Low-luminosity Active Galactic Nuclei Research Articles

CLEAR: Spatially Resolved Emission Lines and Active Galactic Nuclei at 0.6 < z < 1.3

We investigate spatially resolved emission-line ratios in a sample of 219 galaxies (0.6 < z < 1.3) detected using the G102 grism on the Hubble Space Telescope Wide Field Camera 3 taken as part of the CANDELS Lyα Emission at Reionization survey to measure ionization profiles and search for low-luminosity active galactic nuclei (AGN). We analyze [O iii] and Hβ emission-line maps, enabling us to spatially resolve the [O iii]/Hβ emission-line ratio across the galaxies in the sample. We compare the [O iii]/Hβ ratio in galaxy centers and outer annular regions to measure ionization differences and investigate the potential of sources with nuclear ionization to host AGN. We investigate some of the individual galaxies that are candidates to host strong nuclear ionization and find that they often have low stellar mass and are undetected in X-rays, as expected for low-luminosity AGN in low-mass galaxies. We do not find evidence for a significant population of off-nuclear AGN or other clumps of off-nuclear ionization. We model the observed distribution of [O iii]/Hβ spatial profiles and find that most galaxies are consistent with a small or zero difference between their nuclear and off-nuclear line ratios, but 6%–16% of galaxies in the sample are likely to host nuclear [O iii]/Hβ that is ∼0.5 dex higher than in their outer regions. This study is limited by large uncertainties in most of the measured [O iii]/Hβ spatial profiles; therefore, deeper data, e.g., from deeper HST/WFC3 programs or from JWST/NIRISS, are needed to more reliably measure the spatially resolved emission-line conditions of individual high-redshift galaxies.

A Candidate for the Least-massive Black Hole in the First 1.1 Billion Years of the Universe

We report a candidate of a low-luminosity active galactic nucleus (AGN) at z = 5 that was selected from the first near-infrared images of the JWST CEERS project. This source, named CEERS-AGN-z5-1 at absolute 1450 Å magnitude M 1450 = −19.5 ± 0.3, was found via a visual selection of compact sources from a catalog of Lyman break galaxies at z > 4, taking advantage of the superb spatial resolution of the JWST/NIRCam images. The 20 photometric data available from CFHT, Hubble Space Telescope, Spitzer, and JWST suggest that the continuum shape of this source is reminiscent of that for an unobscured AGN, and there is a clear color excess in the filters where the redshifted Hβ+[O iii] and Hα are covered. The estimated line luminosity is L Hβ+[O III] = 1043.0 erg s−1 and L Hα = 1042.9 erg s−1 with the corresponding rest-frame equivalent width EWHβ+[O III] = 1100 Å and EWHα = 1600 Å, respectively. Our spectral energy distribution fitting analysis favors the scenario that this object is either a strong broad-line emitter or even a super-Eddington accreting black hole (BH), although a possibility of an extremely young galaxy with moderate dust attenuation is not completely ruled out. The bolometric luminosity, L bol = 2.5 ± 0.3 × 1044 erg s−1, is consistent with those of z < 0.35 broad-line AGNs with M BH ∼ 106 M ⊙ accreting at the Eddington limit. This new AGN population in the first 1.1 billion years of the universe may close the gap between the observed BH mass range at high redshift and that of BH seeds. Spectroscopic confirmation is awaited to secure the redshift and its AGN nature.

Central Star Formation in Early-type Galaxy I Zw 81 in the Bootes Void

The origin of star formation in customarily passively evolving early-type massive galaxies is poorly understood. We present a case study of a massive galaxy, I Zw 81, inside the Bootes void. The void galaxy is known to host active galactic nuclei (AGN). Our detailed 2D decomposition of the surface brightness distribution in the Canada France Hawaii Telescope (CFHT) g and r bands revealed multiple structural components such as a nuclear point source, a bar, a ring, and an inner exponential disk followed by an outer low surface brightness disk. I Zw 81 turns out to be a disk-dominated galaxy with lenticular morphology. The modeling of the multiwavelength spectral energy distribution shows that the galaxy is star-forming (SF), and belongs to the blue cloud. We find that the optical (g−r) color of the bar is bluer than the disks, and the far- and near-ultraviolet emission inside the galaxy observed with Imaging Telescope onboard AstroSat is concentrated in the central few kpc region enclosing the bar. The strong bar might be playing a pivotal role in driving the gas inflow and causing SF activity in tandem with the minor merger-like interactions as evident from the deep CFHT data. The low-luminosity AGN is insufficient to quench the central SF. The results are peculiar from the standpoint of a massive barred lenticular galaxy.

Multifrequency VLBI Observations of the M84 Inner Jet/Counterjet

Observational studies of the innermost regions of the edge-on jets in nearby active galactic nuclei (AGN) are crucial to understanding their kinematics and morphology. For the inner jet of the nearby low-luminosity AGN in M84, we present new high-sensitivity observations with very long baseline Interferometry since 2019, as well as archival Very Long Baseline Array observations in 2014. We find that the compact core in M84 has an inverted-to-flat spectrum from 1.5–88 GHz. Based on the turnover frequency of 4.2 ± 0.2 GHz in the spectrum, we estimated a magnetic field strength of 1 ∼ 10 mG and an electron number density of ∼105 cm−3 in the core region. Three inner jet components within ∼3 mas of the core are identified and traced in the images at 22 GHz, whose apparent speeds are 0.11 c, 0.27 c, and 0.32 c, respectively. We calculate the viewing angle of for the inner jet based on the proper motion and the flux ratio of jet to counterjet. A propagating sinusoidal model with a wavelength of ∼3.4 mas is used to fit the helical morphology of the jet extended to 20 mas (∼2.2 × 104 Schwarzschild radii).

Hadronuclear interactions in the jet of low TeV luminosity AGN: Implications for the low-state very-high-energy gamma-ray emission

In the study of radiation mechanisms of AGNs' jets, hadronuclear (pp) interactions are commonly neglected, because the number density of cold protons in the jet is considered insufficient. Very recently, our previous work proves that pp interactions in the low TeV luminosity AGNs, which have potential to generate detectable very-high-energy (VHE) emission, could be important. Based on this, the one-zone pp model is employed to study low-state quasi-simultaneous spectral energy distributions of a sample of low TeV luminosity AGNs in this work. Our modeling results show that the gamma-ray generated in pp interactions can explain the observed TeV spectra and has contribution to higher energy band that could be detected by the Large High Altitude Air Shower Observatory (LHAASO). In the sample, we suggest that M 87, Mrk 421 and Mrk 501 are the most likely objects to be detected by LHAASO in the near future. Other possible origins of VHE emission are also briefly discussed.

Collimation of the Relativistic Jet in the Quasar 3C 273

The collimation of relativistic jets launched from the vicinity of supermassive black holes (SMBHs) at the centers of active galactic nuclei (AGNs) is one of the key questions to understand the nature of AGN jets. However, little is known about the detailed jet structure for AGN like quasars since very high angular resolutions are required to resolve these objects. We present very long baseline interferometry (VLBI) observations of the archetypical quasar 3C 273 at 86 GHz, performed with the Global Millimeter VLBI Array, for the first time including the Atacama Large Millimeter/submillimeter Array. Our observations achieve a high angular resolution down to ∼60 μas, resolving the innermost part of the jet ever on scales of ∼105 Schwarzschild radii. Our observations, including close-in-time High Sensitivity Array observations of 3C 273 at 15, 22, and 43 GHz, suggest that the inner jet collimates parabolically, while the outer jet expands conically, similar to jets from other nearby low-luminosity AGNs. We discovered the jet collimation break around 107 Schwarzschild radii, providing the first compelling evidence for structural transition in a quasar jet. The location of the collimation break for 3C 273 is farther downstream from the sphere of gravitational influence (SGI) from the central SMBH. With the results for other AGN jets, our results show that the end of the collimation zone in AGN jets is governed not only by the SGI of the SMBH but also by the more diverse properties of the central nuclei.

Evidence That Shocks Destroy Small PAH Molecules in Low-luminosity Active Galactic Nuclei

We combined mapping-mode mid-infrared Spitzer spectra with complementary infrared imaging to perform a spatially resolved study of polycyclic aromatic hydrocarbon (PAH) emission from the central regions of 66 nearby galaxies, roughly evenly divided into star-forming systems and low-luminosity active galactic nuclei (AGNs). In conjunction with similar measurements available for quasars, we aim to understand the physical properties of PAHs across a broad range of black hole accretion power, with the goal of identifying observational diagnostics that can be used to probe the effect of AGNs on the host galaxy. Whereas the PAH emission correlates tightly with far-ultraviolet luminosity in star-forming regions, the spatially resolved regions of AGNs tend to be PAH deficient. Moreover, AGN regions exhibit on average smaller PAH 6.2 μm/7.7 μm and larger PAH 11.3 μm/7.7 μm band ratios. Although the current data are highly restrictive, they suggest that these anomalous PAH band ratios cannot be explained by the effects of the AGN radiation field alone. Instead, they hint that small grains may be destroyed by the combined effects of radiative processes and shocks, which are plausibly linked to jets and outflows preferentially associated with highly sub-Eddington, radiatively inefficient AGNs. While quasars also present a PAH deficit and unusual PAH band ratios, their characteristics differ in detail compared to those observed in more weakly accreting AGNs, a possible indicator of fundamental differences in their modes of energy feedback.

IFU observations of the inner 200 pc of NGC 4546: gas rotation, non-circular motions, and ionized outflows

ABSTRACT We present a detailed analysis of the ionized gas distribution and kinematics in the inner ∼ 200 pc of NGC 4546, host of a low-luminosity active galactic nucleus (LLAGN). Using GMOS−IFU observations, with a spectral coverage of 4736–6806 Å and an angular resolution of 0.7 arcsec, we confirm that the nuclear emission is consistent with photoionization by an AGN, while the gas in the circumnuclear region may be ionized by hot low-mass evolved stars. The gas kinematics in the central region of NGC 4546 presents three components: (i) a disc with major axis oriented along a position angle of 43° ± 3°, counter rotating relative to the stellar disc; (ii) non-circular motions, evidenced by residual velocities of up to 60 km s−1, likely associated with a previous capture of a dwarf satellite by NGC 4546; and (iii) nuclear outflows in ionized gas, identified as a broad component (σ ∼ 320 km s−1) in the line profiles, with a mass outflow rate of $\dot{M}_{\rm out} = 0.3 \pm 0.1$ M⊙ yr−1 and a total mass of Mout = (9.2 ± 0.8) × 103 M⊙ in ionized gas, corresponding to less than 3 per cent of the total mass of ionized gas in the inner 200 pc of NGC 4546. The kinetic efficiency of the outflow is roughly 0.1 per cent, which is smaller than the outflow coupling efficiencies predicted by theoretical studies to AGN feedback become efficient in suppressing star formation in the host galaxy.

Modeling Nearby Low-Luminosity Active-Galactic-Nucleus Jet Images at All VLBI Scales

Relativistic jets from nearby low-luminosity active-galactic-nuclei (LLAGN) were observed by Very-Long Baseline Interferometry (VLBI) across many orders of magnitude in space, from milliparsec to sub-parsec scales, and from the jet base in the vicinity of black holes to the jet collimation and acceleration regions. With the improved resolution for VLBI observations, resolved VLBI jet morphologies provide valuable opportunities for testing and constraining black hole jet physics. In this review, we summarize and discuss the current progress of modeling nearby LLAGN jet images from horizon scales to large scales, including the construction of jet models and the assumed emission details. Illustrative examples for jet image modeling are also given to demonstrate how jet image features may vary with the underlying physics.

Lepto-hadronic Jet-disk Model for the Multiwavelength SED of M87

The low-luminosity active galactic nuclei M87, archetype of Fanaroff–Riley I radio galaxies, was observed in a historically quiet state in 2017. While one-zone leptonic jet models alone cannot explain the core radio-to-gamma-ray spectrum, we explore a hybrid jet-disk scenario. In this work, we model the overall spectral energy distribution of M87's core with a dominating one-zone lepto-hadronic jet component, coupled with the contribution from the accretion flow. We find close-to-equipartition parameter sets for which the jet component fits the radio-to-optical data as well as the gamma-ray band, while the accretion flow mainly contributes to the X-ray band. The effects of gamma-ray absorption by the extragalactic background light during the propagation toward Earth are probed and are found to be negligible for this model. The neutrino flux produced by such scenarios is also calculated, but remains below the current instruments’ sensitivity.

Spatially resolved properties of early-type group-dominant galaxies with MUSE: gas content, ionization mechanisms, and metallicity gradients

ABSTRACT With the goal of a thorough investigation of the ionized gas and its origin in early-type group-dominant galaxies, we present archival MUSE data for 18 galaxies from the Complete Local-Volume Groups Sample (CLoGS). These data allowed us to study the spatially resolved warm gas properties, including the morphology of the ionized gas, EW(H α), and kinematics as well as the gas-phase metallicity (12 + log(O/H)) of these systems. In order to distinguish between different ionization mechanisms, we used the emission-line ratios [O iii]/H β and [N ii]/H α in the BPT diagrams and EW(H α). We find that the ionization sources in our sample have variable impacts at different radii; central regions are more influenced by low-luminosity active galactic nuclei, while extended regions of low-ionization nuclear emission-line region-like emission are ionized by other mechanisms with post-asymptotic giant branch stars photoionization likely contributing significantly. We classified our sample into three H α +[N ii] emission morphology types. We calculate the gas-phase metallicity assuming several methods and ionization sources. In general, 12 + log(O/H) decreases with radius from the centre for all galaxies, independently of nebular morphology type, indicating a metallicity gradient in the abundance profiles. Interestingly, the more extended filamentary structures and all extranuclear star-forming regions present shallow metallicity gradients. Within the uncertainties these extended structures can be considered chemically homogeneous. We suggest that group-dominant galaxies in our sample likely acquired their cold gas in the past as a consequence of one or more mechanisms, e.g. gas-clouds or satellite mergers/accretion and/or cooling flows that contribute to the growth of the ionized gas structures.

On Planet Formation around Supermassive Black Holes and Grain Disruption Barriers by Radiative Torques

It has recently been suggested that planets can form by dust coagulation in the tori of active galactic nuclei (AGN) with low luminosities of L bol ≲ 1042 erg s−1, constituting a new class of exoplanets orbiting supermassive black holes called blanets. However, large dust grains in the AGN torus may be rotationally disrupted by the radiative torque disruption (RATD) mechanism due to AGN radiation feedback, which would prevent blanet formation. To test this scenario, we adopt a simple smooth and a clumpy dust/gas distribution inside the torus region to study the effect of RATD on the evolution of composite dust grains in the midplane of the torus. We found that grain growth and then blanet formation are possible in the smooth torus model. However, in the clumpy torus model, grain growth will be strongly constrained by RATD, assuming the gas density distribution as adopted by Wada et al. We also found that icy grain mantles inside clumps are quickly detached from the grain cores by rotational desorption, reducing the sticking coefficient between icy grains and the coagulation efficiency. The grain rotational disruption and ice desorption occur on timescales much shorter than the growth time up to a factor of ∼104, which are the new barriers that grain growth must overcome to form blanets. Further studies with more realistic AGN models are required to constrain better the effect of RATD on grain growth and blanet formation hypothesis around low-luminosity AGN.

NGC 3147: a prototypical low-luminosity active galactic nucleus with double-peaked optical and ultraviolet lines

ABSTRACT A previous narrow-slit (0.1 arcsec) Hubble Space Telescope observation unveiled a broad relativistic Hα profile in NGC 3147, a low-luminosity (Lbol ∼ 1042 erg s−1), low-Eddington ratio (Lbol/LEdd ∼ 10−4) active galactic nucleus (AGN), formerly believed to be a candidate true type 2 AGN intrinsically lacking the broad-line region. The new observations presented here confirm the double-peaked profile of the Hα line, which further shows variability both in flux and in the inner radius of the emitting disc with respect to the previous epoch. Similar disc line profiles are also found in prominent ultraviolet (UV) lines, in particular Lyα and C iv. The new data also allow us to build a simultaneous subarcsec optical-to-X-ray spectral energy distribution of NGC 3147, which is characterized by the absence of a thermal UV bump, and an emission peak in the X-rays. The resulting very flat αox = −0.82 is typical of low-luminosity AGNs, and is in good agreement with the extrapolation to low luminosities of the well-known trend with luminosity observed in a standard AGN. Indeed, we are possibly observing the accretion disc emission in NGC 3147 in the optical, close to the expected peak. On the other hand, the steep −2 UV power law may be Comptonization of that cold disc by a warm corona, what is instead generally observed as a ‘soft excess’ in a more luminous AGN.

Active galactic nuclei signatures in Red Geyser galaxies from Gemini GMOS-IFU observations

ABSTRACT Red Geysers are quiescent galaxies with galactic scale ionized outflows, likely due to low-luminosity active galactic nuclei (AGN). We used Gemini GMOS-IFU observations of the inner ∼1–3 kpc of nine Red Geysers selected from the MaNGA survey to study the gas ionization and kinematics. The emission-line ratios suggest the presence of Seyfert/LINER (Low Ionization Nuclear Emission Region) nuclei in all sources. Two galaxies show Hα equivalent width ( Hα EW) larger than 3 Å (indicative of AGN ionization) within an aperture 2 ${_{.}^{\prime\prime}}$5 of diameter (1.3–3.7 kpc at the distance of galaxies) for MaNGA data, while with the higher resolution GMOS data, four galaxies present Hα EW&amp;gt;3 Å within an aperture equal to the angular resolution (0.3–0.9 kpc). For two objects with GMOS-IFU data, the Hα EW is lower than 3 Å but larger than 1.5 Å, most probably due to a faint AGN. The spatially resolved electron density maps show values between 100 and 3000 cm −3 and are consistent with those determined in other studies. The large (MaNGA) and the nuclear scale (GMOS-IFU) gas velocity fields are misaligned, with a kinematic position angle difference between 12° and 60°. The [N ii] λ6583 emission-line profiles are asymmetrical, with blue wings on the redshifted side of the velocity field and red wings on the blueshifted side. Our results support previous indications that the gas in Red Geysers is ionized by an AGN, at least in their central region, with the presence of outflows, likely originating in a precessing accretion disc.

Unexplored outflows in nearby low luminosity AGNs

Context. Multi-phase outflows play a central role in galaxy evolution shaping the properties of galaxies. Understanding outflows and their effects in low luminosity active galactic nuclei (AGNs), such as low ionisation nuclear emission line regions (LINERs), is essential. LINERs bridge the gap between normal and active galaxies, being the most numerous AGN population in the local Universe. Aims. Our goal is to analyse the kinematics and ionisation mechanisms of the multi-phase gas of NGC 1052, the prototypical LINER, in order to detect and map the ionised and neutral phases of the putative outflow. Methods. We obtained Very Large Telescope MUSE and Gran Telescopio Canarias MEGARA optical integral field spectroscopy data for NGC 1052. In addition to stellar kinematics maps, by modelling spectral lines with multiple Gaussian components, we obtained flux, kinematic, and excitation maps of both ionised and neutral gas. Results. The stars are distributed in a dynamically hot disc (V/σ ∼ 1.2), with a centrally peaked velocity dispersion map (σc = 201 ± 10 km s−1) and large observed velocity amplitudes (ΔV = 167 ± 19 km s−1). The ionised gas, probed by the primary component is detected up to ∼30″ (∼3.3 kpc) mostly in the polar direction with blue and red velocities (∣V∣ &lt; 250 km s−1). The velocity dispersion map shows a notable enhancement (σ &gt; 90 km s−1) crossing the galaxy along the major axis of rotation in the central 10″. The secondary component has a bipolar morphology, velocity dispersion larger than 150 km s−1, and velocities up to 660 km s−1. A third component is detected with MUSE (and barely with MEGARA), but it is not spatially resolved. The broad-line region (BLR) component (used to model the broad Hα emission only) has a full width at half maximum of 2427 ± 332 and 2350 ± 470 km s−1 for MUSE and MEGARA data, respectively. The maps of the NaD absorption indicate optically thick neutral gas with complex kinematics. The velocity field is consistent with a slow rotating disc (ΔV = 77 ± 12 km s−1), but the velocity dispersion map is off-centred without any counterpart in the (centrally peaked) flux map. Conclusions. We found evidence of an ionised gas outflow (secondary component) with a mass of 1.6 ± 0.6 × 105 M⊙, and mass rate of 0.4 ± 0.2 M⊙ yr−1. The outflow is propagating in a cocoon of gas with enhanced turbulence and might be triggering the onset of kiloparsec-scale buoyant bubbles (polar emission), both probed by the primary component. Taking into account the energy and kinetic power of the outflow (1.3 ± 0.9 × 1053 erg and 8.8 ± 3.5 × 1040 erg s−1, respectively) as well as its alignment with both the jet and the cocoon, and that the gas is collisionally ionised (due to gas compression), we consider that the most likely power source of the outflow is the jet, although some contribution from the AGN is possible. The hints of the presence of a neutral gas outflow are weak.

Are Active Galactic Nuclei in Post-starburst Galaxies Driving the Change or Along for the Ride?

We present an analysis of 10 ks snapshot Chandra observations of 12 shocked post-starburst galaxies, which provide a window into the unresolved question of active galactic nuclei (AGN) activity in post-starburst galaxies and its role in the transition of galaxies from active star formation to quiescence. While seven of the 12 galaxies have statistically significant detections (with two more marginal detections), the brightest only obtained 10 photons. Given the wide variety of hardness ratios in this sample, we chose to pursue a forward-modeling approach to constrain the intrinsic luminosity and obscuration of these galaxies, rather than stacking. We constrain the intrinsic luminosity of obscured power laws based on the total number of counts and spectral shape, itself mostly set by the obscuration, with hardness ratios consistent with the data. We also tested thermal models. While all the galaxies have power-law models consistent with their observations, a third of the galaxies are better fit as an obscured power law and another third are better fit as thermal emission. If these post-starburst galaxies, early in their transition, contain AGNs, then these are mostly confined to lower obscuration (N H ≤ 1023 cm−2) and lower luminosity (L 2−10 keV ≤ 1042 erg s−1). Two galaxies, however, are clearly best fit as significantly obscured AGNs. At least half of this sample shows evidence of at least low-luminosity AGN activity, though none could radiatively drive out the remaining molecular gas reservoirs. Therefore, these AGNs are more likely along for the ride, having been fed gas by the same processes driving the transition.

A Resolved Search for AGN in the Centers of Nearby Galaxies with WISE

We investigate the mid-infrared detection of active galactic nucleus (AGN) in nearby galaxies and the effects of contamination by starlight and dust emission from the host galaxy. We used a sample of ∼14,000 nearby galaxies from the z = 0 Multiwavelength Galaxy Synthesis to study the effect of isolating the galaxy center on AGN detection. We identify 39 known AGN and 9 additional previously selected AGN candidates using color selection in WISE Bands 1 and 2. Most AGN identified using the smallest apertures around the nucleus are lost from the WISE color selection at larger aperture radii. This suggests that when low spatial resolution or integrated galaxy measurements are used to identify AGN in the mid-IR, lower luminosity AGN may go undetected due to contamination from the galaxy’s dust emission and starlight.

Improved Model of X-Ray Emission from Hot Accretion Flows

We have developed an improved model of X-ray emission from optically thin, two-temperature accretion flows, kerrflow, using an exact Monte Carlo treatment of global Comptonization as well as with a fully general relativistic description of both the radiative and hydrodynamic processes. It also includes pion-decay electrons, whose synchrotron emission dominates the seed photon yield at in flows around supermassive black holes. We consider in detail the dependence of the model spectra on the black hole spin, the electron heating efficiency, the plasma magnetization, and the accretion rate, and we discuss the feasibility of constraining these parameters by analyzing X-ray spectra of nearby low-luminosity active galactic nuclei. We note some degeneracies that hinder precise estimations of these parameters when individual X-ray spectra are analyzed. These degeneracies are eliminated when several spectra from a given source are fitted jointly, which then allows us to reliably measure the model parameters. We find significant differences with previous spectral models of hot-flow emission, related with the computational methods for Comptonization. Finally, we briefly consider and discuss the dependence on the viscosity parameter and on the outflow strength.

Variability Selected Active Galactic Nuclei from ASAS-SN Survey: Constraining the Low Luminosity AGN Population

Low luminosity active galactic nuclei (LLAGN) probe accretion physics in the low Eddington regime can provide additional clues about galaxy evolution. AGN variability is ubiquitous and thus provides a reliable tool for finding AGN. We analyze the All-Sky Automated Survey for SuperNovae light curves of 1218 galaxies with g < 14 mag and Sloan Digital Sky Survey spectra in search of AGN. We find 37 objects that are both variable and have AGN-like structure functions, which is about 3% of the sample. The majority of the variability selected AGN are LLAGN with Eddington ratios ranging from 10−4 to 10−2. We thus estimate the fraction of LLAGN in the population of galaxies as 2% down to a median Eddington ratio of 2 × 10−3. Combining the BPT line ratio AGN diagnostics and the broad-line AGN, up to ∼60% of the AGN candidates are confirmed spectroscopically. The BPT diagnostics also classified 10%–30% of the candidates as star-forming galaxies rather than AGN.

COSMOS2020: Ubiquitous AGN Activity of Massive Quiescent Galaxies at 0 &lt; z &lt; 5 Revealed by X-Ray and Radio Stacking

Abstract We characterize the average X-ray and radio properties of quiescent galaxies (QGs) with log ( M ⋆ / M ⊙ ) &gt; 10 at 0 &lt; z &lt; 5. QGs are photometrically selected from the latest COSMOS2020 catalog. We conduct the stacking analysis of X-ray images of the Chandra COSMOS Legacy Survey for individually undetected QGs. Thanks to the large sample and deep images, the stacked X-ray signal is significantly detected up to z ∼ 5. The average X-ray luminosity cannot be explained by the X-ray luminosity of X-ray binaries, suggesting that the low-luminosity active galactic nuclei (AGNs) ubiquitously exist in QGs. Moreover, the X-ray AGN luminosity of QGs at z &gt; 1.5 is higher than that of star-forming galaxies (SFGs), derived in the same manner as QGs. The stacking analysis of the VLA-COSMOS images is conducted for the identical sample, and the radio signal for QGs is also detected up to z ∼ 5. We find that the radio AGN luminosity of QGs at z &gt; 1.5 is also higher than SFGs, which is in good agreement with the X-ray analysis. The enhanced AGN activity in QGs suggested by the individual analysis in the X-ray and radio wavelength supports its important role for quenching at high redshift. Their enhanced AGN activity is less obvious at z &lt; 1.5, which can be interpreted as an increasing role of others at lower redshifts, such as environmental quenching.