL-North field) using a likelihood ratio method, which determines the counterparts based on their positions and their optical properties. The multi-component sources were matched visually with the aid of a computer code: Multi-Catalog Visual Cross-Matching. A flux density cut above 1 mJy selects AGN hosts with a high purity in terms of star formation contamination based on the available source counts. After cross-matching and elimination of observational biases arising from survey incompletenesses, the number of remaining sources was 1150. We constructed the rest-frame 1.4 GHz radio luminosity functions of these sources using the maximum volume method. This survey allows us to probe luminosities of 23 ≲ log(L1.4 GHz[W Hz−1]) ≲ 28 up to redshifts of z ≈ 2.1. Our results are consistent with the results from the literature in which AGN are comprised of two differently evolving populations, where the high luminosity end of the luminosity functions evolves more strongly than the low-luminosity end.

AGN anisotropic radiative feedback set by black hole spin

ABSTRACT We consider the impact of anisotropic radiation on the active galactic nucleus (AGN) radiative dusty feedback. The radiation pattern originating from the accretion disc is determined by the central black hole (BH) spin. Here we analyse how such BH spin-induced angular dependence affects the dynamics and energetics of the radiation pressure-driven outflows, as well as AGN obscuration and BH accretion. In addition, we explore the effect of a spatially varying dust-to-gas ratio on the outflow propagation. We obtain two distinct trends for high-spin and low-spin objects, providing a direct connection between anisotropic feedback and BH spin. In the case of maximum spin, powerful quasi-spherical outflows can propagate on large scales, at all inclination angles with fairly uniform energetics. In contrast, in the case of zero spin, only weaker bipolar outflows can be driven in the polar directions. As a result, high BH spins can efficiently clear out the obscuring gas from most directions, whereas low BH spins can only remove dusty gas from the polar regions, hence also determining the overall AGN obscuration geometry. Due to such anisotropic feedback, high BH spins can prevent accretion of gas from most directions (except in the equatorial plane), while low BH spins allow inflows to proceed from a wider range of directions. This may have important implications for the BH growth in the early Universe. Anisotropic radiative dusty feedback, ruled by the BH spin, may thus play a major role in shaping AGN evolution over cosmic time.

Investigation on young radio AGNs based on SDSS spectroscopy

ABSTRACT The gigahertz peaked spectrum (GPS) sources, compact steep spectrum (CSS) radio sources, and high-frequency peaker (HFP) radio sources are thought to be young radio active galactic nuclei (AGNs) at the early stage of AGN evolution. We investigated the optical properties of the largest sample of 126 young radio AGNs based on the spectra in SDSS DR12. We find that the black hole masses MBH range from 107.32 to 109.84$\rm M_{\odot }$ and the Eddington ratios Redd vary from 10−4.93 to 100.37, suggesting that young radio AGNs have various accretion activities and not all are accreting at high accretion rate. Our young radio sources generally follow the evolutionary trend towards large-scale radio galaxies with increasing linear size and decreasing accretion rate in the radio power–linear size diagram. The radio properties of low-luminosity young radio AGNs with low Redd are discussed. The line width of [O iii] λ5007 core (σ[O iii]) is found to be a good surrogate of stellar velocity dispersion σ*. The radio luminosity $L_{\rm 5\, GHz}$ correlates strongly with [O iii] core luminosity L[O iii], suggesting that radio activity and accretion are closely related in young radio sources. We find one object that can be defined as a narrow-line Seyfert 1 galaxy, representing a population of young AGNs with both young jet and early accretion activity. The optical variabilities of 15 quasars with multi-epoch spectroscopy were investigated. Our results show that the optical variability in young AGN quasars presents low variations ($\leqslant \! 60{{\ \rm per\ cent}}$) similar to the normal radio-quiet quasars.

AGN evolution as seen in spectral lines: The case of narrow-line Seyfert 1s

AbstractLine profiles can provide fundamental information on the physics of active galactic nuclei (AGN). In the case of narrow-line Seyfert 1 galaxies (NLS1s) this is of particular importance since past studies revealed how their permitted line profiles are well reproduced by a Lorentzian function instead of a Gaussian. This has been explained with different properties of the broad-line region (BLR), which may present more pronounced turbulent motions in NLS1s with respect to other AGN. We investigated the line profiles in a recent large NLS1 sample classified using SDSS, and we divided the sources into two subsamples according to their line shapes, Gaussian or Lorentzian. The line profiles seem to separate all the properties of NLS1s. Black hole mass, Eddington ratio, [OIII] luminosity, and Fe II strength are all very different in the Lorentzian and Gaussian samples, as well as their position on the quasar main sequence. We interpret this in terms of evolution within the class of NLS1s. The Lorentzian sources may be the youngest objects, while Gaussian profiles may be typically associated to more evolved objects. Further detailed spectroscopic studies are needed to fully confirm our hypothesis.

A global view of the inner accretion and ejection flow around super massive black holes

Context.Understanding the physics and geometry of accretion and ejection around super massive black holes (SMBHs) is important to understand the evolution of active galactic nuclei (AGN) and therefore of the large scale structures of the Universe.Aims.We aim at providing a simple, coherent, and global view of the sub-parsec accretion and ejection flow in AGN with varying Eddington ratio,ṁ, and black hole mass,MBH.Methods.We made use of theoretical insights, results of numerical simulations, as well as UV and X-ray observations to review the inner regions of AGN by including different accretion and ejection modes, with special emphasis on the role of radiation in driving powerful accretion disk winds from the inner regions around the central SMBH.Results.We propose fiveṁregimes where the physics of the inner accretion and ejection flow around SMBHs is expected to change, and that correspond observationally to quiescent and inactive galaxies; low luminosity AGN (LLAGN); Seyferts and mini-broad absorption line quasars (mini-BAL QSOs); narrow line Seyfert 1 galaxies (NLS1s) and broad absorption line quasars (BAL QSOs); and super-Eddington sources. We include in this scenario radiation-driven disk winds, which are strong in the highṁ, largeMBHregime, and possibly present but likely weak in the moderateṁ, smallMBHregime.Conclusions.A great diversity of the accretion/ejection flows in AGN can be explained to a good degree by varying just two fundamental properties: the Eddington ratioṁand the black hole massMBH, and by the inclusion of accretion disk winds that can naturally be launched by the radiation emitted from luminous accretion disks.

Modelling the AGN broad line region using single-epoch spectra – I. The test case of Arp 151

ABSTRACT We show that individual (single-epoch) spectra of active galactic nuclei (AGNs) can constrain some of the geometry and dynamics of the AGN broad line region. Studies of the cosmic influence of supermassive black holes are limited by the current large uncertainties in the determination of black hole masses. One dominant limitation is the unknown geometry, dynamics, and line-of-sight inclination of the broad line region, used to probe the central black hole mass. Recent progress has been made to constrain the spatial and kinematic structure of the broad line region using dynamical modelling of AGN monitoring data and an underlying physical model for the broad line region. In this work we test the ability of a modified version of this dynamical modelling code to constrain the broad line region structure using single-epoch spectra. We test our modelling code on single-epoch spectra of nearby Arp 151 by comparing our results with those obtained with monitoring data of this same object. We find that a significant fraction of the broad line region parameters can indeed be adequately constrained, with uncertainties that are comparable to, or at most a factor of approximately a few higher than those obtained from modelling of monitoring data. Considering the wealth of available single-epoch spectroscopic observations, this method is promising for establishing the overall AGN population trends in the geometry and dynamics of the broad line region. This method can be applied to spectra of AGNs at low and high redshift making it valuable for studies of cosmological black hole and AGN evolution.

The evolving X-ray spectrum of active galactic nuclei: evidence for an increasing reflection fraction with redshift

The cosmic X-ray background (XRB) spectrum and active galaxy number counts encode essential information about the spectral evolution of active galactic nuclei (AGNs) and have been successfully modeled by XRB synthesis models for many years. Recent measurements of the $8$--$24$ keV AGN number counts by NuSTAR and Swift-BAT are unable to be simultaneously described by existing XRB synthesis models, indicating a fundamental breakdown in our understanding of AGN evolution. Here we show that the $8$--$24$ keV AGN number counts can be successfully modeled with an XRB synthesis model in which the reflection strength ($R$) in the spectral model increases with redshift. We show that an increase in reflection strength with redshift is a natural outcome of (1) connecting $R$ to the incidence of high column density gas around AGNs, and (2) the growth in the AGN obscured fraction to higher redshifts. In addition to the redshift evolution of $R$, we also find tentative evidence that an increasing Compton-thick fraction with $z$ is necessary to describe the $8$--$24$ keV AGN number counts determined by NuSTAR. These results show that, in contrast to the traditional orientation-based AGN unification model, the nature and covering factor of the absorbing gas and dust around AGNs evolve over time and must be connected to physical processes in the AGN environment. Future XRB synthesis models must allow for the redshift evolution of AGN spectral parameters. This approach may reveal fundamental connections between AGNs and their host galaxies.

Identifying the 3FHL Catalog. III. Results of the CTIO-COSMOS Optical Spectroscopy Campaign

Active galactic Nuclei (AGNs) with their relativistic jets pointed towards the observer, form a subclass of luminous gamma-ray sources commonly known as blazars. The study of blazars is essential to improve our understanding on the AGNs emission mechanisms and evolution, as well as to map the extragalactic background light. To do so, however, one needs to correctly classify and measure a redshift for a large sample of these sources. The Third {\it Fermi}--LAT Catalog of High-Energy Sources (3FHL) contains $\approx1160$ blazars reported at energies greater than $10$\,GeV. However $\sim$25\% of these sources are unclassified and $\sim$50\% lack of redshift information. To increase the spectral completeness of the 3FHL catalog, we are working on an optical spectroscopic follow up campaign using 4--m and 8--m telescopes. In this paper, we present the results of the second part of this campaign, where we observed 23 blazars using the 4\,$m$ telescope at CTIO in Chile. We report all the 23 sources to be classified as BL Lacs, a confirmed redshift measurement for 3 sources, a redshift lower limit for 2 sources and a tentative redshift measurement for 3 sources.

The XXL Survey

Context. We present the results of a study of the active galactic nucleus (AGN) density in a homogeneous and well-studied sample of 167 bona fide X-ray galaxy clusters (0.1 < z < 0.5) from the XXL Survey, from the cluster core to the outskirts (up to 6r500). The results can provide evidence of the physical mechanisms that drive AGN and galaxy evolution within clusters, testing the efficiency of ram pressure gas stripping and galaxy merging in dense environments. Aims. The XXL cluster sample mostly comprises poor and moderately rich structures (M = 1013–4 × 1014 M⊙), a poorly studied population that bridges the gap between optically selected groups and massive X-ray selected clusters. Our aim is to statistically study the demographics of cluster AGNs as a function of cluster mass and host galaxy position. Methods. To investigate the effect of the environment on AGN activity, we computed the fraction of spectroscopically confirmed X-ray AGNs (LX [0.5-10 keV] > 1042 erg cm−1) in bright cluster galaxies with Mi* − 2 < M < Mi* + 1, up to 6r500 radius. The corresponding field fraction was computed from 200 mock cluster catalogues with reshuffled positions within the XXL fields. To study the mass dependence and the evolution of the AGN population, we further divided the sample into low- and high-mass clusters (below and above 1014M⊙, respectively) and two redshift bins (0.1–0.28 and 0.28–0.5). Results. We detect a significant excess of X-ray AGNs, at the 95% confidence level, in low-mass clusters between 0.5r500 and 2r500, which drops to the field value within the cluster cores (r < 0.5r500). In contrast, high-mass clusters present a decreasing AGN fraction towards the cluster centres, in agreement with previous studies. The high AGN fraction in the outskirts is caused by low-luminosity AGNs, up to LX [0.5-10 keV] = 1043 erg cm−1. It can be explained by a higher galaxy merging rate in low-mass clusters, where velocity dispersions are not high enough to prevent galaxy interactions and merging. Ram pressure stripping is possible in the cores of all our clusters, but probably stronger in deeper gravitational potentials. Compared with previous studies of massive or high-redshift clusters, we conclude that the AGN fraction in cluster galaxies anti-correlates strongly with cluster mass. The AGN fraction also increases with redshift, but at the same rate with the respective fraction in field galaxies.