Multiple Active Galactic Nuclei Research Articles

Dual Active Galactic Nuclei: Precursors of Binary Supermassive Black Hole Formation and Mergers

The presence of dual active galactic nuclei (AGNs) on scales of a few tens of kiloparsecs can be used to study merger-induced accretion on supermassive black holes (SMBHs) and offer insights about SMBH mergers, using dual AGNs as merger precursors. This study uses the Romulus25 cosmological simulation to investigate the properties and evolution of dual AGNs. We first analyze the properties of AGNs (L bol > 1043 erg s−1) and their neighboring SMBHs (any SMBHs closer than 30 pkpc to an AGN) at z ≤ 2. This is our underlying population. We then applied the luminosity threshold of L bol > 1043 erg s−1 to the neighboring SMBHs thereby identifying dual and multiple AGNs. Our findings indicate an increase in the number of both single and dual AGNs from lower to higher redshifts. We also find that the number of dual AGNs with separations of 0.5–4 kpc is twice the number of duals with separations of 4–30 kpc. All dual AGNs in our sample resulted from major mergers. Compared to single AGNs, duals have a lower black hole-to-halo mass ratio. We found that the properties of dual AGN host halos, including halo mass, stellar mass, star formation rate, and gas mass, are generally consistent with those of single AGN halos, albeit tending toward the higher end of their respective property ranges. Our analysis uncovered a diverse array of evolutionary patterns among dual AGNs, including rapidly evolving systems, slower ones, and instances where SMBH mergers are ineffective.

New multiple AGN systems with subarcsec separation: Confirmation of candidates selected via the novel GMP method

The existence of multiple active galactic nuclei (AGNs) at small projected distances on the sky is due to either the presence of multiple, inspiraling supermassive black holes, or to gravitational lensing of a single AGN. Both phenomena allow us to address important astrophysical and cosmological questions. However, few kiloparsec-separation multiple AGNs are currently known. Recently, the newly developed Gaia multi-peak (GMP) method provided numerous new candidate members of these populations. We present spatially resolved, integral-field spectroscopy of a sample of four GMP-selected multiple AGN candidates. In all of these systems, we detect two or more components with subarcsec separations. We find that two of the systems are dual AGNs, one is either an intrinsic triple or a lensed dual AGN, while the last system is a chance alignment of an AGN and a star. Our observations double the number of confirmed multiple AGNs at projected separations below 7 kpc at z > 0.5, present the first detection of a possible triple AGN in a single galaxy at z > 0.5, and successfully test the GMP method as a novel technique to discover previously unknown multiple AGNs.

Unveiling the population of dual and lensed active galactic nuclei at sub-arcsec separations

All cosmological models of structure formation predict the existence of a widespread population of dual supermassive black holes in-spiralling inside their common host galaxy, eventually merging and giving rise to intense gravitational waves. These systems can be identified as dual AGNs at kilo parsec separations, but only very few have been confirmed at z>0.5. The appearance of multiple AGNs at small angular separations can also be due to gravitational lensing of single AGNs, which are themselves very important systems for many astrophysical topics. Here we present a novel technique, dubbed the Gaia Multipeak (GMP) method, to obtain large and reliable samples of dual/lensed AGN candidates with sub-arcsec separations by looking for AGNs showing multiple peaks in the light profiles observed by the Gaia satellite. All the GMP-selected sources with high resolution images (26 from the HST archive and 5 from dedicated adaptive-optics assisted imaging at the Large Binocular Telescope) show multiple components with sub-arcsec separation pointing toward a very high reliability of the method. By sampling separations down to ~2 kpc at z>1, this method allows us to probe the physical processes that drive the inspiralling of a pair of SMBHs inside a single galaxy.

Determining active galactic nucleus luminosity histories using present-day outflow properties: a neural network-based approach

ABSTRACT Large-scale outflows driven by active galactic nuclei (AGNs) can have a profound influence on their host galaxies. The outflow properties themselves depend sensitively on the history of AGN energy injection during the lifetime of the outflow. Most observed outflows have dynamical time-scales longer than the typical AGN episode duration, i.e. they have been inflated by multiple AGN episodes. Here, we present a neural network-based approach to inferring the most likely duty cycle and other properties of AGN based on the observable properties of their massive outflows. Our model recovers the AGN parameters of simulated outflows with typical errors $\lt 25{{\ \rm per\ cent}}$. We apply the method to a sample of 59 real molecular outflows and show that a large fraction of them have been inflated by AGN shining with a rather high duty cycle δAGN > 0.2. This result suggests that nuclear activity in galaxies is clustered hierarchically in time, with long phases of more frequent activity composed of many short activity episodes. We predict that $\sim \! 19{{\ \rm per\ cent}}$ of galaxies should have AGN-driven outflows, but half of them are fossils – this is consistent with currently available data. We discuss the possibilities to investigate AGN luminosity histories during outflow lifetimes and suggest ways to use our software to test other physical models of AGN outflows. The source code of all of the software used here is made public.

Quasars with Proper Motions and the Link to Double and Multiple AGNs

Gaia used a large sample of photometrically selected active galactic nuclei (AGNs) and quasars to remove the residual spin of its global proper motion system in order to achieve a maximally inertial reference frame. A small fraction of these reference objects have statistically significant astrometric proper motions in Gaia EDR3. We compile a source sample of 105,593 high-fidelity AGNs with accurate spectroscopically determined redshifts above 0.5 from the SDSS and normalized proper motions below 4. The rate of genuinely perturbed proper motions is at least 0.17%. A smaller high completeness sample of 152 quasars with excess proper motions at a confidence level of 0.9995 is examined in detail. Pan-STARRS images and Gaia-resolved pairs reveal that 29% of the sample are either double sources or gravitationally lensed quasars. An Anderson–Darling test on parameters of a smaller high-reliability sample and their statistical controls reveals 17 significant factors that favor multiplicity and multi-source structure as the main cause of perturbed astrometry. Using a nearest-neighbor distance statistical analysis and counts of close companions in Gaia on a much larger initial sample of AGNs, an excess of closely separated sources in Gaia is detected. At least 0.33% of all optical quasars are genuinely double or multiply imaged. We provide a list of 44 candidate double or multiple AGNs and four previously known gravitational lenses. Many proper motion quasars may be more closely separated, unresolved doubles exhibiting the variability imposed motion effect, and a smaller fraction may be chance alignments with foreground stars causing weak gravitational lensing.

Dual AGN in the Horizon-AGN simulation and their link to galaxy and massive black hole mergers, with an excursus on multiple AGN

ABSTRACT The occurrence of dual active galactic nuclei (AGN) on scales of a few tens of kpc can be used to study merger-induced accretion on massive black holes (MBHs) and to derive clues on MBH mergers, using dual AGN as a parent population of precursors. We investigate the properties of dual AGN in the cosmological simulation Horizon-AGN. We create catalogs of dual AGN selected with distance and luminosity criteria, plus sub-catalogs where further mass cuts are applied. We divide the sample into dual AGN hosted in different galaxies, on the way to a merger, and into those hosted in one galaxy, after the galaxy merger has happened. We find that the relation between MBH and galaxy mass is similar to that of general AGN population and we compare the properties of dual AGN also with a control sample, discussing differences and similarities in masses and Eddington ratios. The typical mass ratio of galaxy mergers associated to dual AGN is 0.2, with mass loss in the smaller galaxy decreasing the mass ratio as the merger progresses. Between 30 and 80 per cent of dual AGN with separations between 4 and 30 kpc can be matched to an ensuing MBH merger. The dual AGN fraction increases with redshift and with separation threshold, although above 50 kpc the increase of multiple AGN limits that of duals. Multiple AGN are generally associated with massive haloes, and mass loss of satellites shapes the galaxy–halo relation.

A Multiwavelength Study of ELAN Environments (AMUSE2). Mass Budget, Satellites Spin Alignment, and Gas Infall in a Massive z ∼ 3 Quasar Host Halo

The systematic targeting of extended Lyα emission around high-redshift quasars resulted in the discovery of rare and bright Enormous Lyα Nebulae (ELANe) associated with multiple active galactic nuclei (AGNs). We initiate here “a multiwavelength study of ELAN environments” (AMUSE2) focusing on the ELAN around the z ∼ 3 quasar SDSS J1040+1020, aka the Fabulous ELAN. We report on VLT/HAWK-I, APEX/LABOCA, JCMT/SCUBA-2, SMA/850μm, and ALMA CO(5-4), and 2 mm observations and compare them to previously published VLT/MUSE data. The continuum and line detections enable a first estimate of the star formation rates, dust, stellar, and molecular gas masses in four objects associated with the ELAN (three AGNs and one Lyα emitter), confirming that the quasar host is the most star-forming (star formation rate of ∼500 M ⊙ yr−1) and massive galaxy (M star ∼ 1011 M ⊙) in the system, and thus can be assumed as central. All four embedded objects have similar molecular gas reservoirs ( M ⊙), resulting in short depletion timescales. This fact together with the estimated total dark matter halo mass, M DM = (0.8–2) × 1013 M ⊙, imply that this ELAN will evolve into a giant elliptical galaxy. Consistently, the constraint on the baryonic mass budget for the whole system indicates that the majority of baryons should reside in a massive warm/hot reservoir (up to 1012 M ⊙), needed to complete the baryons count. Additionally, we discuss signatures of gas infall on the compact objects as traced by Lyα radiative transfer effects and the evidence for the alignment between the satellites’ spins and their directions to the central.

Hosts and triggers of AGNs in the Local Universe

Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compared optically selected active galactic nuclei (AGNs) with control star-forming and quiescent galaxies on galactic and inter-halo scales, and larger. We find that AGNs are preferentially found in two specific stages of galaxy evolution: in the starburst and ‘green valley’ phases. We also find that the stellar population of their host galaxies is quite independent of stellar mass, which is not the case for more typical galaxies. Combining galaxy-galaxy lensing and galaxy clustering on large scales, we measured the mass of AGN host halos. The typical halo mass is about 1012 h−1 M⊙, similar to the characteristic mass in the stellar mass-halo mass relation (SHMR). For a given stellar mass, AGN host galaxies and star-forming galaxies share the same SHMR, while quiescent galaxies have more massive halos. Clustering analyses on halo scales reveals that AGNs are surrounded by a larger number of satellites (with stellar mass down to 1/1000 of the mass of the central galaxy) than star-forming galaxies and that galaxies with a greater stellar velocity dispersion have a greater number of satellites. The number of satellites also increase with halo mass, reaching unity around 1012 h−1 M⊙. Our results suggest a scenario in which the interaction of the central galaxy with the satellites triggers an early episode of starburst and AGN activity, followed by multiple AGN cycles driven by the non-axisymmetric structure produced by the interaction. The feedback from the starburst and AGN reduces the amount of cold gas for fueling the central black hole, producing a characteristic halo mass scale, that is, ∼1012 h−1 M⊙, where the AGN fraction peaks.

AGN Triality of Triple Mergers: Detection of Faint X-Ray Point Sources

Abstract We present results from our X-ray analysis of a systematic search for triple active galactic nuclei (AGNs) in nearby (z < 0.077) triple galaxy mergers. We analyze archival Chandra observations of seven triple galaxy mergers with baymax (Bayesian Analysis of Multiple AGNs in X-rays), fitting each observation with single, dual, and triple X-ray point-source models. In doing so, we conclude that one triple merger has one X-ray point source (SDSS J0858+1822, although it is unlikely to be an AGN), five triple mergers are likely composed of two X-ray point sources (NGC 3341, SDSS J1027+1749, SDSS J1631+2352, SDSS J1708+2153, and SDSS J2356−1016), and one system is composed of three X-ray point sources (SDSS J0849+1114). By fitting the individual X-ray spectra of each point source, we analyze the 2−7 keV luminosities, as well as the levels of obscuration associated with each potential AGN. We find that 4/5 dual X-ray point-source systems have primary and secondary point sources with bright X-ray luminosities (L 2−7kev > 1040 erg s−1), possibly associated with four new undetected dual AGNs. The dual and triple-point-source systems are found to have physical separations between 3 and 9 kpc and flux ratios between 2 × 10−3 and 0.84. A multiwavelength analysis to determine the origin of the X-ray point sources discovered in this work is presented in our companion paper (Foord et al. 2020c).

AGN Triality of Triple Mergers: Multiwavelength Classifications

Abstract We present results from a multiwavelength analysis searching for multiple active galactic nucleus (AGN) systems in nearby (z < 0.077) triple galaxy mergers. Combining archival Chandra, Sloan Digital Data Survey (SDSS), Wide-field Infrared Survey Explorer (WISE), and Very Large Array observations, we quantify the rate of nearby triple AGNs, as well as investigate possible connections between supermassive black hole accretion and merger environments. Analyzing the multiwavelength observations of seven triple galaxy mergers, we find that one triple merger has a single AGN (NGC 3341); we discover, for the first time, four likely dual AGNs (SDSS J1027+1749, SDSS J1631+2352, SDSS J1708+2153, and SDSS J2356−1016); we confirm one triple-AGN system, SDSS J0849+1114; and one triple merger in our sample remains ambiguous (SDSS J0858+1822). Analyzing the WISE data, we find a trend of increasing N H (associated with the primary AGN) as a function of increasing W1 – W2 color, reflecting that the motions of gas and dust are coupled in merging environments, where large amounts of both can be funneled into the active central region during mergers. Additionally, we find that the one triple-AGN system in our sample has the highest levels of N H and W1 – W2 color, while the dual-AGN candidates all have lower levels; these results are consistent with theoretical merger simulations that suggest that higher levels of nuclear gas are more likely to activate AGNs in mergers.

Multiple AGN activity during the BCG assembly of XDCPJ0044.0-2033 at z ∼ 1.6

ABSTRACT Undisturbed galaxy clusters are characterized by a massive and large elliptical galaxy at their centre, i.e. the brightest cluster galaxy (BCG). How these central galaxies form is still debated. According to most models, a typical epoch for their assembly is $z$ ∼ 1–2. We have performed a detailed multiwavelength analysis of the core of XMM–Newton Distant Cluster Project (XDCP) J0044.0-2033 (XDCP0044), one of the most massive and densest galaxy clusters currently known at redshift $z$ ∼ 1.6, whose central galaxy population shows high star formation compared to lower z clusters and an X-ray active galactic nuclei (AGN) located close to its centre. SINFONI J-, H-, and KMOS YJ-, H-bands spectroscopic data have been analysed, together with deep archival HST photometric data in F105W, F140W, and F160W bands, Chandra X-ray, radio JVLA data at 1–2 GHz, and ALMA band-6 observations. In the very central region of the cluster (∼70 kpc × 70 kpc), two systems of interacting galaxies have been identified and studied (Complex A and B), with a total of seven confirmed cluster members. These galaxies show perturbed morphologies and three of them show signs of AGN activity. In particular, two type-1 AGN with typical broad lines have been found at the centre of each complex (both of them X-ray obscured and highly accreting with $\rm \lambda _{Edd}\sim 0.4-0.6$), while a type-2 AGN has been discovered in Complex A. The AGN at the centre of Complex B is also detected in X-ray, while the other two are spatially related to radio emission. The three AGN provide one of the closest AGN triple at $z$ > 1 revealed so far with a minimum (maximum) projected distance of 10 (40) kpc. The observation of high star formation, merger signatures, and nuclear activity in the core of XDCP0044 suggests that all these processes are key ingredients in shaping the nascent BCG. According to our data, XDCP0044 could form a typical massive galaxy of $M_{\star }\sim 10^{12} \, \mathrm{M}_{\odot }$, hosting a black hole of $\rm 2 \times 10^8\!-\!10^9 \, \mathrm{M}_{\odot }$, in a time-scale of the order of ∼2.5 Gyr.

Multiplicity functions of quasars: predictions from the MassiveBlackII simulation

ABSTRACT We examine multiple active galactic nucleus (AGN) systems (triples and quadruples, in particular) in the MassiveBlackII simulation over a redshift range of 0.06 ≲ z ≲ 4. We identify AGN systems (with bolometric luminosity $L_{\mathrm{bol}}\gt 10^{42}~\mathrm{erg\, s}^{-1}$) at different scales (defined by the maximum distance between member AGNs) to determine the AGN multiplicity functions. This is defined as the volume/surface density of AGN systems per unit richness R, the number of AGNs in a system. We find that gravitationally bound multiple AGN systems tend to populate scales of ${\lesssim}0.7~\mathrm{cMpc}\, h^{-1}$; this corresponds to angular separations of ≲100 arcsec and a line-of-sight velocity difference ${\lesssim}200~\mathrm{km\, s}^{-1}$. The simulation contains ∼10 and ∼100 triples/quadruples per deg2 up to depths of DESI (g ≲ 24) and LSST (g ≲ 26) imaging, respectively; at least $20{{\ \rm per\ cent}}$ of these should be detectable in spectroscopic surveys. The simulated quasar ($L_{\mathrm{bol}}\gt 10^{44}~\mathrm{erg\, s}^{-1}$) triples and quadruples predominantly exist at 1.5 ≲ z ≲ 3. Their members have black hole masses $10^{6.5}\lesssim M_{\mathrm{ bh}}\lesssim 10^{9}~\mathrm{M}_{\odot }\, h^{-1}$ and live in separate (one central and multiple satellite) galaxies with stellar masses $10^{10}\lesssim M_{*}\lesssim 10^{12}~\mathrm{M}_{\odot }\, h^{-1}$. They live in the most massive haloes (e.g. ${\sim}10^{13}~\mathrm{M}_{\odot }\, h^{-1}$ at z = 2.5; ${\sim}10^{14}~\mathrm{M}_{\odot }\, h^{-1}$ at z = 1) in the simulation. Their detections provide an exciting prospect for understanding massive black hole growth and their merger rates in galaxies in the era of multimessenger astronomy.

A Catalog of AGN Host Galaxies Observed with HST/ACS: Correlations between Star Formation and AGN Activity

Abstract We present the Advanced Camera for Surveys Active Galactic Nuclei (ACS-AGN) Catalog, a catalog of 2585 active galactic nucleus (AGN) host galaxies that are at redshifts 0.2 < z < 2.5 and that were imaged with the Hubble Space Telescope’s Advanced Camera for Surveys (ACS). Using the ACS General Catalog (ACS-GC) as our initial sample, we select an AGN subsample using Spitzer and Chandra data along with their respective established AGN selection criteria. We then gather further multiwavelength photometric data in order to construct spectral energy distributions (SEDs). Using these SEDs, we are able to derive multiple AGN and host galaxy properties, such as star formation rate (SFR), AGN luminosity, stellar mass, and nuclear column density. From these data, we show that AGN host galaxies tend to lie below the star-forming main sequence, with X-ray-selected AGN host galaxies being more offset than IR-selected AGN host galaxies. This suggests that there is some process, possibly negative feedback, in AGN host galaxies causing decreased star formation. We also demonstrate that there is a positive trend between the SFR and AGN luminosity in AGN host galaxies, in individual redshift bins, and across all redshift bins, and that both are correlated with the stellar mass of their galaxies. This points toward an underlying link between the stellar mass, stellar growth, and supermassive black hole growth in a galaxy.

A Bayesian Analysis of SDSS J0914+0853, a Low-mass Dual AGN Candidate

Abstract We present the first results from Bayesian AnalYsis of Multiple AGN in X-rays (BAYMAX), a tool that uses a Bayesian framework to quantitatively evaluate whether a given Chandra observation is more likely a single or dual point source. Although the most robust method of determining the presence of dual active galactic nuclei (AGNs) is to use X-ray observations, only sources that are widely separated relative to the instrument's point-spread function are easy to identify. It becomes increasingly difficult to distinguish dual AGNs from single AGNs when the separation is on the order of Chandra's angular resolution (<1″). Using likelihood models for single and dual point sources, BAYMAX quantitatively evaluates the likelihood of an AGN for a given source. Specifically, we present results from BAYMAX analyzing the lowest-mass dual AGN candidate to date, SDSS J0914+0853, where archival Chandra data shows a possible secondary AGN ∼ 0.″3 from the primary. Analyzing a new 50 ks Chandra observation, results from BAYMAX shows that SDSS J0914+0853 is most likely a single AGN with a Bayes factor of 13.5 in favor of a single point source model. Further, posterior distributions from the dual point source model are consistent with emission from a single AGN. We find a very low probability of SDSS J0914+0853 being a dual AGN system with a flux ratio f > 0.3 and separation r > 0.″3. Overall, BAYMAX will be an important tool for correctly classifying candidate dual AGNs in the literature, as well as studying the dual AGN population where past spatial resolution limits have prevented systematic analyses.

Overdensity of submillimeter galaxies around the z ≃ 2.3 MAMMOTH-1 nebula

In the hierarchical model of structure formation, giant elliptical galaxies form through merging processes within the highest density peaks known as protoclusters. While high-redshift radio galaxies usually pinpoint the location of these environments, we have recently discovered at z ∼ 2−3 three enormous (> 200 kpc) Lyman-α nebulae (ELANe) that host multiple active galactic nuclei (AGN) and that are surrounded by overdensities of Lyman-α emitters (LAE). These regions are prime candidates for massive protoclusters in the early stages of assembly. To characterize the star-forming activity within these rare structures – both on ELAN and protocluster scales – we have initiated an observational campaign with the James Clerk Maxwell Telescope (JCMT) and the Atacama Pathfinder EXperiment (APEX) telescopes. In this paper we report on sensitive SCUBA-2/JCMT 850 and 450 μm observations of a ∼128 arcmin2 field comprising the ELAN MAMMOTH-1, together with the peak of the hosting BOSS1441 LAE overdensity at z = 2.32. These observations unveil 4.0 ± 1.3 times higher source counts at 850 μm with respect to blank fields, likely confirming the presence of an overdensity also in obscured tracers. We find a strong detection at 850 μm associated with the continuum source embedded within the ELAN MAMMOTH-1, which – together with the available data from the literature – allow us to constrain the spectral energy distribution of this source to be of an ultra-luminous infrared galaxy (ULIRG) with a far-infrared luminosity of LFIRSF = 2.4−2.1+7.4×1012 L⊙, and hosting an obscured AGN. Such a source is thus able to power a hard photoionization plus outflow scenario to explain the extended Lyman-α, He IIλ1640, and C IVλ1549 emission, and their kinematics. In addition, the two brightest detections at 850 μm (f850 > 18 mJy) sit at the density peak of the LAEs’ overdensity, likely pinpointing the core of the protocluster. Future multiwavelength and spectroscopic datasets targeting the full extent of the BOSS1441 overdensity have the potential to firmly characterize a cosmic nursery of giant elliptical galaxies, and ultimately of a massive cluster.

Unveiling multiple AGN activity in galaxy mergers

In this paper we present an overview of the MAGNA (Multiple AGN Activity) project aiming at a comprehensive study of multiple super‐massive black hole systems. With the main goal of characterizing the sources in merging systems at different stages of evolution, we selected a sample of objects optically classified as multiple systems on the basis of emission line diagnostics and started a massive multiband observational campaign. Here we report on the discovery of the exceptionally high active galactic nuclei (AGN) density compact group SDSS (Sloan Digital Sky Survey) J0959+1259. A multiband study suggests that strong interactions are taking place among its galaxies through tidal forces, and therefore this system represents a case study for physical mechanisms that trigger nuclear activity and star formation. We also present a preliminary analysis of the multiple AGN system SDSS J1038+3921.

New insights into the evolution of the FR I radio galaxy 3C 270 (NGC 4261) from VLA and GMRT radio observations

We present the Giant Metrewave Radio Telescope 240 MHz observations of the nearby luminous Fanaroff–Riley type I radio source 3C 270, in the group-central elliptical NGC 4261. Combining these data with reprocessed Very Large Array 1.55 and 4.8 GHz observations, we produce spectral-index maps that reveal a constant spectral index along the jets and a gradual steepening from the ends of the jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted to the integrated spectrum of the source gives an asymptotic low-frequency index of |$\alpha _{\rm {inj}}=0.53_{-0.02}^{+0.01}$|⁠, while JP models fitted to the observed spectral index trend along the lobes allow us to estimate radiative ages of ∼29 and ∼37 Myr for the west and east lobes, respectively. Our age estimates are a factor of 2 lower than the 75-Myr upper limit derived from X-ray data. We find unlikely the scenario of an early supersonic phase in which the lobe expanded into the ISM at approximately Mach 6 (3500 km s−1), and suggest that either the source underwent multiple active galactic nuclei outbursts with possible large changes in jet power, or possibly that the source age that we find is due to a backflow that transports young electrons from the jet tips through the lobes towards the nucleus relatively quickly. We calculate that in the lobes the energy ratio of non-radiating to radiating particles is ∼4–24 indicating significant gas entrainment. If the lobes are in pressure balance with their surroundings, the total energy required to heat the entrained material is 1058 erg, ∼40 per cent of the total enthalpy of the lobes.

A multiwavelength view of cooling versus AGN heating in the X-ray luminous cool-core of Abell 3581★

We report the results of a multi-wavelength study of the nearby galaxy group, Abell 3581 (z=0.0218). This system hosts the most luminous cool core of any nearby group and exhibits active radio mode feedback from the super-massive black hole in its brightest group galaxy, IC 4374. The brightest galaxy has suffered multiple active galactic nucleus outbursts, blowing bubbles into the surrounding hot gas, which have resulted in the uplift of cool ionised gas into the surrounding hot intragroup medium. High velocities, indicative of an outflow, are observed close to the nucleus and coincident with the radio jet. Thin dusty filaments accompany the uplifted, ionised gas. No extended star formation is observed, however, a young cluster is detected just north of the nucleus. The direction of rise of the bubbles has changed between outbursts. This directional change is likely due to sloshing motions of the intragroup medium. These sloshing motions also appear to be actively stripping the X-ray cool core, as indicated by a spiraling cold front of high metallicity, low temperature, low entropy gas.

COMPARISONS OF COSMOLOGICAL MAGNETOHYDRODYNAMIC GALAXY CLUSTER SIMULATIONS TO RADIO OBSERVATIONS

Radio observations of galaxy clusters show that there are $\mu$G magnetic fields permeating the intra-cluster medium (ICM), but it is hard to accurately constrain the strength and structure of the magnetic fields without the help of advanced computer simulations. We present qualitative comparisons of synthetic VLA observations of simulated galaxy clusters to radio observations of Faraday Rotation Measure (RM) and radio halos. The cluster formation is modeled using adaptive mesh refinement (AMR) magneto-hydrodynamic (MHD) simulations with the assumption that the initial magnetic fields are injected into the ICM by active galactic nuclei (AGNs) at high redshift. In addition to simulated clusters in Xu et al. (2010, 2011), we present a new simulation with magnetic field injections from multiple AGNs. We find that the cluster with multiple injection sources is magnetized to a similar level as in previous simulations with a single AGN. The RM profiles from simulated clusters, both $|RM|$ and the dispersion of RM ($\sigma_{RM}$), are consistent at a first-order with the radial distribution from observations. The correlations between the $\sigma_{RM}$ and X-ray surface brightness from simulations are in a broad agreement with the observations, although there is an indication that the simulated clusters could be slightly over-dense and less magnetized with respect to those in the observed sample. In addition, the simulated radio halos agree with the observed correlations between the radio power versus the cluster X-ray luminosity and between the radio power versus the radio halo size. These studies show that the cluster wide magnetic fields that originate from AGNs and are then amplified by the ICM turbulence (Xu et al. 2010) match observations of magnetic fields in galaxy clusters.

THE SECOND CATALOG OF ACTIVE GALACTIC NUCLEI DETECTED BY THEFERMILARGE AREA TELESCOPE

The second catalog of active galactic nuclei (AGNs) detected by the Fermi Large Area Telescope (LAT) in two years of scientific operation is presented. The Second LAT AGN Catalog (2LAC) includes 1017 gamma-ray sources located at high Galactic latitudes (|b|>10{\deg}) that are detected with a test statistic greater than 25 and associated statistically with AGNs. However some of these are affected by analysis issues and some are associated with multiple AGNs. Consequently we define a clean sample which includes 886 AGNs, comprising 395 BL Lacertae objects (BL Lacs), 310 flat-spectrum radio quasars (FSRQs), 157 candidate blazars of unknown type (i.e., with broad-band blazar characteristics but with no optical spectral measurement yet), eight misaligned AGNs, four narrow-line Seyfert 1 (NLS1s), 10 AGNs of other types and two starburst galaxies. Where possible, the blazars have been further classified based on their spectral energy distributions (SEDs) as archival radio, optical, and X-ray data permit. While almost all FSRQs have a synchrotron-peak frequency 10^14 Hz, about half of the BL Lacs have a synchrotron-peak frequency >10^15 Hz. The 2LAC represents a significant improvement relative to the First LAT AGN Catalog (1LAC), with 52% more associated sources. The full characterization of the newly detected sources will require more broad-band data. Various properties, such as gamma-ray fluxes and photon power law spectral indices, redshifts, gamma-ray luminosities, variability, and archival radio luminosities---and their correlations are presented and discussed for the different blazar classes. The general trends observed in 1LAC are confirmed.