Binary Active Galactic Nuclei Research Articles

VaDAR: Varstrometry for Dual AGN Using Radio Interferometry

Binary and dual active galactic nuclei (AGNs) are an important observational tool for studying the formation and dynamical evolution of galaxies and supermassive black holes. An entirely new method for identifying possible AGN pairs makes use of the exquisite positional accuracy of Gaia to detect astrometrically variable quasars, in tandem with the high spatial resolution of the Karl G. Jansky Very Large Array (VLA). We present a new pilot study of radio observations of 18 quasars (0.8 ≤ z ≤ 2.9), selected from the Sloan Digital Sky Survey DR16Q and matched with the Gaia DR3. All 18 targets are identified by their excess astrometric noise in Gaia. We targeted these 18 quasars with the VLA at 2–4 GHz (S band) and 8–12 GHz (X band), providing resolutions of 0.″65 and 0.″2, respectively, in order to constrain the origin of this variability. We combine these data with ancillary radio survey data and perform radio spectral modeling. The new observations are used to constrain the driver of the excess astrometric noise. We find that ∼44% of the target sample is likely to be either candidate dual AGN or gravitationally lensed quasars. Ultimately, we use this new strategy to help identify and understand this sample of astrometrically variable quasars, demonstrating the potential of this method for systematically identifying kiloparsec-scale dual quasars.

Multiwavelength scrutiny of X-ray sources in dwarf galaxies: ULXs versus AGNs

ABSTRACT Owing to their quiet evolutionary histories, nearby dwarf galaxies (stellar masses $M_\star \lesssim 3 \times 10^9 \, \mathrm{M}_\odot$) have the potential to teach us about the mechanism(s) that ‘seeded’ the growth of supermassive black holes, and also how the first stellar mass black holes formed and interacted with their environments. Here, we present high spatial resolution observations of three dwarf galaxies in the X-ray (Chandra), the optical/near-infrared (Hubble Space Telescope), and the radio (Karl G. Jansky Very Large Array). These three galaxies were previously identified as hosting candidate active galactic nuclei on the basis of lower resolution X-ray imaging. With our new observations, we find that X-ray sources in two galaxies (SDSS J121326.01+543631.6 and SDSS J122111.29+173819.1) are off-nuclear and lack corresponding radio emission, implying they are likely luminous X-ray binaries. The third galaxy (Mrk 1434) contains two X-ray sources (each with LX ≈ 1040 erg s−1) separated by 2.8 arcsec, has a low metallicity [12 + log(O/H) = 7.8], and emits nebular He ii λ4686 line emission. The northern source has spatially coincident point-like radio emission at 9.0 GHz and extended radio emission at 5.5 GHz. We discuss X-ray binary interpretations (where an ultraluminous X-ray source blows a ‘radio bubble’) and active galactic nucleus interpretations (where an $\approx 4\times 10^5 \, \mathrm{M}_\odot$ black hole launches a jet). In either case, we find that the He ii emission cannot be photoionized by the X-ray source, unless the source was ≈30–90 times more luminous several hundred years ago.

The X-shaped Radio Galaxy J0725+5835 is Associated with an AGN Pair

X-shaped radio galaxies (XRGs) are those that exhibit two pairs of unaligned radio lobes (main radio lobes and wings). One of the promising models for the peculiar morphology is jet reorientation. To clarify this, we conducted a 5 GHz observation with the European VLBI Network (EVN) of XRG J0725+5835, which resembles the archetypal binary active galactic nuclei (AGNs) 0402+379 in radio morphology, but it is larger in angular size. In our observation, two milliarcsecond-scale radio components with nonthermal radio emission are detected. Each of them coincides with an optical counterpart with similar photometric redshift and (optical and infrared) magnitude, corresponding to dual active nuclei. Furthermore, with the improved Very Large Array (VLA) images, we find a bridge between the two radio cores and a jet bending in the region surrounding the companion galaxy. This further supports the interplay between the main and companion galaxies. In addition, we also report the discovery of an arcsecond-scale jet in the companion. Given the projected separation of ∼100 kpc between the main and companion galaxies, XRG J0725+5835 is likely associated with a dual jetted-AGN system. In both EVN and VLA observations, we find signatures that the jet is changing its direction, which is likely responsible for the X-shaped morphology. For the origin of jet reorientation, several scenarios are discussed.

Investigating the Accretion Nature of Binary Supermassive Black Hole Candidate SDSS J025214.67−002813.7

Abstract We present results of a multiwavelength analysis of SDSS J025214.67−002813.7, a system that has been previously classified as a binary active galactic nucleus (AGN) candidate based on periodic signals detected in the optical light curves. We use available radio−X-ray observations of the system to investigate the true accretion nature. Analyzing new observations from XMM-Newton and NuSTAR, we characterize the X-ray emission and search for evidence of circumbinary accretion. Although the 0.5–10 keV spectrum shows evidence of an additional soft emission component, possibly due to extended emission from hot nuclear gas, we find the spectral shape is consistent with that of a single AGN. Compiling a full multiwavelength spectral energy distribution (SED), we also search for signs of circumbinary accretion, such as a “notch” in the continuum due to the presence of minidisks. We find that the radio–optical emission agrees with the SED of a standard, radio-quiet, AGN; however, there is a large deficit in emission blueward of ∼1400 Å. Although this deficit in emission can plausibly be attributed to a binary AGN system, we find that the SED of SDSS J0252−0028 is better explained by emission from a reddened, single AGN. However, future studies of the expected hard X-ray emission associated with binary AGNs (especially in the unequal-mass regime) will allow for more rigorous analyses of the binary AGN hypothesis.

Revealing dual radio sources in a sub-kpc-scale binary active galactic nucleus candidate

ABSTRACT We present new imaging of a sub-kpc-scale binary active galactic nucleus (AGN) candidate from the Karl G. Jansky Very Large Array (VLA) and the Multi-Element Radio Linked Interferometer Network (e-MERLIN). Two unresolved radio sources of similar luminosity around 1022 WHz−1 are identified in ∼35 h of e-MERLIN 6 cm imaging. These radio sources have an angular separation of 0.19 ± 0.06 arcsec and position angle (PA) of 22° ± 10°, corresponding to a projected separation of 0.95 ± 0.29 kpc at the epoch of the source. Our results suggest the presence of a kpc-scale active black hole pair hosted by two galaxies in the late stage of a merger at z = 0.35. This work follows Woo et al., which presented two optical sources with a similar separation and PA, and a velocity separation of 200 km s−1. Our target adds to the currently limited sample of close-separation binary AGNs, which will aid in understanding the frequency of mergers and the stochastic gravitational wave background.

Binary AGNs simulations with radiation pressure reveal a new duty cycle, and a reduction of gravitational torque, through ‘minitori’ structures

ABSTRACT We produce the first set of radiation hydrodynamics simulations of binary active galactic nuclei (AGNs) at parsec-scale separation in scale-model simulations. We use SPH for hydrodynamics, and raytracing to calculate optical depths and radiation pressure from the two AGNs. We confirm that, without radiation pressure, the sign of gravitational torque is sensitive to the binary parameters, although in one of our two orbital configurations the binary should coalesce in a time-scale of <109 yr. However, radiation pressure quickly destroys the ‘minitori’ around each supermassive black hole (SMBH), drastically reducing gravitational torques and accretion, and greatly increasing the coalescence time-scale. Our simulations suggest a new ‘minitorus’ duty cycle with a time-scale of ∼10 binary periods (∼106 yr when scaling our models to a total binary mass of 2 × 107 M⊙). The growth and blow-out phases of the ‘minitori’ are of similar time-scales, and thus we expect about half of observed binary SMBHs to be active, in at least one component. The ‘minitorus’ structure provides asymmetries that could be observed by infrared interferometry.

The Deepest Chandra View of RBS 797: Evidence for Two Pairs of Equidistant X-ray Cavities

Abstract We present the first results of a deep Chandra observation of the galaxy cluster RBS 797 whose previous X-ray studies revealed two pronounced X-ray cavities in the east–west (E–W) direction. Follow-up VLA radio observations of the central active galactic nucleus (AGN) uncovered different jet and lobe orientations, with radio lobes filling the E–W cavities and perpendicular jets showing emission in the north–south (N–S) direction over the same scale (≈30 kpc). With the new ∼427 ks total exposure, we report the detection of two additional, symmetric X-ray cavities in the N–S direction at nearly the same radial distance as the E–W ones. The newly discovered N–S cavities are associated with the radio emission detected at 1.4 and 4.8 GHz in archival VLA data, making RBS 797 the first galaxy cluster found to have four equidistant, centrally symmetric, radio-filled cavities. We derive the dynamical and radiative ages of the four cavities from X-ray and radio data, respectively, finding that the two outbursts are approximately coeval, with an age difference of ⪅10 Myr between the E–W and N–S cavities. We discuss two scenarios for the origin of the two perpendicular, equidistant cavity systems: either the presence of a binary AGN that is excavating coeval pairs of cavities in perpendicular directions or a fast (<10 Myr) jet reorientation event that produced subsequent, misaligned outbursts.

Gravitational self-lensing in populations of massive black hole binaries

ABSTRACT The community may be on the verge of detecting low-frequency gravitational waves from massive black hole binaries (MBHBs), but no examples of binary active galactic nuclei (AGN) have been confirmed. Because MBHBs are intrinsically rare, the most promising detection methods utilize photometric data from all-sky surveys. Gravitational self-lensing has recently been proposed as a method of detecting AGN in close separation binaries. In this study, we calculate the detectability of lensing signatures in realistic populations of simulated MBHBs. Within our model assumptions, we find that VRO’s LSST should be able to detect tens to hundreds of self-lensing binaries, with the rate uncertainty depending primarily on the orientation of AGN discs relative to their binary orbits. Roughly a quarter of lensing detectable systems should also show detectable Doppler boosting signatures. If AGN discs tend to be aligned with the orbit, lensing signatures are very nearly achromatic, while in misaligned configurations, the bluer optical bands are lensed more than redder ones. Whether substantial obscuring material (e.g. a dusty torus) will be present in close binaries remains uncertain, but our estimates suggest that a substantial fraction of systems would still be observable in this case.

Spatially Resolved C iii] λ1909 Emission in Haro 11

Abstract The C iii] 1909 (hereafter C iii]) line is the strongest ultraviolet emission line after Lyα and is therefore of interest to high-redshift studies of star-forming (SF) galaxies near the epoch of reionization. It is thought that C iii] emission is strongest in galaxies with subsolar metallicity and low mass; however, spectral observations of numerous such galaxies at high and low redshift produce inconclusive or even contradictory results. We present the first-ever C iii] imaging, obtained with HST/STIS for the low-redshift SF galaxy Haro 11. Cluster parameters like stellar mass, dust fraction and attenuation, and ionization parameter, obtained through spectral energy distribution fitting, show no correlation with the C iii] equivalent width (EW), which may be due to a combination of the limitation of the models and the age homogeneity of the cluster population. Comparing the ratio of C iii] emission line flux from individual clusters to that of Hα, [O iii], and [O ii], we find that the clusters with the highest EW(C iii]) can be reconciled only with Cloudy models with an extremely high C/O ratio of ≥1.4(C/O)⊙ for an ionizing population of single stars, binary stars, or a mixture of binary stars and active galactic nuclei. Given the pointlike nature of strong C iii], the integrated total strength of EW(C iii]) becomes dependent on the morphology of the galaxy, which would explain the large scatter in EW(C iii]) strengths observed in galaxies with otherwise similar SF properties and of similarly low metallicity and stellar mass.

Basic considerations for the observability of kinematically offset binary AGN

ABSTRACT The gravitational waves from massive black hole (MBH) binaries are expected to be detected by pulsar timing arrays in the next few years. While they are a promising source for multimessenger observations as binary active galactic nuclei (AGNs), few convincing candidates have been identified in electromagnetic surveys. One approach to identifying candidates has been through spectroscopic surveys searching for offsets or time-dependent offsets of broad emission lines (BLs), which may be characteristic of Doppler shifts from binary orbital motion. In this study, we predict the parameter space of MBH binaries that should be kinematically detectable. There is a delicate trade-off between requiring binary separations to be large enough for BL regions to remain attached to one of the AGNs, but also small enough such that their orbital velocity is detectable. We find that kinematic signatures are only observable for the lower mass secondary AGN, for binaries with total masses above about $10^8 \, \textrm {M}_{\odot }$, and separations between 0.1 and 1 pc. We motivate our usage of a kinematic offset sensitivity of 103 km s−1, and a sensitivity to changing offsets of 102 km s−1. With these parameters, and an Eddington ratio of 0.1, we find that $0.5{{\ \rm per\ cent}}$ of binaries have detectable offsets, and only $0.03{{\ \rm per\ cent}}$ have detectable velocity changes. Overall, kinematic binary signatures should be expected in fewer than one in 104 AGNs. Better characterizing the intrinsic variability of BLs is crucial to understanding and vetting MBH binary candidates. This requires multi-epoch spectroscopy of large populations of AGNs over a variety of time-scales.

J1110+4817 – a compact symmetric object candidate revisited

ABSTRACT Compact symmetric objects (CSOs) are radio-emitting active galactic nuclei (AGNs) typically with a double-lobed radio structure confined to within 1 kpc. CSOs represent the earliest evolutionary phase of jetted AGNs. Some of them may eventually evolve into large-scale extended double sources, while others stall within the host galaxy and die out, depending on the longevity of nuclear activity, the jet power, and parameters of the surrounding galactic environment. Studying CSOs is a useful tool for understanding the evolution of the galaxies and the interactions between the jets and the medium of the host galaxy. Based on milliarcsec-resolution imaging observations using very long baseline interferometry (VLBI), it is not always straightforward to distinguish between a compact double-lobed or a core–jet structure. The quasar J1110+4817 was considered a CSO candidate in the literature earlier, but because of the lack of clear evidence, it could not be securely classified as a CSO. Here we present a comprehensive analysis of archival multifrequency VLBI observations combined with accurate Gaia optical astrometric information. Lower frequency VLBI images reveal an extended radio feature nearly perpendicular to the main structural axis of the source, apparently emanating from the brighter northern feature, which is rare among the known CSOs. While the presence of a binary AGN system cannot be fully excluded, the most plausible explanation is that J1110+4817 is a CSO.

Inward bound: the incredible journey of massive black holes as they pair and merge – I. The effect of mass ratio in flattened rotating galactic nuclei

ABSTRACT Understanding how supermassive black holes (SMBHs) pair and merge helps to inform predictions of off-centre, dual, and binary active galactic nuclei (AGNs), and provides key insights into how SMBHs grow and co-evolve with their galaxy hosts. As the loudest known gravitational wave source, binary SMBH mergers also hold centrestage for the Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave observatory set to launch in 2034. Here, we continue our work to characterize SMBH binary formation and evolution through increasingly more realistic high-resolution direct N-body simulations, focusing on the effect of SMBH mass ratio, orientation, and eccentricity within a rotating and flattened stellar host. During the dynamical friction phase, we found a prolonged orbital decay for retrograde SMBHs and swift pairing time-scales for prograde SMBHs compared to their counterparts in non-rotating models, an effect that becomes more pronounced for smaller mass ratios Msec/Mprim = q. During this pairing phase, the eccentricity dramatically increases for retrograde configurations, but as the binary forms, the orbital plane flips so that it is almost perfectly prograde, which stifles the rapid eccentricity growth. In prograde configurations, SMBH binaries form and remain at comparatively low eccentricities. As in our prior work, we note that the centre of mass of a prograde SMBH binary itself settles into an orbit about the centre of the galaxy. Since even the initially retrograde binaries flip their orbital plane, we expect few binaries in rotating systems to reside at rest in the dynamic centre of the host galaxy, though this effect becomes smaller as q decreases.

SDSS-IV MaNGA: Galaxy Pair Fraction and Correlated Active Galactic Nuclei

Abstract We have identified 105 galaxy pairs at z ∼ 0.04 with the MaNGA integral-field spectroscopic data. The pairs have projected separations between 1 and 30 kpc, and are selected to have radial velocity offsets less than 600 km s−1 and stellar mass ratio between 0.1 and 1. The pair fraction increases with both the physical size of the integral-field unit and the stellar mass, consistent with theoretical expectations. We provide the best-fit analytical function of the pair fraction and find that ∼3% of M* galaxies are in close pairs. For both isolated galaxies and paired galaxies, active galactic nuclei (AGNs) are selected using emission-line ratios and Hα equivalent widths measured inside apertures at a fixed physical size. We find AGNs in ∼24% of the paired galaxies and binary AGNs in ∼13% of the pairs. To account for the selection biases in both the pair sample and the MaNGA sample, we compare the AGN comoving volume densities with those expected from the mass- and redshift-dependent AGN fractions. We find a strong (∼5×) excess of binary AGNs over random pairing and a mild (∼20%) deficit of single AGNs. The binary AGN excess increases from ∼2× to ∼6× as the projected separation decreases from 10–30 to 1–10 kpc. Our results indicate that the pairing of galaxies preserves the AGN duty cycle in individual galaxies but increases the population of binary AGNs through correlated activities. We suggest tidally induced galactic-scale shocks and AGN cross-ionization as two plausible channels to produce low-luminosity narrow-line-selected binary AGNs.

Searching for dual active galactic nuclei

Binary or dual active galactic nuclei (DAGN) are expected from galaxy formation theories. However, confirmed DAGN are rare and finding these systems has proved to be challenging. Recent systematic searches for DAGN using double-peaked emission lines have yielded several new detections. As have the studies of samples of merging galaxies. In this paper, we present an updated list of DAGN compiled from published data. We also present preliminary results from our ongoing Expanded Very Large Array (EVLA) radio study of eight double- peaked emission-line AGN (DPAGN). One of the sample galaxy shows an S-shaped radio jet. Using new and archival data, we have successfully fitted a precessing jet model to this radio source. We find that the jet precession could be due to a binary AGN with a super-massive black-hole (SMBH) separation of 0.02 pc or a single AGN with a tilted accretion disk. We have found that another sample galaxy, which is undergoing a merger, has two radio cores with a projected separation of 5.6 kpc. We discuss the preliminary results from our radio study.

Dual Active Galactic Nuclei in Nearby Galaxies

Galaxy mergers play a crucial role in the formation of massive galaxies and the buildup of their bulges. An important aspect of the merging process is the in-spiral of the supermassive black-holes (SMBHs) to the centre of the merger remnant and the eventual formation of a SMBH binary. If both the SMBHs are accreting they will form a dual or binary active galactic nucleus (DAGN). The final merger remnant is usually very bright and shows enhanced star formation. In this paper we summarise the current sample of DAGN from previous studies and describe methods that can be used to identify strong DAGN candidates from optical and spectroscopic surveys. These methods depend on the Doppler separation of the double peaked AGN emission lines, the nuclear velocity dispersion of the galaxies and their optical/UV colours. We describe two high resolution, radio observations of DAGN candidates that have been selected based on their double peaked optical emission lines (DPAGN). We also examine whether DAGN host galaxies have higher star formation rates (SFRs) compared to merging galaxies that do not appear to have DAGN. We find that the SFR is not higher for DAGN host galaxies. This suggests that the SFRs in DAGN host galaxies is due to the merging process itself and not related to the presence of two AGN in the system.

A possible binary AGN in Mrk 622?

Abstract Mrk 622 is a Compton thick active galactic nuclei (AGN) and a double-peaked narrow emission line galaxy, thus a dual AGN candidate. In this work, new optical long-slit spectroscopic observations clearly show that this object is rather a triple peaked narrow emission line galaxy, with both blue and red shifted narrow emission lines, as well as a much narrower emission line centred at the host galaxy systemic velocity. The average velocity offset between the blue and red shifted components is ∼500 km s−1, which is producing the apparent double-peaked emission lines. These two components are in the loci of AGN in the Baldwin, Phillips & Terlevich diagrams and are found to be spatially separated by ∼76 pc. Analysis of the optical spatially resolved spectroscopic observations presented in this work favours that Mrk 622 is a system consisting of a composite AGN amidst a binary AGN candidate, likely the result of a recent merger. This notwithstanding, outflows from a starburst, or single AGN could also explain the triple nature of the emission lines.

Gravitational-wave localization alone can probe origin of stellar-mass black hole mergers

The recent discovery of gravitational waves from stellar-mass binary black hole mergers by the Laser Interferometer Gravitational-wave Observatory opened the door to alternative probes of stellar and galactic evolution, cosmology and fundamental physics. Probing the origin of binary black hole mergers will be difficult due to the expected lack of electromagnetic emission and limited localization accuracy. Associations with rare host galaxy types—such as active galactic nuclei—can nevertheless be identified statistically through spatial correlation. Here we establish the feasibility of statistically proving the connection between binary black hole mergers and active galactic nuclei as hosts, even if only a sub-population of mergers originate from active galactic nuclei. Our results are the demonstration that the limited localization of gravitational waves, previously written off as not useful to distinguish progenitor channels, can in fact contribute key information, broadening the range of astrophysical questions probed by binary black hole observations.

Finding binary active galactic nuclei candidates by the centroid shift in imaging surveys

In Liu (2015), we propose selecting binary active galactic nuclei (AGNs) candidates using the centroid shift of the images, which is induced by the non-synchronous variations of the two nuclei. In this paper, a known binary AGN (SDSS J233635.75-010733.7) is employed to verify the ability of this method. Using 162 exposures in the $R$ band of \textit{Palomar Transient Factory} (PTF), an excess of dispersion in the positional distribution of the binary AGN is detected, though the two nuclei cannot be resolved in the images of PTF. We also propose a new method to compare the position of the binary AGN in PTF $g$ and $R$ band and find the difference is highly significant even only with 20 exposures. This new method is efficient for two nuclei with different spectral energy distributions, e.g., type I + type II AGN or off-set AGN. Large-scale surveys, e.g., the Panoramic Survey Telescope and Rapid Response System and the Large Synoptic Survey Telescope, are expected to discover a large sample of binary AGN candidates with these methods.

BINARY ACTIVE GALACTIC NUCLEI IN STRIPE 82: CONSTRAINTS ON SYNCHRONIZED BLACK HOLE ACCRETION IN MAJOR MERGERS

Representing simultaneous black hole accretion during a merger, binary active galactic nuclei (AGNs) could provide valuable observational constraints to models of galaxy mergers and AGN triggering. High-resolution radio interferometer imaging offers a promising method to identify a large and uniform sample of binary AGNs, because it probes a generic feature of nuclear activity and is free from dust obscuration. Our previous search yielded 52 strong candidates of kpc-scale binaries over the 92 deg$^2$ of the Sloan Digital Sky Survey (SDSS) Stripe 82 area with 2"-resolution Very Large Array (VLA) images. Here we present 0.3"-resolution VLA 6 GHz observations for six candidates that have complete optical spectroscopy. The new data confirm the binary nature of four candidates and identify the other two as line-of-sight projections of radio structures from single AGNs. The four binary AGNs at $z \sim 0.1$ reside in major mergers with projected separations of 4.2-12 kpc. Optical spectral modeling shows that their hosts have stellar masses between $10.3 < log(M_\star/M_\odot) < 11.5$ and velocity dispersions between $120 < \sigma_\star < 320$ km/s. The radio emission is compact ($<$0.4") and show steep spectrum ($-1.8 < \alpha < -0.5$) at 6 GHz. The host galaxy properties and the Eddington-scaled accretion rates broadly correlate with the excitation state, similar to the general radio-AGN population at low redshifts. Our estimated binary AGN fraction indicates that simultaneous accretion occurs $>23^{+15}_{-8}$% of the time when a kpc-scale galaxy pair is detectable as a radio-AGN. The high duty cycle of the binary phase strongly suggests that major mergers can trigger and synchronize black hole accretion.

Finding binary active galactic nuclei by the centroid shift in imaging surveys

The census of binary active galactic nuclei (AGNs) is important in order to understand the merging history of galaxies and the triggering of AGNs. However, there is still no efficient method for selecting the candidates of binary AGNs. The non-synchronous variations of the two AGNs in one binary system will induce the shift of the image centroid. Since the astrometric error is normally much smaller than the angular resolution of telescopes, it is possible to detect such shifts even in the unresolved system via multi-epoch observations. We perform some simulations and find that hundreds of observations are required to discover compact binary AGNs. This method is suitable for the future large-scale surveys, e.g., the Large Synoptic Survey Telescope, and it might lead to a large sample of binary AGNs with a 1-2 yr survey.