arXiv subjects

Laura Blecha

Publications and source records attributed to Laura Blecha.

At least 91 records · Page 5Linked to original sources

arXiv2022

Electromagnetic Counterparts to Massive Black Hole Mergers

The next two decades are expected to open the door to the first coincident detections of electromagnetic (EM) and gravitational wave (GW) signatures associated with massive black hole (MBH) binaries heading for coalescence. These detections will launch a new era of multimessenger astrophysics by expanding this growing field to the low-frequency GW regime and will provide an unprecedented understanding of the evolution of MBHs and galaxies. They will also constitute fundamentally new probes of cosmology and would enable unique tests of gravity. The aim of this Living Review is to provide an introduction to this research topic by presenting a summary of key findings, physical processes and ideas pertaining to EM counterparts to MBH mergers as they are known at the time of this writing. We review current observational evidence for close MBH binaries, discuss relevant physical processes and timescales, and summarize the possible EM counterparts to GWs in the precursor, coalescence, and afterglow stages of a MBH merger. We also describe open questions and discuss future prospects in this dynamic and quick-paced research area.

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arXiv2022

Probing the $z\gtrsim6$ quasars in a universe with IllustrisTNG physics: Impact of gas-based black hole seeding models

We explore implications of a range of black hole (BH) seeding prescriptions on the formation of the brightest $z\gtrsim6$ quasars in cosmological hydrodynamic simulations. The underlying galaxy formation model is the same as in IllustrisTNG. Using constrained initial conditions, we study the growth of BHs in rare overdense regions (forming $\gtrsim10^{12}M_{\odot}/h$ halos by $z=7$) using a $(9~\mathrm{Mpc}/h)^3$ simulated volume. BH growth is maximal within halos that are compact and have a low tidal field. For these halos, we consider an array of gas-based seeding prescriptions wherein $M_{\mathrm{seed}}=10^4-10^6~M_{\odot}/h$ seeds are inserted in halos above critical thresholds for halo mass and dense, metal-poor gas mass (defined as $\tilde{M}_{\mathrm{h}}$ and $\tilde{M}_{\mathrm{sf,mp}}$, respectively, in units of $M_{\mathrm{seed}}$). We find that a seed model with $\tilde{M}_{\mathrm{sf,mp}}=5$ and $\tilde{M}_{\mathrm{h}}=3000$ successfully produces a $z\sim6$ quasar with $\sim10^9~M_{\odot}$ mass and $\sim10^{47}~\mathrm{ergs~s^ {-1}}$ luminosity. BH mergers play a crucial role at $z\gtrsim9$, causing an early boost in BH mass at a time when accretion-driven BH growth is negligible. When more stringent seeding conditions are applied (for e.g., $\tilde{M}_{\mathrm{sf,mp}}=1000$), the relative paucity of BH seeds results in a much lower merger rate. In this case, $z\gtrsim6$ quasars can only be formed if we enhance the maximum allowed BH accretion rates (by factors $\gtrsim10$) compared to the accretion model used in IllustrisTNG. This can be achieved either by allowing for super-Eddington accretion, or by reducing the radiative efficiency. Our results show that progenitors of $z\sim6$ quasars have distinct BH merger histories for different seeding models, which will be distinguishable with LISA observations.

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arXiv2022

Simulations of black hole fueling in isolated and merging galaxies with an explicit, multiphase ISM

We study gas inflows onto supermassive black holes using hydrodynamics simulations of isolated galaxies and idealized galaxy mergers with an explicit, multiphase interstellar medium (ISM). Our simulations use the recently developed ISM and stellar evolution model called Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE). We implement a novel super-Lagrangian refinement scheme that increases the gas mass resolution in the immediate neighborhood of the black holes (BHs) to accurately resolve gas accretion. We do not include black hole feedback in our simulations. We find that the complex and turbulent nature of the SMUGGLE ISM leads to highly variable BH accretion. BH growth in SMUGGLE converges at gas mass resolutions $\lesssim3\times10^3{\rm M_\odot}$. We show that the low resolution simulations combined with the super-Lagrangian refinement scheme are able to produce central gas dynamics and BH accretion rates very similar to that of the uniform high resolution simulations. We further explore BH fueling by simulating galaxy mergers. The interaction between the galaxies causes an inflow of gas towards the galactic centres and results in elevated and bursty star formation. The peak gas densities near the BHs increase by orders of magnitude resulting in enhanced accretion. Our results support the idea that galaxy mergers can trigger AGN activity, although the instantaneous accretion rate depends strongly on the local ISM. We also show that the level of merger-induced enhancement of BH fueling predicted by the SMUGGLE model is much smaller compared to the predictions by simulations using an effective equation of state model of the ISM.

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arXiv2021

Impact of gas based seeding on supermassive black hole populations at $z\geq7$

Deciphering the formation of supermassive black holes~(SMBHs) is a key science goal for upcoming observational facilities. In most theoretical channels proposed so far, the seed formation depends crucially on local gas conditions. We systematically characterize the impact of a range of gas based black hole seeding prescriptions on SMBH populations using cosmological simulations. Seeds of mass $M_{\mathrm{seed}}\sim 10^3-10^{6}~M_{\odot}/h$ are placed in halos that exceed critical thresholds for star-forming, metal-poor gas mass and halo mass (defined as $\tilde{M}_{\mathrm{sf,mp}}$ and $\tilde{M}_{\mathrm{h}}$, respectively, in units of $M_{\mathrm{seed}}$). We quantify the impact of these parameters on the properties of $z\geq7$ SMBHs. Lower seed masses produce much higher BH merger rates (by factors of $\sim10$ and $\sim1000$ at $z\sim7$ and $z\sim15$, respectively). For fixed seed mass, we find that $\tilde{M}_{\mathrm{h}}$ has the strongest impact on the BH population at high redshift ($z\gtrsim15$, where a factor of 10 increase in $\tilde{M}_{\mathrm{h}}$ suppresses merger rates by $\gtrsim 100$). At lower redshift ($z\lesssim15$), we find that $\tilde{M}_{\mathrm{sf,mp}}$ has a larger impact on the BH population. Increasing $\tilde{M}_{\mathrm{sf,mp}}$ from $5-150$ suppresses the merger rates by factors of $\sim8$ at $z\sim7-15$. This suggests that the seeding criteria explored here could leave distinct imprints on the redshift distribution of LISA merger rates. In contrast, AGN luminosity functions are much less sensitive to seeding criteria, varying by factors $\lesssim2-3$ within the seed parameters we have explored. Such variations will be challenging to probe even with future sensitive instruments such as Lynx or JWST. Overall, our systematic parameter study provides a useful benchmark for development of seed models for large-volume cosmological simulations.

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arXiv2021

Impact of gas spin and Lyman-Werner flux on black hole seed formation in cosmological simulations: implications for direct collapse

Direct collapse black holes~(BH) are promising candidates for producing massive $z\gtrsim 6$ quasars, but their formation requires fine-tuned conditions. In this work, we use cosmological zoom simulations to study systematically the impact of requiring: 1) low gas angular momentum, and 2) a minimum incident Lyman-Werner~(LW) flux in order to form BH seeds. We probe the formation of seeds (with initial masses of $M_{\rm seed} \sim 10^4$ - $10^6 M_{\odot}/h)$ in halos with a total mass $> 3000\times M_{\mathrm{seed}}$ and a dense, metal poor gas mass $> 5\times M_{\mathrm{seed}}$. We find that the seed-forming halos have a prior history of star formation and metal enrichment, but contain pockets of dense, metal poor gas. When seeding is further restricted to halos with low gas spins, the number of seeds formed is suppressed by factors of $\sim6$ compared to the baseline model, regardless of the seed mass. Seed formation is much more strongly impacted if the dense, metal poor gas is required to have a critical LW flux ($J_{\mathrm{crit}}$). Even for $J_{\mathrm{crit}}$ values as low as $50J_{21}$, no $8\times10^{5}M_{\odot}/h$ seeds are formed. While lower mass ($1.25\times10^{4},1\times10^{5} M_{\odot}/h$) seeds do form, they are strongly suppressed~(by factors of $\sim10-100$) compared to the baseline model at gas mass resolutions of $\sim10^4~M_{\odot}/h$ (with even stronger suppression at higher resolutions). As a result, BH merger rates are also similarly suppressed. Since early BH growth is dominated by mergers in our models, no seeds are able to grow to the supermassive regime~($\gtrsim10^6 M_{\odot}/h$) by $z=7$. Our results hint that producing the bulk of the $z\gtrsim6$ supermassive BH population may require alternate seeding scenarios that do not depend on the LW flux, early BH growth dominated by rapid or super-Eddington accretion, or a combination of these possibilities.

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arXiv2021

Relics of Supermassive Black Hole Seeds: The Discovery of an Accreting Black Hole in an Optically Normal, Low Metallicity Dwarf Galaxy

The detection and characterization of supermassive black holes (SMBHs) in local low mass galaxies is crucial to our understanding of the origins of SMBHs. This statement assumes that low mass galaxies have had a relatively quiet cosmic history, so that their black holes have not undergone significant growth and therefore can be treated as relics of the original SMBH seeds. While recent studies have found optical signatures of active galactic nuclei (AGNs) in a growing population of dwarf galaxies, these studies are biased against low metallicity and relatively merger-free galaxies, thus missing precisely the demographic in which to search for the relics of SMBH seeds. Here, we report the detection of the [\ion{Si}{6}]1.963~$μ$m coronal line (CL), a robust indicator of an AGN in the galaxy SDSS~J160135.95+311353.7, a nearby ($z=0.031$) low metallicity galaxy with a stellar mass approximately an order of magnitude lower than the LMC ($M_*\approx10^{8.56}$~M$_\odot$) and no optical evidence for an AGN. The AGN bolometric luminosity implied by the CL detection is $\approx10^{42}$~erg~s$^{-1}$, precisely what is predicted from its near-infrared continuum emission based on well-studied AGNs. Our results are consistent with a black hole of mass $\approx~10^5$~M$_\odot$, in line with expectations based on its stellar mass. This is the first time a near-infrared CL has been detected in a low mass, low metallicity galaxy with no optical evidence for AGN activity, providing confirmation of the utility of infrared CLs in finding AGNs in low mass galaxies when optical diagnostics fail. These observations highlight a powerful avenue of investigation to hunt for low mass black holes in the JWST era.

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arXiv2021

2MASX J00423991+3017515: An offset active galactic nucleus in an interacting system

We present a spectroscopic and imaging study of an abnormal active galactic nucleus (AGN), 2MASX J00423991+3017515. This AGN is newly identified in the hard X-rays by the Swift BAT All-Sky survey and found in an edge-on disk galaxy interacting with a nearby companion. Here, we analyze the first optical spectra obtained for this system (taken in 2011 and 2016), high-resolution imaging taken with the Hubble Space Telescope and Chandra X-ray Observatory, and 1" imaging with the Very Large Array. Two unique properties are revealed: the peaks of the broad Balmer emission lines (associated with gas orbiting very near the supermassive black hole) are blue shifted from the corresponding narrow line emission and host galaxy absorption by 1540 km/s, and the AGN is spatially displaced from the apparent center of its host galaxy by 3.8 kpc. We explore several scenarios to explain these features, along with other anomalies, and propose that 2MASX J00423991+3017515 may be an AGN with an unusually strong wind residing in a uniquely configured major merger, or that it is an AGN recoiling from either a gravitational "slingshot" in a three-body interaction or from a kick due to the asymmetric emission of gravitational waves following the coalescence of two progenitor supermassive black holes.

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arXiv2021

Massive black hole binary inspiral and spin evolution in a cosmological framework

Massive black hole (MBH) binary inspiral time scales are uncertain, and their spins are even more poorly constrained. Spin misalignment, along with unequal mass ratios and spin magnitudes, introduces asymmetry in the gravitational radiation, which imparts a recoil kick to the merged MBH. Understanding how MBH binary spins evolve is crucial for determining their recoil velocities, their gravitational wave (GW) waveforms detectable with LISA, as well as their post-merger retention rate in galaxies and thus their subsequent merger rate. Here we present a novel study that introduces a sub-resolution model for gas- and GW-driven MBH binary spin evolution using a population of accreting MBHs from the Illustris cosmological hydrodynamics simulations. We also model sub-resolution binary inspiral via dynamical friction, stellar scattering, viscous gas drag, and GW emission. Our model assumes differential accretion, which causes greater alignment of the secondary MBH spin in unequal-mass mergers. We find that 47% of the MBHs in our population merge by $z=0$. Of these, 19% have misaligned primaries and 10% have misaligned secondaries at the time of merger in our (conservative) fiducial model. The MBH misalignment fraction depends strongly on the accretion disc parameters, however. Reducing accretion rates by a factor of 100, in a thicker disc, yields 79% and 42% misalignment for primaries and secondaries, respectively. Even in the fiducial model, more than 12% of binaries experience recoils of $>500$ km s$^{-1}$, which could displace them at least temporarily from galactic nuclei. We additionally find that a significant number of systems experience strong precession.

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arXiv2020

Supermassive black hole fueling in IllustrisTNG: Impact of environment

We study the association between active galactic nuclei (AGN) and environment at scales of $0.01-1\ h^{-1}$Mpc in the IllustrisTNG (TNG100) simulated universe. We identify supermassive black hole (BH) pairs and multiples within scales of 0.01, 0.1, & 1 $h^{-1}$Mpc and examine their AGN activity in relation to randomly-selected pairs and multiples. The number density of BHs in TNG100 is $n=0.06\,h^3$Mpc$^{-3}$ at $z\lesssim1.5$ ($n=0.02\,h^3$ Mpc$^{-3}$ at $z=3$). About $\sim10$% and $\sim1$% of them live in pairs and multiples, respectively, within 0.1 $h^{-1}$Mpc scales. We find that BH systems have enhanced likelihood (up to factors of 3-6) of containing high Eddington ratio ($η\gtrsim0.7$) AGN compared to random pairs and multiples. Conversely, the likelihood of an AGN to live in 0.1$h^{-1}$Mpc scale systems is also higher (by factors $\sim4$ for $η\gtrsim0.7$) compared to random pairs and multiples. We also estimate that $\sim10$% of ultra-hard X-ray selected AGN in TNG100 have detectable 2-10 keV AGN companions on $0.1\ h^{-1}$Mpc scales, in agreement with observations. On larger spatial scales ($\sim 1$ $h^{-1}$Mpc), however, no significant enhancement is associated with BH pairs and multiples, even at high Eddington ratios. The enhancement of AGN activity in rich, small-scale ($\lesssim0.1$ $h^{-1}$Mpc) environments is therefore likely to be driven by galaxy interactions and mergers. Nonetheless, the overall percentage of AGN that live in $\lesssim0.1$ $h^{-1}$Mpc scale multiples is still subdominant (at most $\sim40$% for the highest Eddington ratio AGN). Furthermore, the enhancement in Eddington ratios of BH systems(as well as merging BHs) is only up to factors of $\sim2-3$. Thus, our results support the existence of a merger-AGN connection, but they also suggest that mergers and interactions play a relatively minor role in fueling the AGN population as a whole.

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arXiv2020

The X-ray View of Merger-Induced AGN Activity at Low Redshift

Galaxy mergers are predicted to trigger accretion onto the central supermassive black holes, with the highest rates occurring during final coalescence. Previously, we have shown elevated rates of both optical and mid-IR selected active galactic nuclei (AGN) in post-mergers, but to date the prevalence of X-ray AGN has not been examined in the same systematic way. We present XMM-Newton data of 43 post-merger galaxies selected from the Sloan Digital Sky Survey along with 430 non-interacting control galaxies matched in stellar mass, redshift, and environment in order to test for an excess of hard X-ray (2-10 keV) emission in post-mergers attributable to triggered AGN. We find 2 X-ray detections in the post-mergers (4.7^{+9.3}_{-3.8}%) and 9 in the controls (2.1^{+1.5}_{-1.0}%), an excess of 2.22^{+4.44}_{-2.22}, where the confidence intervals are 90%. While we therefore do not find statistically significant evidence for an X-ray AGN excess in post-mergers (p = 0.26), we find a factor of ~17 excess of mid-IR AGN in our sample, consistent with past work and inconsistent with the observed X-ray excess (p = 2.7 x 10^{-4}). Dominant, luminous AGN are therefore more frequent in post-mergers, and the lack of a comparable excess of 2-10 keV X-ray AGN suggests that AGN in post-mergers are more likely to be heavily obscured. Our results are consistent with the post-merger stage being characterised by enhanced AGN fueling, heavy AGN obscuration, and more intrinsically luminous AGN, in line with theoretical predictions.

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arXiv2020

The BAT AGN Spectroscopic Survey -- XVIII. Searching for Supermassive Black Hole Binaries in the X-rays

Theory predicts that a supermassive black hole binary (SMBHB) could be observed as a luminous active galactic nucleus (AGN) that periodically varies on the order of its orbital timescale. In X-rays, periodic variations could be caused by mechanisms including relativistic Doppler boosting and shocks. Here we present the first systematic search for periodic AGNs using $941$ hard X-ray light curves (14-195 keV) from the first 105 months of the Swift Burst Alert Telescope (BAT) survey (2004-2013). We do not find evidence for periodic AGNs in Swift-BAT, including the previously reported SMBHB candidate MCG+11$-$11$-$032. We find that the null detection is consistent with the combination of the upper-limit binary population in AGNs in our adopted model, their expected periodic variability amplitudes, and the BAT survey characteristics. We have also investigated the detectability of SMBHBs against normal AGN X-ray variability in the context of the eROSITA survey. Under our assumptions of a binary population and the periodic signals they produce which have long periods of hundreds of days, up to $13$% true periodic binaries can be robustly distinguished from normal variable AGNs with the ideal uniform sampling. However, we demonstrate that realistic eROSITA sampling is likely to be insensitive to long-period binaries because longer observing gaps reduce their detectability. In contrast, large observing gaps do not diminish the prospect of detecting binaries of short, few-day periods, as 19% can be successfully recovered, the vast majority of which can be identified by the first half of the survey.

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arXiv2020

Multi-wavelength observations of SDSS J105621.45+313822.1, a broad-line, low-metallicity AGN

In contrast to massive galaxies with Solar or super-Solar gas phase metallicities, very few Active Galactic Nuclei (AGN) are found in low-metallicity dwarf galaxies. Such a population could provide insight into the origins of supermassive black holes. Here we report near-infrared spectroscopic and X-ray observations of SDSS J105621.45+313822.1, a low-mass, low-metallicity galaxy with optical narrow line ratios consistent with star forming galaxies but a broad H$α$ line and mid-infrared colors consistent with an AGN. We detect the [Si VI] 1.96$μ$m coronal line and a broad Pa$α$ line with a FWHM of $850 \pm 25$~km~s$^{-1}$. Together with the optical broad lines and coronal lines seen in the SDSS spectrum, we confirm the presence of a highly accreting black hole with mass $(2.2 \pm 1.3) \times 10^{6}$~M$_{\odot}$, with a bolometric luminosity of $\approx10^{44}$~erg~s$^{-1}$ based on the coronal line luminosity, implying a highly accreting AGN. Chandra observations reveal a weak nuclear point source with $L_{\textrm{X,2-10 keV}} = (2.3 \pm 1.2) \times 10^{41}$~erg~s$^{-1}$, $\sim 2$ orders of magnitude lower than that predicted by the mid-infrared luminosity, suggesting that the AGN is highly obscured despite showing broad lines in the optical spectrum. The low X-ray luminosity and optical narrow line ratios of J1056+3138 highlight the limitations of commonly employed diagnostics in the hunt for AGNs in the low metallicity low mass regime.

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arXiv2019

Probing Massive Black Hole Binary Populations with LISA

ESA and NASA are moving forward with plans to launch LISA around 2034. With data from the Illustris cosmological simulation, we provide analysis of LISA detection rates accompanied by characterization of the merging massive black hole population. Massive black holes of total mass $\sim10^5-10^{10} M_\odot$ are the focus of this study. We evolve Illustris massive black hole mergers, which form at separations on the order of the simulation resolution ($\sim$kpc scales), through coalescence with two different treatments for the binary massive black hole evolutionary process. The coalescence times of the population, as well as physical properties of the black holes, form a statistical basis for each evolutionary treatment. From these bases, we Monte Carlo synthesize many realizations of the merging massive black hole population to build mock LISA detection catalogs. We analyze how our massive black hole binary evolutionary models affect detection rates and the associated parameter distributions measured by LISA. With our models, we find massive black hole binary detection rates with LISA of $\sim0.5-1$ yr$^{-1}$ for massive black holes with masses greater than $10^5M_\odot$. This should be treated as a lower limit primarily because our massive black hole sample does not include masses below $10^5M_\odot$, which may significantly add to the observed rate. We suggest reasons why we predict lower detection rates compared to much of the literature.

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arXiv2019

A Triple AGN in a Mid-Infrared Selected Late Stage Galaxy Merger

The co-evolution of galaxies and the supermassive black holes (SMBHs) at their centers via hierarchical galaxy mergers is a key prediction of $Λ$CDM cosmology. As gas and dust are funneled to the SMBHs during the merger, the SMBHs light up as active galactic nuclei (AGNs). In some cases, a merger of two galaxies can encounter a third galaxy, leading to a triple merger, which would manifest as a triple AGN if all three SMBHs are simultaneously accreting. Using high-spatial resolution X-ray, near-IR, and optical spectroscopic diagnostics, we report here a compelling case of an AGN triplet with mutual separations < 10 kpc in the advanced merger SDSS J084905.51+111447.2 at z = 0.077. The system exhibits three nuclear X-ray sources, optical spectroscopic line ratios consistent with AGN in each nucleus, a high excitation near-IR coronal line in one nucleus, and broad Pa$α$ detections in two nuclei. Hard X-ray spectral fitting reveals a high column density along the line of sight, consistent with the picture of late-stage mergers hosting heavily absorbed AGNs. Our multiwavelength diagnostics support a triple AGN scenario, and we rule out alternative explanations such as star formation activity, shock-driven emission, and emission from fewer than three AGN. The dynamics of gravitationally bound triple SMBH systems can dramatically reduce binary SMBH inspiral timescales, providing a possible means to surmount the "Final Parsec Problem." AGN triplets in advanced mergers are the only observational forerunner to bound triple SMBH systems and thus offer a glimpse of the accretion activity and environments of the AGNs prior to the gravitationally-bound triple phase.

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arXiv2019

Buried Black Hole Growth in IR-selected Mergers: New Results from Chandra

Observations and theoretical simulations suggest that a significant fraction of merger-triggered accretion onto supermassive black holes is highly obscured, particularly in late-stage galaxy mergers, when the black hole is expected to grow most rapidly. Starting with the Wide-Field Infrared Survey Explorer all-sky survey, we identified a population of galaxies whose morphologies suggest ongoing interaction and which exhibit red mid-infrared colors often associated with powerful active galactic nuclei (AGNs). In a follow-up to our pilot study, we now present Chandra/ACIS and XMM-Newton X-ray observations for the full sample of the brightest 15 IR-preselected mergers. All mergers reveal at least one nuclear X-ray source, with 8 out of 15 systems exhibiting dual nuclear X-ray sources, highly suggestive of single and dual AGNs. Combining these X-ray results with optical line ratios and with near-IR coronal emission line diagnostics, obtained with the near-IR spectrographs on the Large Binocular Telescope, we confirm that 13 out of the 15 mergers host AGNs, two of which host dual AGNs. Several of these AGNs are not detected in the optical. All X-ray sources appear X-ray weak relative to their mid-infrared continuum, and of the nine X-ray sources with sufficient counts for spectral analysis, eight reveal strong evidence of high absorption with column densities of $N_\mathrm{H} \gtrsim 10^{23}$~cm$^{-2}$. These observations demonstrate that a significant population of single and dual AGNs are missed by optical studies, due to high absorption, adding to the growing body of evidence that the epoch of peak black hole growth in mergers occurs in a highly obscured phase.

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arXiv2019

Cosmic evolution of supermassive black holes: A view into the next two decades

The discoveries made over the past 20 years by Chandra and XMM-Newton surveys in conjunction with multiwavelength imaging and spectroscopic data available in the same fields have significantly changed the view of the supermassive black hole (SMBH) and galaxy connection. These discoveries have opened up several exciting questions that are beyond the capabilities of current X-ray telescopes and will need to be addressed by observatories in the next two decades. As new observatories peer into the early Universe, we will begin to understand the physics and demographics of SMBH infancy (at $z>6$) and investigate the influence of their accretion on the formation of the first galaxies ($§$ 2.1). We will also be able to understand the accretion and evolution over the cosmic history (at $z\sim$1-6) of the full population of black holes in galaxies, including low accretion rate, heavily obscured AGNs at luminosities beyond the reach of current X-ray surveys ($§$2.2 and $§$2.3), enabling us to resolve the connection between SMBH growth and their environment.

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arXiv2019

Detecting Offset Active Galactic Nuclei

Gravitational wave (GW) and gravitational slingshot recoil kicks, which are natural products of SMBH evolution in merging galaxies, can produce active galactic "nuclei" that are offset from the centers of their host galaxies. Detections of offset AGN would provide key constraints on SMBH binary mass and spin evolution and on GW event rates. Although numerous offset AGN candidates have been identified, none have been definitively confirmed. Multi-wavelength observations with next-generation telescopes, including systematic large-area surveys, will provide unprecedented opportunities to identify and confirm candidate offset AGN from sub-parsec to kiloparsec scales. We highlight ways in which these observations will open a new avenue for multi-messenger studies in the dawn of low-frequency (~ nHz - mHz) GW astronomy.

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arXiv2019

Multi-Messenger Astrophysics with Pulsar Timing Arrays

Pulsar timing arrays (PTAs) are on the verge of detecting low-frequency gravitational waves (GWs) from supermassive black hole binaries (SMBHBs). With continued observations of a large sample of millisecond pulsars, PTAs will reach this major milestone within the next decade. Already, SMBHB candidates are being identified by electromagnetic surveys in ever-increasing numbers; upcoming surveys will enhance our ability to detect and verify candidates, and will be instrumental in identifying the host galaxies of GW sources. Multi-messenger (GW and electromagnetic) observations of SMBHBs will revolutionize our understanding of the co-evolution of SMBHs with their host galaxies, the dynamical interactions between binaries and their galactic environments, and the fundamental physics of accretion. Multi-messenger observations can also make SMBHBs 'standard sirens' for cosmological distance measurements out to $z\simeq0.5$. LIGO has already ushered in breakthrough insights in our knowledge of black holes. The multi-messenger detection of SMBHBs with PTAs will be a breakthrough in the years $2020-2030$ and beyond, and prepare us for LISA to help complete our views of black hole demographics and evolution at higher redshifts.

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