Galactic Nuclei Research Articles

Multiwavelength investigations of PKS 2300−18: S-shaped radio quasar with precessing jets and double peaked broad emission line spectrum

Abstract S-shaped radio galaxy jets are prime sources for investigating the dynamic interplay between the central active galactic nucleus, the jets, and the ambient intergalactic medium. These sources are excellent candidates for studying jet precession, as their S-shaped inversion symmetry strongly indicates underlying precession. We present a multiwavelength analysis of the giant inversion-symmetric S-shaped radio galaxy PKS 2300−18, which spans 0.76 Mpc. The host is a quasar at a redshift of 0.128, displaying disturbed optical morphology due to an ongoing merger with a companion galaxy. We conducted a broadband radio spectral study using multifrequency data ranging from 183 MHz to 6 GHz, incorporating dedicated observations with the uGMRT and JVLA alongside archival radio data. Particle injection model was fitted to the spectra of different regions of the source to perform ageing analysis, which was supplemented with a kinematic jet precession model. The ageing analysis revealed a maximum plasma age of ∼ 40 Myr, while the jet precession model indicated a precession period of ∼ 12 Myr. ROSAT data revealed an X-ray halo of Mpc size, and from Chandra the AGN X-ray spectrum was modelled using thermal and power law components. The optical spectrum displaying double peaked broad emission lines was modelled, indicating complex broad line region kinematics at the centre with the possibility of a binary SMBH. We present the results of our multiwavelength analysis of the source, spanning scales from a few light days to a few Mpc, and discuss its potential evolutionary path.

Velocity-resolved Reverberation Mapping of Changing-look Active Galactic Nucleus NGC 4151 during Outburst Stage. II. Results of Four Seasons of Observation

We present the results of a four-year velocity-resolved reverberation mapping (RM) campaign of the changing-look active galactic nucleus (CL-AGN) NGC 4151 during its outburst phase. By measuring the time lags of the Hα, Hβ, Hγ, He i, and He ii emission lines, we confirm a stratified broad-line region (BLR) structure that aligns with predictions from photoionization models. Intriguingly, we observed an “anti-breathing” phenomenon, where the lags of broad emission lines decreased with increasing luminosity, contrary to the typical expectation. This anomaly may be attributed to the influence of the ultraviolet-optical lag or nonvirialized motions in the BLR gas. Velocity-resolved RM and ionization mapping analyses revealed rapid and significant changes in the BLR geometry and kinematics on timescales of less than a year, which cannot be interpreted by any single mechanism, such as an inhomogeneous BLR, variations in radiation pressure, or changes in the illuminated ionizing field. Additionally, the Hβ lags of NGC 4151 and other CL-AGNs agree with the radius–luminosity relationship established for AGNs with low accretion rates, implying that the CL phenomenon is more likely driven by intrinsic changes in the accretion rate rather than obscuration. These findings provide new insights into the complex internal processes of CL-AGNs and highlight the importance of long-term, multiline RM for understanding BLR structures, geometry, and kinematics.

Could the Interband Lag of Active Galactic Nucleus Vary Randomly?

The interband lags among the optical broad-band continua of active galactic nuclei (AGNs) have been intensively explored over the past decade. However, the nature of the lags remains under debate. Here, utilizing two distinct scenarios for AGN variability, i.e., the thermal fluctuation of accretion disk and the reprocessing of both the accretion disk and clouds in the broad line region, we show that, owing to the random nature of AGN variability, the interband lags of an individual AGN would vary from one campaign with a finite baseline to another. Specifically, the thermal fluctuation scenario implies larger variations in the lags than the reprocessing scenario. Moreover, the former predicts a positive correlation between the lag and variation amplitude, while the latter does not result in such a correlation. For both scenarios, averaging the lags of an individual AGN measured with repeated and nonoverlapping campaigns would give rise to a stable lag, which is larger for a longer baseline and gets to saturation for a sufficiently long baseline. However, obtaining the stable lag for an individual AGN is very time-consuming. Alternatively, it can be equivalently inferred by averaging the lags of a sample of AGNs with similar physical properties, and thus can be properly compared with predictions of AGN models. In addition, several new observational tests suggested by our simulations are discussed, as well as the role of the deep high-cadence surveys of the Wide Field Survey Telescope in enriching our knowledge of the lags.

Nuclear burning in an accretion flow around a stellar-mass black hole embedded within an AGN disc

ABSTRACT A stellar-mass black hole, embedded within the accretion disc of an active galactic nuclei (AGN), has the potential to accrete gas at a rate that can reach approximately ${\sim}10^9$ times the Eddington limit. This study explores the potential for nuclear burning in the rapidly accreting flow towards this black hole and studies how nucleosynthesis affects metal production. Using numerical methods, we have obtained the disc structure while considering nuclear burning and assessed the stability of the disc. In contrast to gas accretion onto the surface of a neutron star or white dwarf, the disc remains stable against the thermal and secular instabilities because advection cooling offsets the nuclear heating effects. The absence of a solid surface for a black hole prevents excessive mass accumulation in the inner disc region. Notably, nuclear fusion predominantly takes place in the inner disc region, resulting in substantial burning of $\rm ^{12}C$ and $\rm ^{3}He$, particularly for black holes around $M = 10\,{\rm M}_\odot$ with accretion rates exceeding approximately ${\sim}10^7$ times the Eddington rate. The ejection of carbon-depleted gas through outflows can lead to an increase in the mass ratio of oxygen or nitrogen to carbon, which may be reflected in observed line ratios such as N v/C iv and O iv/C iv. Consequently, these elevated spectral line ratios could be interpreted as indications of supersolar metallicity in the broad-line region.

The SDSS-V Black Hole Mapper Reverberation Mapping Project: A Kinematically Variable Broad-line Region and Consequences for the Masses of Luminous Quasars

We present a velocity-resolved reverberation mapping analysis of the hypervariable quasar RM160 (SDSS J141041.25+531849.0) at z = 0.359 with 153 spectroscopic epochs of data representing a 10 yr baseline (2013–2023). We split the baseline into two regimes based on the 3× flux increase in the light curve: a “low state” phase during the years 2013–2019 and a “high state” phase during the years 2022–2023. The velocity-resolved lag profiles (VRLPs) indicate that gas with different kinematics dominates the line emission in different states. The Hβ VRLP begins with a signature of inflow onto the broad-line region (BLR) in the low state, while in the high state it is flatter with less signature of inflow. The Hα VRLP begins consistent with a virialized BLR in the low state, while in the high state shows a signature of inflow. The differences in the kinematics between the Balmer lines and between the low state and the high state suggests complex BLR dynamics. We find that the BLR radius and velocity (both FWHM and σ) do not obey a constant virial product throughout the monitoring period. We find that the BLR lags and continuum luminosity are correlated, consistent with rapid response of the BLR gas to the illuminating continuum. The BLR kinematic profile changes in unpredictable ways that are not related to continuum changes and reverberation lag. Our observations indicate that nonvirial kinematics can significantly contribute to observed line profiles, suggesting caution for black hole mass estimation in luminous and highly varying quasars like RM160.

Revisiting the Hβ Size–Luminosity Relation Using a Uniform Reverberation-mapping Analysis

We revisit the relation between active galactic nucleus (AGN) broad-line region (BLR) size and luminosity by conducting a uniform Hβ reverberation-mapping analysis for 212 AGNs with archival light curves. Our analysis incorporates three different lag measurement methods, including the interpolated cross-correlation function (ICCF), JAVELIN, and PyROA, alongside a consistently defined lag-searching window and an alias removal procedure. We find that ICCF, albeit with larger uncertainties compared to other methods, is the most reliable method based on our visual inspection of the matches between Hβ and the shifted continuum light curves. Combining this sample with the 32 AGNs from Seoul National University AGN Monitoring Project, we obtain the best-fit relation between the BLR size (R BLR) and the continuum luminosity at 5100 Å (L 5100) with a slope significantly flatter than 0.5. By selecting a subsample of 157 AGNs with the best-quality lag measurements using a set of quantitative criteria and visual inspection, we find a consistent slope and a slightly decreased intrinsic scatter. We further investigate the effect of luminosity tracers, including L 5100, Hβ luminosity (L Hβ ), [O iii] luminosity (L [O III]), and 2–10 keV hard X-ray luminosity L 2–10keV. We find that sub-Eddington and super-Eddington AGNs exhibit systematic offsets in both R BLR–L 5100 and R BLR–L Hβ relations, while they show comparable R BLR–L [O III] and R BLR–L 2–10keV relations. We discuss the potential causes for these different deviations when employing different luminosity tracers.

AGN STORM 2. X. The Origin of the Interband Continuum Delays in Mrk 817* *Based in part on observations associated with program GO-16196 made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

The local (z = 0.0315) active galactic nucleus (AGN) Mrk 817 was monitored over more than 500 days with space-borne and ground-based instruments as part of a large international campaign, AGN STORM 2. Here, we present a comprehensive analysis of the broadband continuum variations using detailed modeling of the broad line region (BLR), several types of disk winds classified by their optical depth, and new numerical simulations. We find that diffuse continuum (DC) emission, with additional contributions from strong and broad emission lines, can explain the continuum lags observed in this source during high- and low-luminosity phases. Disk illumination by the variable X-ray corona contributes only a small fraction of the observed continuum lags. Our BLR models assume radiation-pressure-confined clouds distributed over a distance of 2–122 light days. We present calculated mean emissivity radii of many emission lines, and DC emission, and suggest a simple, transfer-function-dependent method that ties them to cross-correlation lag determinations. We do not find clear indications for large-optical-depth winds, but identify the signature of lower-column-density winds. In particular, we associate the shortest observed continuum lags with a combination of τ(1 Ryd) ≈ 2 wind and a partly shielded BLR. Even smaller optical depth winds may be associated with X-ray absorption features and with noticeable variations in the widths and lags of several high-ionization lines like He ii and C iv. Finally, we demonstrate the effect of torus dust emission on the observed lags in the i and z bands.

Resonant Friction on Disks in Galactic Nuclei

We argue that resonant friction has a dramatic effect on a disk whose rotation direction is misaligned with that of its host nuclear star cluster. The disk’s gravity causes gravitational perturbation of the cluster that in turn exerts a strong torque back onto the disk. We argue that this torque may be responsible for the observed disruption of the clockwise disk of young stars in the Galactic center, and show in numerical experiments that it produces the observed features in the distribution of the stars’ angular momenta. More generally, we speculate that the rotation of nuclear star clusters has a stabilizing effect on the orientation of transient massive accretion disks around the supermassive black holes residing in their centers, and thus on the directions and magnitudes of the black hole spins.

No Redshift Evolution in the Fe ii/Mg ii Flux Ratios of Quasars across Cosmic Time

The Fe ii/Mg ii emission line flux ratio in quasar spectra serves as a proxy for the relative Fe to α-element abundances in the broad-line regions of quasars. Due to the expected different enrichment timescales of the two elements, they can be used as a cosmic clock in the early Universe. We present a study of the Fe ii/Mg ii ratios in a sample of luminous quasars exploiting high-quality near-IR spectra taken primarily by the XQR-30 program with VLT XSHOOTER. These quasars have a median bolometric luminosity of log(L bol[erg s−1]) ∼ 47.3 and cover a redshift range of z = 6.0–6.6. The median value of the measured Fe ii/Mg ii ratios is ∼7.9 with a normalized median absolute deviation of ∼2.2. In order to trace the cosmic evolution of Fe ii/Mg ii in an unbiased manner, we select two comparison samples of quasars with similar luminosities and high-quality spectra from the literature, one at intermediate redshifts (z = 3.5–4.8) and the other at low redshifts (z = 1.0–2.0). We perform the same spectral analysis for all these quasars, including the usage of the same iron template, the same spectral fitting method, and the same wavelength fitting windows. We find no significant redshift evolution in the Fe ii/Mg ii ratio over the wide redshift range from z = 1 to 6.6. The result is consistent with previous studies and supports the scenario of a rapid iron enrichment in the vicinity of accreting supermassive black holes at high redshift.

Spectrophotometric Reverberation Mapping of Intermediate-mass Black Hole NGC 4395

Understanding the origins of massive black hole seeds and their coevolution with their host galaxy requires studying intermediate-mass black holes (IMBHs) and estimating their mass. However, measuring the masses of these IMBHs is challenging, due to the high-spatial-resolution requirement. Spectrophotometric reverberation monitoring is performed for a low-luminosity Seyfert 1 galaxy, NGC 4395, to measure the size of the broad-line region and black hole mass. The data were collected using the 1.3 m Devasthal fast optical telescope and 3.6 m Devasthal optical telescope at ARIES, Nainital, over two consecutive days in 2022 March. The analysis revealed strong emission lines in the spectra and light curves of the merged 5100 Å spectroscopic continuum flux (f 5100) with the photometric continuum V band and Hα, with fractional variabilities of 6.38% and 6.31% respectively. In comparison to several previous studies with lag estimation <90 minutes, our calculated Hα lag supersedes them by 125.0−6.1+6.2 minutes, using the ICCF and JAVELIN methods. The velocity dispersion (σ line) of the broad-line clouds is measured to be 544.7−25.1+22.4 km s−1, yielding a black hole mass of ∼ 2.2−0.2+0.2×104M⊙ and an Eddington ratio of 0.06.

NGC 7314: X-Ray Study of the Evolving Accretion Properties

We present a comprehensive analysis of the timing and spectral properties of NGC 7314, a Seyfert 1.9 galaxy, using X-ray observations from XMM-Newton, NuSTAR, and RXTE/proportional counter array (PCA). The timing analysis reveals significant variability across different energy bands, with fractional variability values consistent with previous studies. The highly variable soft photons and comparatively less variable high-energy photons imply different origins of these two types. The soft energy photons come from a hot corona near the center, while the high-energy photons are produced by inverse Compton scattering of these primary X-ray photons in a hot plasma away from the central region. The spectral analysis employs various models to characterize the emission components. The results indicate the presence of a soft energy bump, Fe Kα line emission, and a prominent reflection component. The long-term RXTE/PCA data analysis reveals temporal variations in the photon index (Γ) and power-law flux, suggesting evolving emission properties over time. The signature of both broad and narrow Fe Kα emission line features suggested the broad, variable one coming from the accretion disk (∼10−5 pc), while the nonevolving narrow line cannot be well constrained. The absorption feature could originate in a highly ionized region, possibly closer to the broad-line region. The evolution of the inner accretion properties indicates that NGC 7314 could be a potential changing-state active galactic nucleus.

An X-ray flaring event and a variable soft X-ray excess in the Seyfert LCRS B040659.9–385922 as detected with eROSITA

Context. Extreme continuum variability in extragalactic nuclear sources can indicate extreme changes in accretion flows onto supermassive black holes. Aims. We explore the multiwavelength nature of a continuum flare in the Seyfert LCRS B040659.9−385922. The all-sky X-ray surveys conducted by the Spectrum Roentgen Gamma (SRG)/extended ROentgen Survey with an Imaging Telescope Array (eROSITA) showed that its X-ray flux increased by a factor of roughly five over six months, and concurrent optical photometric monitoring with the Asteroid Terrestrial-impact Last Alert System (ATLAS) showed a simultaneous increase. Methods. We complemented the eROSITA and ATLAS data by triggering a multiwavelength follow-up monitoring program (X-ray Multi-Mirror Mission: XMM-Newton, Neutron star Interior Composition Explorer: NICER; optical spectroscopy) to study the evolution of the accretion disk, broad-line region, and X-ray corona. During the campaign, X-ray and optical continuum flux subsided over roughly six months. Our campaign includes two XMM-Newton observations, one taken near the peak of this flare and the other taken when the flare had subsided. Results. The soft X-ray excess in both XMM-Newton observations was power law-like (distinctly nonthermal). Using a simple power law, we observed that the photon index of the soft excess varies from a steep value of Γ ∼ 2.7 at the flare peak to a relatively flatter value of Γ ∼ 2.2 as the flare subsided. We successfully modeled the broadband optical/UV/X-ray spectral energy distribution at both the flare peak and post-flare times with the AGNSED model, incorporating thermal disk emission into the optical/UV and warm thermal Comptonization in the soft X-rays. The accretion rate falls by roughly 2.5, and the radius of the hot Comptonizing region increases from the flaring state to the post-flare state. Additionally, from the optical spectral observations, we find that the broad He IIλ4686 emission line fades significantly as the optical/UV/X-ray continuum fades, which could indicate a substantial flare of disk emission above 54 eV. We also observed a redshifted broad component in the Hβ emission line that is present during the high flux state of the source and disappears in subsequent observations. Conclusions. A sudden strong increase in the local accretion rate in this source manifested itself via an increase in accretion disk emission and in thermal Comptonized emission in the soft X-rays, which subsequently faded. The redshifted broad Balmer component could be associated with a transient kinematic component distinct from that comprising the rest of the broad-line region.

Electromagnetic signatures of black hole clusters in the center of super-Eddington galaxies

Supermassive black holes (SMBHs) at the centers of active galaxies are fed by accretion disks that radiate from the infrared or optical to the X-ray bands. Several types of objects can orbit SMBHs, including massive stars, neutron stars, clouds from the broad- and narrow-line regions, and X-ray binaries. Isolated black holes with a stellar origin (BHs of ∼10 M⊙) should also be present in large numbers within the central parsec of the galaxies. These BHs are expected to form a cluster around the SMBH as a result of the enhanced star formation rate in the inner galactic region and the BH migration caused by gravitational dynamical friction. However, except for occasional microlensing effects on background stars or gravitational waves from binary BH mergers, the presence of a BH population is hard to verify. In this paper, we explore the possibility of detecting electromagnetic signatures of a central cluster of BHs when the accretion rate onto the central SMBH is greater than the Eddington rate. In these supercritical systems, the accretion disk launches powerful winds that interact with the objects orbiting the SMBH. Isolated BHs can capture matter from this dense wind, leading to the formation of small accretion disks around them. If jets are produced in these single microquasars, they could be sites of particle acceleration to relativistic energies. These particles in turn are expected to cool by various radiative processes. Therefore, the wind of the SMBH might illuminate the BHs through the production of both thermal and nonthermal radiation.

Determining the Extents, Geometries, and Kinematics of Narrow-line Region Outflows in Nearby Seyfert Galaxies

Outflowing gas from supermassive black holes in the centers of active galaxies has been postulated as a major contributor to galactic evolution. To explore the interaction between narrow-line region (NLR) outflows and their host galaxies, we use Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) spectra and Wide Field Camera 3 (WFC3) images of 15 nearby (z < 0.02) active galactic nuclei (AGN) to determine the extents and geometries of their NLRs. We combine new HST WFC3 continuum and [O iii] λ5007 images of 11 AGN with four archival AGN to match existing spectra from HST STIS. For the six AGN with suitable long-slit coverage of their NLRs, we use isophotal fitting of ground-based images, continuum-subtracted [O iii] images, and the STIS spectra, to resolve, measure, and de-project the gas kinematics to the plane of the host galaxy disk and distinguish NLR outflows from galaxy rotation and/or kinematically disturbed gas. We find an average [O iii] extent of ∼680 pc with a correlation between gas extent and [O iii] luminosity of R [O III] ∝ L[OIII]0.39 . The measured extents depend strongly on the depth of the [O iii] images, highlighting the importance of adopting uniform thresholds when analyzing scaling relationships. The outflows reach from 39% to 88% of the full NLR extents, and we find that all six of the AGN with STIS coverage of their entire NLRs show strong kinematic evidence for outflows, despite previous uncertainty for these AGN. This suggests that NLR outflows are ubiquitous in moderate-luminosity AGN and that standard criteria for kinematic modeling are essential for identifying outflows.

A link to the past: characterizing wandering black holes in Milky Way-type galaxies

Abstract A population of non-stellar black holes (≳100 M⊙) has been long predicted to wander the Milky Way. We aim to characterize this population by using the L-Galaxies semi-analytical model applied on top of the high resolution Millennium-II merger trees. Our results predict ∼10 wandering black holes with masses ∼2 × 103 M⊙ in a typical z = 0 Milky Way galaxy, accounting for $\sim 2\%$ of the total non-stellar black hole mass budget of the galaxy. We find that the locations of these wanderers correlate with their formation scenario. While the ones concentrated at ≲1 kpc from the galactic nucleus on the disc come from past galactic mergers, the ones formed as a consequence of ejections due to gravitational recoils or the disruption of satellite galaxies are typically located at ≳100 kpc. Such small and large distances might explain the absence of strong observational evidence for wandering black holes in the Milky Way. Our results also indicate that $\sim 67\%$ of the wandering population is conformed by the leftovers of black hole seeds that had little to no growth since their formation. We find that wandering black holes that are leftover seeds become wanderers at an earlier time with respect to grown seeds, and also come from more metal-poor galaxies. Finally, we show that the number of wandering black holes in a Milky Way-type galaxy depends on the seeding efficiency.

Feedback-regulated seed black hole growth in star-forming molecular clouds and galactic nuclei

Context. The detection of supermassive black holes (SMBHs) in high-redshift luminous quasars may require a phase of rapid accretion, and as a precondition, substantial gas influx toward seed black holes (BHs) from kiloparsec or parsec scales. Our previous research demonstrated the plausibility of such gas supply for BH seeds within star-forming giant molecular clouds (GMCs) with high surface density (∼104 M⊙ pc−2), facilitating “hyper-Eddington” accretion via efficient feeding by dense clumps, which are driven by turbulence and stellar feedback. Aims. This article presents an investigation of the impacts of feedback from accreting BHs on this process, including radiation, mechanical jets, and highly relativistic cosmic rays. Methods. We ran a suite of numerical simulations to explore diverse parameter spaces of BH feedback, including the subgrid accretion model, feedback energy efficiency, mass loading factor, and initial metallicity. Results. Using radiative feedback models inferred from the slim disk, we find that hyper-Eddington accretion is still achievable, yielding BH bolometric luminosities of as high as 1041 − 1044 erg/s, depending on the GMC properties and specific feedback model assumed. We find that the maximum possible mass growth of seed BHs (ΔMmax BH) is regulated by the momentum-deposition rate from BH feedback, ṗfeedback/(ṀBHc), which leads to an analytic scaling that agrees well with simulations. This scenario predicts the rapid formation of ∼104 M⊙ intermediate-massive BHs (IMBHs) from stellar-mass BHs within ∼1 Myr. Furthermore, we examine the impacts of subgrid accretion models and how BH feedback may influence star formation within these cloud complexes.

Multi-epoch UV–X-Ray Spectral Study of NGC 4151 with AstroSat

We present a multiwavelength spectral study of NGC 4151 based on five epochs of simultaneous AstroSat observations in the near-ultraviolet (NUV) to hard X-ray band (∼0.005–80 keV) during 2017–2018. We derived the intrinsic accretion disk continuum after correcting for internal and Galactic extinction, contributions from broad- and narrow-line regions, and emission from the host galaxy. We found a bluer continuum at brighter UV flux, possibly due to variations in the accretion disk continuum or the UV reddening. We estimated the intrinsic reddening, E(B − V) ∼ 0.4, using high-resolution Hubble Space Telescope (HST)/STIS spectrum acquired in 2000 March. We used thermal Comptonization, neutral and ionized absorption, and X-ray reflection to model the X-ray spectra. We obtained the X-ray absorbing neutral column varying between N H ∼1.2 and 3.4 × 1023 cm−2, which are ∼100 times larger than that estimated from UV extinction, assuming the Galactic dust-to-gas ratio. To reconcile this discrepancy, we propose two plausible configurations of the obscurer: (a) a two-zone obscurer consisting of dust-free and dusty regions, divided by the sublimation radius, or (b) a two-phase obscurer consisting of clumpy, dense clouds embedded in a low-density medium, resulting in a scenario where a few dense clouds obscure the compact X-ray source substantially, while the bulk of UV emission arising from the extended accretion disk passes through the low-density medium. Furthermore, we find a positive correlation between the X-ray absorption column and NUV − far-UV color and UV flux, indicative of enhanced winds possibly driven by the “bluer-when-brighter” UV continuum.

A Closer Look at Dwarf Galaxies Exhibiting Mid-infrared Variability: Active Galactic Nuclei Confirmation and Comparison With Nonvariable Dwarf Galaxies

Detecting active black holes in dwarf galaxies has proven to be a challenge due to their small size and weak electromagnetic signatures. Mid-infrared variability has emerged as a promising tool that can be used to detect active low-mass black holes in dwarf galaxies. We analyzed 10.4 yr of photometry from the AllWISE/NEOWISE multiepoch catalogs, identifying 25 objects with active galactic nuclei (AGN)-like variability. Independent confirmation of AGN activity was found in 68% of these objects using optical and near-infrared diagnostics. Notably, we discover a near-infrared coronal line [S ix] λ 1.252 μm in J1205, the galaxy with the lowest stellar mass (log M * = 7.5 M ⊙) and low metallicity (12 + log(O/H) = 7.46) in our sample. Additionally, we find broad Paα potentially from the broad-line region in two targets, and their implied black hole masses are consistent with black hole-stellar mass relations. Comparing nonvariable galaxies with similar stellar masses and Wide-field Infrared Survey Explorer W1 − W2 colors, we find no clear trends between variability and large-scale galaxy properties. However, we find that AGN activity likely causes redder W1 − W2 colors in variable targets, while for the nonvariable galaxies, the contribution stems from strong star formation activity. A high incidence of optical broad lines was also observed in variable targets. Our results suggest that mid-infrared variability is an effective method for detecting AGN activity in low-mass galaxies and can help uncover a larger sample of active low-mass (<106 M ⊙) black holes in the Universe.

Broad-line Region of the Quasar PG 2130+099. II. Doubling the Size Over Four Years?

Over the past three decades, multiple reverberation mapping (RM) campaigns conducted for the quasar PG 2130+099 have exhibited inconsistent findings with time delays ranging from ∼10 to ∼200 days. To achieve a comprehensive understanding of the geometry and dynamics of the broad-line region (BLR) in PG 2130+099, we continued an ongoing high-cadence RM monitoring campaign using the Calar Alto Observatory 2.2 m optical telescope for an extra four years from 2019 to 2022. We measured the time lags of several broad emission lines (including He ii, He i, Hβ, and Fe ii) with respect to the 5100 Å continuum, and their time lags continuously vary through the years. Especially, the Hβ time lags exhibited approximately a factor of 2 increase in the last two years. Additionally, the velocity-resolved time delays of the broad Hβ emission line reveal a back-and-forth change between signs of virial motion and inflow in the BLR. The combination of negligible (∼10%) continuum change and substantial time-lag variation (over 2 times) results in a significant scatter in the intrinsic R Hβ –L 5100 relationship for PG 2130+099. Taking the consistent changes in the continuum variability time scale and the size of the BLR into account, we tentatively propose that the changes in the measurement of the BLR size may be affected by “geometric dilution”

Windy or Not: Radio Parsec-scale Evidence for a Broad-line Region Wind in Radio-quiet Quasars

Does a broad-line region (BLR) wind in radio-quiet (RQ) active galactic nuclei (AGN) extend to parsec scales and produce radio emission? We explore the correlations between a parsec-scale radio wind and the BLR wind in a sample of 19 RQ Palomar–Green quasars. The radio wind is defined based on the spectral slope and the compactness of the emission at 1.5–5 GHz, and the BLR wind is defined by the excess blue wing in the C iv emission line profile. The five objects with both radio and BLR wind indicators are found at high Eddington ratios, L/L Edd (≥0.66), and eight of the nine objects with neither radio nor BLR winds reside at low L/L Edd (≤0.28). This suggests that the BLR wind and the radio wind in RQ AGN are related to a radiation-pressure-driven wind. Evidence for free–free absorption by AGN photoionized gas, which flattens the spectral slope, is found in two objects. Radio outflows in three low-L/L Edd (0.05–0.12) objects are likely from a low-power jet, as suggested by additional evidence. The presence of a mild equatorial BLR wind in four intermediate-L/L Edd (0.2–0.4) objects can be tested with future spectropolarimetry.