Low-luminosity Active Galactic Nuclei Research Articles

Observational constraints on the stellar recycled gas in active galactic nuclei feeding

Abstract Near-infrared long-slit spectroscopy has been used to study the stellar population (SP) of the low luminosity active galactic nuclei (AGN) and matched analogues (LLAMA) sample. To perform the SP fits we have employed the X-shooter simple stellar population models together with the starlight code. Our main conclusions are: The star formation history of the AGNs is very complex, presenting many episodes of star formation during their lifetimes. In general, AGN hosts have higher fractions of intermediate-age SP (light-weighted mean ages, <t > L ≲ 4.5 Gyr) when compared with their analogues (<t > L ≲ 8.0 Gyr). AGN are more affected by reddening and require significant fractions of featureless continuum and hot dust components. The ratio between the AGN radiated energy and the gravitational potential energy of the molecular gas (ERad/EPG) for the AGN is compared with the <t > L and a possible anti-correlation is observed. This suggests that the AGN is affecting the star formation in these galaxies, in the sense that more energetic AGN (log(ERad/EPG) ≳ 3) tend to host nuclear younger SP (<t > L ≲4Gyr). We found that the recent (t <2 Gyr) returned (recycled) stellar mass is higher in AGN than in the controls. We also provide evidence that the mass loss of stars would be enough to feed the AGN, thus providing observational constraints for models that predict that AGN feeding is partially due to the recycled gas from dying stars.

Exploring Low-mass Black Holes through Tidal Disruption Events in the Early Universe: Perspectives in the Era of the JWST, Roman Space Telescope, and LSST Surveys

The James Webb Space Telescope (JWST) has recently uncovered the presence of low-luminosity active galactic nuclei (AGNs) at z = 4–11. Spectroscopic observations have provided estimates of the nuclear black hole (BH) masses for these sources, extending the low-mass boundary down to M • ∼ 106–7 M ⊙. Despite this breakthrough, the observed lowest mass of BHs is still ≳1–2 orders of magnitude heavier than the predicted mass range of their seed population, thereby leaving the initial mass distribution of massive BHs poorly constrained. In this paper, we focus on UV-to-optical (in the rest frame) flares of stellar tidal disruption events (TDEs) embedded in low-luminosity AGNs as a tool for exploring low-mass BH populations with ≲104–6 M ⊙. We provide an estimate of the TDE rate over z = 4–11, associated with the properties of JWST-detected AGN host galaxies, and we find that deep and wide survey programs with JWST and the Roman Space Telescope (RST) can detect and identify TDEs up to z ≃ 4–7. The predicted detection numbers of TDEs at z > 4 in 1 yr are NTDE∼2–10(0.2–2) for the JADES-Medium (and COSMOS-Web) survey with JWST and NTDE∼2–10(8–50) for the deep (and wide) tiers of the High Latitude Time Domain Survey with RST. We further discuss survey strategies for hunting for transient high-redshift TDEs in wide-field surveys with RST, as well as a joint observation campaign with the Vera C. Rubin Observatory for enhancing the detection number. The high-redshift TDE search will give us a unique opportunity to probe the mass distribution of early BH populations.

ReveaLLAGN 0: First Look at JWST MIRI Data of Sombrero and NGC 1052

We present the first results from the Revealing Low-Luminosity Active Galactic Nuclei (ReveaLLAGN) survey, a JWST survey of seven nearby LLAGNs. We focus on two observations with the Mid-Infrared Instrument (MIRI)’s Medium-Resolution Spectrometer of the nuclei of NGC 1052 and Sombrero (NGC 4594/M104). We also compare these data to public JWST data of higher-luminosity AGNs, NGC 7319 and NGC 7469. JWST clearly separates the AGN spectrum from the galaxy light even in Sombrero, the faintest target in our survey; the AGN components have very red spectra. We find that the emission-line widths in both NGC 1052 and Sombrero increase with increasing ionization potential, with FWHM > 1000 km s−1 for lines with ionization potential ≳ 50 eV. These lines are also significantly blueshifted in both LLAGNs. The high-ionization-potential lines in NGC 7319 show neither broad widths nor significant blueshifts. Many of the lower-ionization-potential emission lines in Sombrero show significant blue wings extending >1000 km s−1. These features and the emission-line maps in both galaxies are consistent with outflows along the jet direction. Sombrero has the lowest-luminosity high-ionization-potential lines ([Ne v] and [O iv]) ever measured in the mid-infrared, but the relative strengths of these lines are consistent with higher-luminosity AGNs. On the other hand, the [Ne v] emission is much weaker relative to the [Ne iii] and [Ne ii] lines of higher-luminosity AGNs. These initial results show the great promise that JWST holds for identifying and studying the physical nature of LLAGNs.

Tracing the rise of supermassive black holes

We report the identification of 64 new candidates of compact galaxies, potentially hosting faint quasars with bolometric luminosities of Lbol = 1043–1046 erg s−1, residing in the reionization epoch within the redshift range of 6 ≲ z ≲ 8. These candidates were selected by harnessing the rich multiband datasets provided by the emerging JWST-driven extragalactic surveys, focusing on COSMOS-Web, as well as JADES, UNCOVER, CEERS, and PRIMER. Our search strategy includes two stages: applying stringent photometric cuts to catalog-level data and detailed spectral energy distribution fitting. These techniques effectively isolate the quasar candidates while mitigating contamination from low-redshift interlopers, such as brown dwarfs and nearby galaxies. The selected candidates indicate physical traits compatible with low-luminosity active galactic nuclei, likely hosting ≈105–107 M⊙ supermassive black holes (SMBHs) living in galaxies with stellar masses of ≈108–1010 M⊙. The SMBHs selected in this study, on average, exhibit an elevated mass compared to their hosts, with the mass ratio distribution slightly higher than those of galaxies in the local Universe. As with other high-z studies, this is at least in part due to the selection method for these quasars. An extensive Monte Carlo analysis provides compelling evidence that heavy black hole seeds from the direct collapse scenario appear to be the preferred pathway to mature this specific subset of SMBHs by z ≈ 7. Notably, most of the selected candidates might have emerged from seeds with masses of ∼105 M⊙, assuming a thin disk accretion with an average Eddington ratio of fEdd = 0.6 ± 0.3 and a radiative efficiency of ϵ = 0.2 ± 0.1. This work underscores the significance of further spectroscopic observations, as the quasar candidates presented here offer exceptional opportunities to delve into the nature of the earliest galaxies and SMBHs that formed during cosmic infancy.

Magnetic flux eruptions at the root of time lags in low-luminosity active galactic nuclei

Sagittarius A$^ is a compact radio source at the center of the Milky Way that has not conclusively shown evidence to support the presence of a relativistic jet. Nevertheless, indirect methods at radio frequencies do indicate consistent outflow signatures. Temporal shifts between features in frequency bands are known as time lags, associated with flares or outflows of the accretion system. It is possible to gain information on the emission and outflow mechanics by interpreting these time lags. By means of a combined general-relativistic magnetrohydrodynamical and radiative transfer modeling approach, we studied the origin of the time lags for magnetically arrested disk models with three black hole spins ($a_ $). We exclusively modeled the emission from the source across a frequency range of $ GHz. Our study also includes a targeted ``slow light'' investigation for one of the best-fitting ``fast light'' windows. We were able to recover the observational time-lag relations in various windows of our simulated light curves. The theoretical interpretation of these most promising time-lag windows is threefold: i) a magnetic flux eruption perturbs the jet-disk boundary and creates a flux tube; ii) the flux tube orbits and creates a clear emission feature; and iii) the flux tube interacts with the jet-disk boundary. The best-fitting windows have an intermediate (i=30$^ inclination and zero black hole spin. The targeted slow light study did not produce better-fitting time lag results, which indicates that the fast versus slow light paradigm is often not intuitively understood and is likely to be influential in timing-sensitive black hole accretion studies. While previous studies have sought to interpret time-lag properties with spherical or jetted expansion models, we show that this picture is too simplistic. Sophisticated general-relativistic magnetrohydrodynamical models consistently capture the observational time-lag behavior, which is rooted in the complex dynamic interplay between the flux tube and coupled disk-jet system.

Investigating Model Dependencies for Obscured Active Galactic Nuclei: A Case Study of NGC 3982

Abstract X-ray spectroscopy of heavily obscured active galactic nuclei (AGN) offers a unique opportunity to study the circumnuclear environment of accreting supermassive black holes. However, individual models describing the obscurer have unique parameter spaces that give distinct parameter posterior distributions when fit to the same data. To assess the impact of model-specific parameter dependencies, we present a case study of the nearby heavily obscured low-luminosity AGN NGC 3982, which has a variety of column density estimations reported in the literature. We fit the same broadband XMM-Newton+NuSTAR spectra of the source with five unique obscuration models and generate posterior parameter distributions for each. By using global parameter exploration, we traverse the full prior-defined parameter space to accurately reproduce complex posterior shapes and inter-parameter degeneracies. The unique model posteriors for the line-of-sight column density are broadly consistent, predicting Compton-thick N H > 1.5 × 1024 cm−2 at the 3σ confidence level. The posterior median intrinsic X-ray luminosity in the 2–10 keV band was found to differ substantially, however, with values in the range log L 2–10 keV/ erg s−1 = 40.9–42.1 for the individual models. We additionally show that the posterior distributions for each model occupy unique regions of their respective multidimensional parameter spaces and how such differences can propagate into the inferred properties of the central engine. We conclude by showcasing the improvement in parameter inference attainable with the High Energy X-ray Probe, with its uniquely broad, simultaneous, and high-sensitivity bandpass of 0.2–80 keV.

The extent and power of ‘maintenance mode’ feedback in MaNGA AGN

ABSTRACT We study the ionized gas kinematics of 293 Active Galactic Nucleus (AGN) hosts as compared to that of 485 control galaxies from the MaNGA–SDSS survey using measurements of the [O iii]$\lambda$5007 Å emission-line profiles, presenting flux, velocity, and W$_{80}$ maps. In 45 per cent of the AGN, a broad component was needed to fit the line profiles wings within the inner few kpc, that we have identified with an outflow. But in most AGN, the profiles are broader than that of their controls over a much more extended region, identified as the ‘kinematically disturbed regions’ (KDRs). We find a positive correlation between the mean $\langle$W$_{80}\rangle$ and L[O iii], supporting that the KDR is due to heating and turbulence of the ISM by outflows and radiation from the AGN. The extent R$_{KDR}$ reaches up to 24 kpc, with a mean ratio to that of the ENLR of 57 per cent. We estimate ionized gas mass flow rates ($\dot{M}_{\rm out}$) and kinetic powers ($\dot{E}_{\rm out}$) both from the AGN broad components and from the W$_{80}$ values, that can be obtained for the whole AGN sample. We find values for $\dot{M}_{\rm out}$ and $\dot{E}_{\rm out}$ that correlate with the AGN luminosity $L_{\mathrm{ bol}}$, populating the low-luminosity end of these known correlations. The mean coupling efficiency between $\dot{E}_{\rm out}$ and AGN luminosity is $\approx$0.02 per cent from the W$_{80}$ values and lower from the broad component. But the large extent of the KDR shows that even low-luminosity AGN can impact the host galaxy along several kpc in a “maintenance mode” feedback.

The Impact of an Active Galactic Nucleus on Polycyclic Aromatic Hydrocarbon Emission in Galaxies: The Case of Ring Galaxy NGC 4138

We present a focused study of radially resolved varying PAH emission in the low-luminosity active galactic nucleus (AGN)-host NGC 4138 using deep Spitzer/infrared spectrograph spectral maps. Using new model PAH spectra, we investigate whether these variations could be associated with changes to the PAH grain size distribution due to photodestruction by the AGN. Separately, we model the effects of the varying radiation field within NGC 4138, and we use this model to predict the corresponding changes in the PAH emission spectrum. We find that PAH band ratios are strongly variable with radius in this galaxy with short-to-long wavelength band ratios peaking in the starburst ring. The changing mix of starlight appears to have a considerable effect on the trends in these band ratios, and our radiation model predicts the shapes of these trends. However, the amplitude of observed variation is ∼2.5 × larger than predicted for some ratios. A cutoff of small grains in the PAH size distribution, as has been suggested for AGN, together with changes in PAH ionization fraction could explain the behavior of the shorter bands, but this model fails to reproduce longer band behaviors. Additionally, we find that short-to-long wavelength PAH band ratios increase slightly within ∼270 pc of the center, suggesting that the AGN may directly influence PAH emission there.

Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks

The conventional accretion disk lore is that magnetized turbulence is the principal angular momentum transport process that drives accretion. However, when dynamically important large-scale magnetic fields thread an accretion disk, they can produce mass and angular momentum outflows, known as winds, that also drive accretion. Yet, the relative importance of turbulent and wind-driven angular momentum transport is still poorly understood. To probe this question, we analyze a long-duration (1.2 × 105 r g /c) simulation of a rapidly rotating (a = 0.9) black hole feeding from a thick (H/r ∼ 0.3), adiabatic, magnetically arrested disk (MAD), whose dynamically important magnetic field regulates mass inflow and drives both uncollimated and collimated outflows (i.e., winds and jets, respectively). By carefully disentangling the various angular momentum transport processes within the system, we demonstrate the novel result that disk winds and disk turbulence both extract roughly equal amounts of angular momentum from the disk. We find cumulative angular momentum and mass accretion outflow rates of L̇∝r0.9 and Ṁ∝r0.4 , respectively. This result suggests that understanding both turbulent and laminar stresses is key to understanding the evolution of systems where geometrically thick MADs can occur, such as the hard state of X-ray binaries, low-luminosity active galactic nuclei, some tidal disruption events, and possibly gamma-ray bursts.

A case for a binary black hole system revealed via quasi-periodic outflows.

Binaries containing a compact object orbiting a supermassive black hole are thought to be precursors of gravitational wave events, but their identification has been extremely challenging. Here, we report quasi-periodic variability in x-ray absorption, which we interpret as quasi-periodic outflows (QPOuts) from a previously low-luminosity active galactic nucleus after an outburst, likely caused by a stellar tidal disruption. We rule out several models based on observed properties and instead show using general relativistic magnetohydrodynamic simulations that QPOuts, separated by roughly 8.3 days, can be explained with an intermediate-mass black hole secondary on a mildly eccentric orbit at a mean distance of about 100 gravitational radii from the primary. Our work suggests that QPOuts could be a new way to identify intermediate/extreme-mass ratio binary candidates.

New insights into the role of AGNs in forming the cluster red sequence

ABSTRACT As a considerable investment of time from various telescope facilities was dedicated toward studying the Spiderweb protocluster at z = 2.2, it so far remains one of the most extensively studied protocluster. We report here the latest results in this field, adding a new dimension to previous research on cluster formation at high redshift. Previous studies have reported a significant overdensity (δ ∼ 10) of massive Hα (+ [N ii])-emitting galaxies in 3700 comoving Mpc3. Many of these were previously considered to be dusty, actively star-forming galaxies, given their rest-frame optical and infrared features. However, this study argues that a third of them are more likely to be ‘passively evolving’ galaxies with low-luminosity active galactic nuclei (AGNs) rather than star-forming galaxies, given the multiwavelength spectral energy distribution (SED) fitting including an AGN component. For their SED-based star formation rates to be valid, bulk of their Hα + [N ii] emission should come from the central AGNs. This difference in interpretation between this work and past studies, including ours, is particularly supported by the recent deep Chandra/X-ray observation. Furthermore, we have spectroscopically confirmed a quiescent nature for one of these AGNs, with its multiple stellar absorption lines but also low-ionization emission lines. This important update provides new insights into the role of AGNs in forming the cluster red sequence observed in the present-day universe.

Revealing the kinematic puzzle of the AGN host NGC 3884: optical integral field spectroscopy unravels stellar and gas motions

ABSTRACT We used optical integral field spectroscopy to analyse the stellar and gas properties of the inner 1.4 kpc radius of NGC 3884, a low-luminosity active galactic nucleus (AGN) host. The observations were performed with Gemini Multi-Object Spectrograph (GMOS)-Integral Field Unit at a seeing of ∼0.85 arcsec (475 pc at the galaxy) that allowed us to map the stellar and gas emission structure and kinematics, for the first time in this galaxy. The stellar motions are consistent with rotation in a disc, with the kinematic position angle (PA) ranging from approximately 0° within 500 pc to 20° beyond 1 kpc, consistent with the photometric PA. We detected extended ionized and neutral gas emission throughout most of the GMOS field of view, with three kinematic components: (i) a disc component with a kinematic PA similar to that of the stars beyond ∼670 pc from the nucleus; (ii) a twist in the PA of up to 60° at a smaller radii that we attribute to gas inflow towards the nucleus; and (iii) an outflow detected as broad components to the emission lines (σ ∼ 250–400 km s−1), with a maximum mass outflow rate of 0.25 ± 0.15 M⊙ yr−1 and a kinetic power corresponding to 0.06 per cent of the AGN bolometric luminosity, possibly being powerful enough to suppress star formation in the galaxy. The observed gas kinematics thus reveals both inflows and outflows in ionized gas.

The Sloan Digital Sky Survey Reverberation Mapping Project: Investigation of Continuum Lag Dependence on Broad-line Contamination and Quasar Properties

This work studies the relationship between accretion-disk size and quasar properties, using a sample of 95 quasars from the Sloan Digital Sky Survey Reverberation Mapping Project with measured lags between the g and i photometric bands. Our sample includes disk lags that are both longer and shorter than predicted by the Shakura and Sunyaev model, requiring explanations that satisfy both cases. Although our quasars each have one lag measurement, we explore the wavelength-dependent effects of diffuse broad-line region (BLR) contamination through our sample’s broad redshift range, 0.1 < z < 1.2. We do not find significant evidence of variable diffuse Fe ii and Balmer nebular emission in the rms spectra, nor from Anderson–Darling tests of quasars in redshift ranges with and without diffuse nebular emission falling in the observed-frame filters. Contrary to previous work, we do not detect a significant correlation between the measured continuum and BLR lags in our luminous quasar sample, similarly suggesting that our continuum lags are not dominated by diffuse nebular emission. Similar to other studies, we find that quasars with larger-than-expected continuum lags have lower 3000 Å luminosities, and we additionally find longer continuum lags with lower X-ray luminosities and black hole masses. Our lack of evidence for diffuse BLR contribution to the lags indicates that the anticorrelation between continuum lag and luminosity is not likely to be due to the Baldwin effect. Instead, these anticorrelations favor models in which the continuum lag increases in lower-luminosity active galactic nuclei, including scenarios featuring magnetic coupling between the accretion disk and X-ray corona, and/or ripples or rims in the disk.

EMU/GAMA: A technique for detecting active galactic nuclei in low mass systems

Abstract We propose a new method for identifying active galactic nuclei (AGN) in low mass ( $\mathrm{M}_*\leq10^{10}\mathrm{M}_\odot$ ) galaxies. This method relies on spectral energy distribution (SED) fitting to identify galaxies whose radio flux density has an excess over that expected from star formation alone. Combining data in the Galaxy and Mass Assembly (GAMA) G23 region from GAMA, Evolutionary Map of the Universe (EMU) early science observations, and Wide-field Infrared Survey Explorer (WISE), we compare this technique with a selection of different AGN diagnostics to explore the similarities and differences in AGN classification. We find that diagnostics based on optical and near-infrared criteria (the standard BPT diagram, the WISE colour criterion, and the mass-excitation, or MEx diagram) tend to favour detection of AGN in high mass, high luminosity systems, while the “ProSpect” SED fitting tool can identify AGN efficiently in low mass systems. We investigate an explanation for this result in the context of proportionally lower mass black holes in lower mass galaxies compared to higher mass galaxies and differing proportions of emission from AGN and star formation dominating the light at optical and infrared wavelengths as a function of galaxy stellar mass. We conclude that SED-derived AGN classification is an efficient approach to identify low mass hosts with low radio luminosity AGN.

WISDOM Project - XVI. The link between circumnuclear molecular gas reservoirs and active galactic nucleus fuelling

ABSTRACT We use high-resolution data from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) project to investigate the connection between circumnuclear gas reservoirs and nuclear activity in a sample of nearby galaxies. Our sample spans a wide range of nuclear activity types including radio galaxies, Seyfert galaxies, low-luminosity active galactic nuclei (AGN) and inactive galaxies. We use measurements of nuclear millimetre continuum emission along with other archival tracers of AGN accretion/activity to investigate previous claims that at, circumnuclear scales (&amp;lt;100 pc), these should correlate with the mass of the cold molecular gas. We find that the molecular gas mass does not correlate with any tracer of nuclear activity. This suggests the level of nuclear activity cannot solely be regulated by the amount of cold gas around the supermassive black hole (SMBH). This indicates that AGN fuelling, that drives gas from the large-scale galaxy to the nuclear regions, is not a ubiquitous process and may vary between AGN type, with time-scale variations likely to be very important. By studying the structure of the central molecular gas reservoirs, we find our galaxies have a range of nuclear molecular gas concentrations. This could indicate that some of our galaxies may have had their circumnuclear regions impacted by AGN feedback, even though they currently have low nuclear activity. Alternatively, the nuclear molecular gas concentrations in our galaxies could instead be set by secular processes.

Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: The Low-luminosity Cases and an Application to Sgr A*

In this paper, we investigate the astrophysical processes of stellar-mass black holes (sMBHs) embedded in advection-dominated accretion flows (ADAFs) of supermassive black holes (SMBHs) in low-luminosity active galactic nuclei. The sMBH is undergoing Bondi accretion at a rate lower than the SMBH. Outflows from the sMBH-ADAF dynamically interact with their surroundings and form a cavity inside the SMBH-ADAF, thereby quenching the accretion onto the sMBH. Rejuvenation of the Bondi accretion is rapidly done by turbulence. These processes give rise to quasi-periodic episodes of sMBH activities and create flickerings from relativistic jets developed by the Blandford–Znajek mechanism if the sMBH is maximally rotating. Accumulating successive sMBH-outflows trigger a viscous instability of the SMBH-ADAF, leading to a flare following a series of flickerings. Recently, the similarity of near-infrared flare’s orbits has been found by GRAVITY/VLTI astrometric observations of Sgr A∗: their loci during the last 4 yr consist of a ring in agreement with the well-determined SMBH mass. We apply the present model to Sgr A*, which shows quasi-periodic flickerings. An sMBH of ∼40M ⊙ is preferred orbiting around the central SMBH of Sgr A* from fitting radio to X-ray continuum. Such an extreme mass ratio inspiraling provides an excellent laboratory for LISA/Taiji/Tianqin detection of mHz gravitational waves with strains of ∼10−17, as well as their polarization.

A fundamental plane of black hole accretion at millimetre wavelengths

ABSTRACT We report the discovery of the ‘mm fundamental plane of black hole accretion’, which is a tight correlation between the nuclear 1 mm luminosity (Lν, mm), the intrinsic 2–10 keV X-ray luminosity (LX, 2–10) and the supermassive black hole (SMBH) mass (MBH) with an intrinsic scatter (σint) of 0.40 dex. The plane is found for a sample of 48 nearby galaxies, most of which are low-luminosity active galactic nuclei. Combining these sources with a sample of high-luminosity (quasar-like) nearby AGN, we show that the plane still holds. We also find that MBH correlates with Lν, mm at a highly significant level, although such correlation is less tight than the mm fundamental plane (σint = 0.51 dex). Crucially, we show that spectral energy distribution (SED) models for both advection-dominated accretion flows (ADAFs) and compact jets can explain the existence of these relations, which are not reproduced by the standard torus-thin accretion disc models usually associated to quasar-like AGN. The ADAF models reproduces the observed relations somewhat better than those for compact jets, although neither provides a perfect fit. Our findings thus suggest that radiatively inefficient accretion processes such as those in ADAFs or compact (and thus possibly young) jets may play a key role in both low- and high-luminosity AGN. This mm fundamental plane also offers a new, rapid method to (indirectly) estimate SMBH masses.

CAHA/PPAK Integral-field Spectroscopic Observations of M81. II. Testing Photoionization Models in a Spatially Resolved LINER

The origin of the low-ionization nuclear emission-line region (LINER) prevalent in local galaxies and its relationship with supermassive black holes have been debated for decades. We perform a comprehensive evaluation of traditional photoionization models against the circumnuclear ionized gas in M81, for which recent CAHA/PPAK integral-field spectroscopic observations reveal a LINER characteristic out to a galactocentric radius of ∼1 kpc. Constructed with the photoionization code cloudy, the models have the novel aspect of their primary parameters being well constrained by extensive observations of a prototypical low-luminosity active galactic nucleus (LLAGN) and an old stellar bulge in M81. Additionally, these models incorporate a reasonably broad range of uncertain nebular properties. It is found that the integrated photoionization by the LLAGN and hot, low-mass stars distributed in the bulge can roughly reproduce the observed radial intensity distributions of the Hα, Hβ, and [N ii] lines, with the bulge stars dominating the ionizing flux at radii ≳200 pc. However, the models generally fail to reproduce a similarly declining profile of the [O iii] line or an accordingly flat profile of the [O iii]/Hβ ratio. This clearly points to a deficiency of ionizing photons in the outer regions despite an extended photoionization source. The discrepancy might be alleviated if much of the observed [O iii] line arose from a bulge-filling, low-density gas surrounding a denser, Hα-emitting disk, or by a higher AGN luminosity in the recent past. The case of M81 has important implications for the ionization mechanism of LINERs and low-ionization emission-line regions in general.

A versatile classification tool for galactic activity using optical and infrared colors

Context.The overwhelming majority of diagnostic tools for galactic activity are focused mainly on the classes of active galaxies. Passive or dormant galaxies are often excluded from these diagnostics, which usually employ emission-line features (e.g., forbidden emission lines). Thus, most of them focus on specific types of activity or only on one activity class, for example active galactic nucleus (AGN) galaxiesAims.In this work we used infrared and optical colors to build an all-inclusive galactic activity diagnostic tool that can discriminate between star-forming, AGN, low-ionization nuclear emission-line region, composite, and passive galaxies, and which can be used in local and low-redshift galaxies.Methods.We used the random forest algorithm to define a new activity diagnostic tool. As the ground truth for the training of the algorithm, we considered galaxies that have been classified based on their optical spectral lines. We explored classification criteria based on infrared colors from the first three WISE bands (bands 1, 2, and 3) supplemented with optical colors from theu, g, andrSDSS bands. From them, we sought the combination with the minimum number of colors that provides optimal results. Furthermore, to mitigate biases related to aperture effects, we introduced a new WISE photometric scheme that combines apertures of different sizes.Results.Using machine learning methods, we developed a diagnostic tool that accommodates both active and passive galaxies under one unified classification scheme using just three colors. We find that the combination ofW1-W2, W2-W3, and g-r colors offers a good performance, while the broad availability of these colors for a large number of galaxies ensures it can be applied to large galaxy samples. The overall accuracy is ~81%, and the achieved completeness for each class is ~81% for star-forming, ~56% for AGN, ~68% for LINER, ~65% for composite, and ~85% for passive galaxies.Conclusions.Our diagnostic represents a significant improvement over existing infrared diagnostics because it includes all types of active galaxies, as well as passive galaxies, extending their application to the local Universe. The inclusion of the optical colors improves its ability to identify low-luminosity AGN galaxies, which are generally confused with star-forming galaxies, and helps us identify cases of starbursts with extreme mid-infrared colors that mimic obscured AGN galaxies, a well-known problem for most infrared diagnostics.

Detection of a ∼0.1c Radio Knot in M81* Associated with a Moderate X-Ray Flare

Through very long baseline interferometry observations of one of the closest low-luminosity active galactic nuclei, M81*, at multiple frequencies of 8.8, 22, and 44 GHz, a bright discrete knot with an unusual low apparent speed ∼0.1c was detected. Combined with the contemporaneous monitoring of X-ray data at 2–10 keV, our data indicate that a moderate X-ray flare happened when the knot was launched from the core region. Three possible origins of the knot are proposed to explain our observational results. They are an episodic jet ejection, a low-speed shock wave, and a possible secondary black hole in a binary system. Future intensive multiwavelength monitoring can help to understand the discrete knot as well as the central black hole better.