arXiv subjects

Ann-Marie Madigan

Publications and source records attributed to Ann-Marie Madigan.

At least 19 recordsLinked to original sources

arXiv2026

JWST Observations of the Double Nucleus in NGC 4486B: Possible Evidence for a Recent Binary SMBH Merger and Recoil

A recent study of the compact elliptical galaxy NGC 4486B using JWST-NIRSpec IFU kinematics confirmed a supermassive black hole (SMBH) of mass $M_{BH}=3.6\pm0.7\times10^8$ (~8% of the stellar mass). In addition to its double nucleus, the nuclear kinematics show pronounced asymmetries: a velocity-dispersion peak displaced by 6 pc from the galaxy center and a ~16 km/s offset in the mean stellar line-of-sight velocity near the SMBH. We examine the origin of the 12 pc double nucleus and these asymmetries and show that the observations favor an SMBH surrounded by an eccentric nuclear disk (END). END formation models require the SMBH to experience a gravitational wave (GW) recoil following a binary SMBH merger. Our orbit-superposition models contain ~50% retrograde stars at the edge of the nuclear region, in striking agreement with END-formation simulations. We infer a pre-merger mass ratio q>0.15 and a recoil kick of ~340 km/s. Our N-body simulations show that with such a kick, the SMBH returns to the center within ~30 Myr. Its flat central core is also consistent with earlier binary black hole scouring. We test two alternative mechanisms-buoyancy-driven oscillations and a pre-merger SMBH binary-but neither reproduces the observed offsets, favoring the GW-kick scenario. Our direct N-body simulations further show that a prograde SMBH binary in a rotating host can stall in a corotation resonance, delaying coalescence. Thus, although NGC 4486B is an old, relaxed galaxy near the Virgo cluster center, its SMBH appears to have merged only recently, making its nucleus a rare nearby laboratory for studying post-merger SMBH dynamics.

astro-ph.GA↗

arXiv2025

A JWST View of the Overmassive Black Hole in NGC 4486B

We present a new stellar dynamical measurement of the supermassive black hole (SMBH) in the compact elliptical galaxy NGC 4486B, based on integral field spectroscopy with JWST/NIRSpec. The two-dimensional kinematic maps reveal a resolved double nucleus and a velocity dispersion peak offset from the photometric center. Utilizing two independent methods-Schwarzschild orbit-superposition and Jeans Anisotropic Modeling-we tightly constrain the black hole mass by fitting the full line-of-sight velocity distribution. Our axisymmetric Schwarzschild models yield a best-fit black hole mass of $M_{BH} = 3.6^{+0.7}_{-0.7} \times 10^8 \, M_{\odot}$, slightly lower but significantly more precise than previous estimates. However, since our models do not account for the non-equilibrium nature of the double nucleus, this value may represent a lower limit. Across all tested dynamical models, the inferred $M_{BH}/M_*$ ratio ranges from ~ 4-13%, providing robust evidence for an overmassive SMBH in NGC 4486B. Combined with the galaxy's location deep within the Virgo Cluster, our results support the interpretation that NGC 4486B is the tidally stripped remnant core of a formerly massive galaxy. As the JWST/NIRSpec field of view is insufficient to constrain the dark matter halo, we incorporate archival ground-based long-slit kinematics extending to 5 arcsec. While this provides some leverage on the dark matter content, the constraints remain relatively weak. We place only an upper limit on the dark matter fraction, with $M_{DM}/M_{*} < 0.5$ within 1 kpc-well beyond the effective radius. The inferred black hole mass remains unchanged with or without a dark matter halo.

astro-ph.GA↗

arXiv2024

On the Formation of S-stars from a Recent Massive Black Hole Merger in the Galactic Center

The Galactic Center hosts a rotating disk of young stars between 0.05 and 0.5 pc of Sgr A*. The ``S-stars'' at a distance $<0.04$ pc, however, are on eccentric orbits with nearly isotropically distributed inclinations. The dynamical origin of the S-star cluster has remained a theoretical challenge. Using a series of $N$-body simulations, we show that a recent massive black hole merger with Sgr A* can self-consistently produce many of the orbital properties of the Galactic nuclear star cluster within 0.5 pc. A black hole merger results in a gravitational wave recoil kick, which causes the surrounding cluster to form an apse-aligned, eccentric disk. We show that stars near the inner edge of an eccentric disk migrate inward and are driven to high eccentricities and inclinations due to secular torques similar to the eccentric Kozai-Lidov mechanism. In our fiducial model, starting with a thin eccentric disk with $e = 0.3$, the initially unoccupied region within $0.04$ pc is populated with high eccentricity, high inclination S-stars within a few Myr. This dynamical channel would suggest that a black hole of mass $2^{+3}_{-1.2} \times 10^5 \ M_{\odot}$ merged with Sgr A* within the last 10 Myr.

astro-ph.GA↗

arXiv2024

Counter-Rotation and Slow Precession in Aligned Eccentric Nuclear Disks due to Gravitational Wave Recoil Kicks

The M31 nucleus contains a supermassive black hole embedded in a massive stellar disk of apsidally-aligned eccentric orbits. It has recently been shown that this disk is slowly precessing at a rate consistent with zero. Here we demonstrate using N-body methods that apsidally-aligned eccentric disks can form with a significant (~0.5) fraction of orbits counter-rotating as the result of a gravitational wave recoil kick of merging supermassive black holes. Higher amplitude kicks map to a larger retrograde fraction in the surrounding stellar population which in turns results in slow precession. We furthermore show that disks with significant counter-rotation are more stable (that is, apsidal-alignment is most pronounced and long lasting), more eccentric, and have the highest rates of stars entering the black hole's tidal disruption radius

astro-ph.GA↗

arXiv2024

Tidal Disruption of Planetesimals from an Eccentric Debris Disk Following a White Dwarf Natal Kick

The surfaces of many white dwarfs are polluted by metals, implying a recent accretion event. The tidal disruption of planetesimals is a viable source of white dwarf pollution and offers a unique window into the composition of exoplanet systems. The question of how planetary material enters the tidal disruption radius of the white dwarf is currently unresolved. Using a series of $N$-body simulations, we explore the response of the surrounding planetesimal debris disk as the white dwarf receives a natal kick caused by anisotropic mass loss on the asymptotic giant branch. We find that the kick can form an apse-aligned, eccentric debris disk in the range 30 to 240 AU which corresponds to the orbits of Neptune, the Kuiper Belt, and the scattered disk in our solar system. In addition, many planetesimals beyond 240 AU flip to counter-rotating orbits. Assuming an isotropic distribution of kicks, we predict that approximately 80% of white dwarf debris disks should exhibit significant apsidal alignment and fraction of counter-rotating orbits. The eccentric disk is able to efficiently and continuously torque planetesimals onto radial, star-grazing orbits. We show that the kick causes both an initial burst in tidal disruption events as well as an extended period of 100 Myr where tidal disruption rates are consistent with observed mass accretion rates on polluted white dwarfs.

astro-ph.EP↗

arXiv2023

How to Flip a Bar

Galactic bars, made up of elongated and aligned stellar orbits, can lose angular momentum via resonant torques with dark matter particles in the halo and slow down. Here we show that if a stellar bar is decelerated to zero rotation speed, it can flip the sign of its angular momentum and reverse rotation direction. We demonstrate this in a collisionless N-body simulation of a galaxy in a live counter-rotating halo. Reversal begins at small radii and propagates outward. The flip generates a kinematically-decoupled core both in the visible galaxy and in the dark matter halo, and counter-rotation generates a large-scale warp of the outer disk with respect to the bar.

astro-ph.GA↗

arXiv2023

Anisotropic Star Clusters around Recoiling Supermassive Black Holes

Gravitational wave recoil kicks from merging supermassive black hole binaries can have a profound effect on the surrounding stellar population. In this work, we study the dynamic and kinematic properties of nuclear star clusters following a recoil kick. We show that these post-kick structures present unique signatures that can provide key insight to observational searches for recoiling supermassive black holes. In Akiba & Madigan (2021), we showed that an in-plane recoil kick turns a circular disk into a lopsided, eccentric disk such as the one we observe in the Andromeda nucleus. Building on this work, here we explore many recoil kick angles as well as initial stellar configurations. For a circular disk of stars, an in-plane kick causes strong apsidal alignment with a significant fraction of the disk becoming retrograde at large radii. If initial orbits are highly eccentric, an in-plane kick forms a bar-like structure made up of two anti-aligned lopsided disks. An out-of-plane kick causes clustering in the argument of periapsis, $ω$, regardless of the initial eccentricity distribution. Initially isotropic configurations form anisotropies in the form of a torus of eccentric orbits oriented perpendicular to the recoil kick. Post-kick surface density and velocity maps are presented in each case to highlight the distinct, observable structures of these systems.

astro-ph.GA↗

arXiv2023

Steeper Scattered Disks Buckle Faster

Disks of low-mass bodies scattered by giant planets to large semi-major axis and constant periapsis orbits are vulnerable to a buckling instability. This instability exponentially grows orbital inclinations, raises periapsis distances, and coherently tilts orbits resulting in clustering of arguments of periapsis. The dynamically hot system is then susceptible to the formation of a lopsided mode. Here we show that the timescale of the buckling instability decreases as the radial surface density of the population becomes more centrally dense, i.e., steeper scattered disks buckle faster. Accounting for differential apsidal precession driven by giant planets, we find that $\sim\!10\,M_\oplus$ is sufficient for a primordial scattered disk in the trans-Neptunian region to have been unstable if $dN \propto a^{-2.5} da$.

astro-ph.EP↗

arXiv2021

A Lopsided Outer Solar System

Axisymmetric disks of eccentric orbits in near-Keplerian potentials are unstable to an out-of-plane buckling. Recently, Zderic et al. (2020) showed that an idealized disk saturates to a lopsided mode. Here we show that this apsidal clustering also occurs in a primordial scattered disk in the outer solar system which includes the orbit-averaged gravitational influence of the giant planets. We explain the dynamics using Lynden-Bell (1979)'s mechanism for bar formation in galaxies. We also show surface density and line of sight velocity plots at different times during the instability, highlighting the formation of concentric circles and spiral arms in velocity space.

astro-ph.EP↗

arXiv2021

On the Formation of an Eccentric Nuclear Disk following the Gravitational Recoil Kick of a Supermassive Black Hole

The anisotropic emission of gravitational waves during the merger of two supermassive black holes can result in a recoil kick of the merged remnant. We show here that eccentric nuclear disks - stellar disks of eccentric, apse-aligned orbits - can directly form as a result. An initially circular disk of stars will align orthogonal to the black hole kick direction with a distinctive 'tick-mark' eccentricity distribution and a spiral pattern in mean anomaly.

astro-ph.GA↗

arXiv2021

Bar-driven Leading Spiral Arms in a Counter-rotating Dark Matter Halo

An overwhelming majority of galactic spiral arms trail with respect to the rotation of the galaxy, though a small sample of leading spiral arms has been observed. The formation of these leading spirals is not well understood. Here we show, using collisionless $N$-body simulations, that a barred disc galaxy in a retrograde dark matter halo can produce long-lived ($\sim3$ Gyr) leading spiral arms. Due to the strong resonant coupling of the disc to the halo, the bar slows rapidly and spiral perturbations are forced ahead of the bar. We predict that such a system, if observed, will also host a dark matter wake oriented perpendicular to the stellar bar. More generally, we propose that any mechanism that rapidly decelerates the stellar bar will allow leading spiral arms to flourish.

astro-ph.GA↗

arXiv2021

Photometry and Kinematics of Self-Gravitating Eccentric Nuclear Disks

The Andromeda Galaxy hosts an elongated nucleus with (at least) two distinct brightness peaks. The double nucleus can be explained by the projection of a thick, apsidally-aligned eccentric nuclear disk of stars in orbit about the central black hole. Several nearby early-type galaxies have similar asymmetric nuclear features, indicating the possible presence of eccentric nuclear disks. We create simulated photometric (surface density) and kinematic (line-of-sight velocity) maps of eccentric nuclear disks using N-body simulations. We image our simulations from various lines of sight in order to classify them as double nuclei, offset nuclei, and centered nuclei. We explore the effects of mass segregation on the photometric maps, finding that heavier stars are concentrated in the brighter peak. The average line-of-sight velocity values are lower in an eccentric nuclear disk than for a circular ring about the supermassive black hole. The velocity dispersion values are higher and peak at the position of the supermassive black hole, which does not typically match the peak in photometry.

astro-ph.GA↗

arXiv2021

The Coupling of Galactic Dark Matter Halos with Stellar Bars

Resonant torques couple stellar bars to dark matter halos. Here we use high-resolution numerical simulations to demonstrate long-term angular momentum transfer between stellar bars and dark matter orbits of varying orientation. We show that bar-driven reversals of dark matter orbit orientations can play a surprisingly large role in the evolution of the bar pattern speed. In predominantly prograde (co-rotating) halos, dark matter orbits become trapped in the stellar bar forming a parallel dark matter bar. This dark matter bar reaches more than double the vertical height of the stellar bar. In halos dominated by retrograde orbits, a dark matter wake forms oriented perpendicular to the stellar bar. These dark matter over-densities provide a novel space to look for dark matter annihilation or decay signals. % We predict that the Milky Way hosts a dark matter bar aligned with the stellar bar as well as a dark matter wake the near-side of which should extend from Galactic center to a galactic longitude of $l \approx 323^\circ$.

astro-ph.GA↗

arXiv2020

Galactic merger implications for eccentric nuclear disks: a mechanism for disk alignment

The nucleus of our nearest, large galactic neighbor, M31, contains an eccentric nuclear disk--a disk of stars on eccentric, apsidally-aligned orbits around a supermassive black hole (SMBH). Previous studies of eccentric nuclear disks considered only an isolated disk, and did not study their dynamics under galaxy mergers (particularly a perturbing SMBH). Here, we present the first study of how eccentric disks are affected by a galactic merger. We perform N-body simulations to study the disk under a range of different possible SMBH initial conditions. A second SMBH in the disk always disrupts it, but more distant SMBHs can shut off differential precession and stabilize the disk. This results in a more aligned disk, nearly uniform eccentricity profile, and suppression of tidal disruption events compared to the isolated disk. We also discuss implications of our work for the presence of a secondary SMBH in M31.

astro-ph.GA↗

arXiv2020

The Hills Mechanism and the Galactic Center S-stars

Our Galactic center contains young stars, including the few million year old clockwise disk between 0.05 and 0.5 pc from the Galactic center, and the S-star cluster of B-type stars at a galactocentric distance of ~0.01 pc. Recent observations suggest the S-stars are remnants of tidally disrupted binaries from the clockwise disk. In particular, Koposov et al. 2020 discovered a hypervelocity star that was ejected from the Galactic center 5 Myr ago, with a velocity vector consistent with the disk. We perform a detailed study of this binary disruption scenario. First, we quantify the plausible range of binary semimajor axes in the disk. Dynamical evaporation of such binaries is dominated by other disk stars rather than by the isotropic stellar population. For the expected range of semimajor axes in the disk, binary tidal disruptions can reproduce the observed S-star semimajor axis distribution. Reproducing the observed thermal eccentricity distribution of the S-stars requires an additional relaxation process. The flight time of the Koposov star suggests that this process must be effective within 10 Myr. We consider three possibilities: (i) scalar resonant relaxation from the observed isotropic star cluster, (ii) torques from the clockwise disk, and (iii) an intermediate-mass black hole. We conclude that the first and third mechanisms are fast enough to reproduce the observed S-star eccentricity distribution. Finally, we show that the primary star from an unequal-mass binary would be deposited at larger semimajor axes than the secondary, possibly explaining the dearth of O stars among the S-stars.

astro-ph.GA↗

arXiv2020

Giant Planet Influence on the Collective Gravity of a Primordial Scattered Disk

Axisymmetric disks of high eccentricity, low mass bodies on near-Keplerian orbits are unstable to an out-of-plane buckling. This "inclination instability" exponentially grows the orbital inclinations, raises perihelia distances and clusters in argument of perihelion. Here we examine the instability in a massive primordial scattered disk including the orbit-averaged gravitational influence of the giant planets. We show that differential apsidal precession induced by the giant planets will suppress the inclination instability unless the primordial mass is $\gtrsim 20$ Earth masses. We also show that the instability should produce a "perihelion gap" at semi-major axes of hundreds of AU, as the orbits of the remnant population are more likely to have extremely large perihelion distances ($\mathcal{O}(100~\rm{AU})$) than intermediate values.

astro-ph.EP↗

arXiv2020

Apsidal Clustering following the Inclination Instability

Disks of low-mass bodies on high-eccentricity orbits in near-Keplerian potentials can be dynamically unstable to buckling out of the plane. In this letter, we present $N$-body simulations of the long-term behavior of such a system, finding apsidal clustering of the orbits in the disk plane. The timescale over which the clustering is maintained increases with number of particles, suggesting that lopsided configurations are stable at large $N$. This discovery may explain the observed apsidal ($\varpi$) clustering of extreme trans-Neptunian Objects in the outer solar system.

astro-ph.EP↗

arXiv2020

Mass Segregation in Eccentric Nuclear Disks: Enhanced Tidal Disruption Event Rates for High Mass Stars

Eccentric nuclear disks (ENDs) are a type of star cluster in which the stars lie on eccentric, apsidally-aligned orbits in a disk around a central supermassive black hole (SMBH). These disks can produce a high rate of tidal disruption events (TDEs) via secular gravitational torques. Previous studies of ENDs have included stars with only one mass. Here, we present the first study of an eccentric nuclear disk with two stellar species. We show that ENDs show radial mass segregation consistent with previous results from other cluster types. Additionally, ENDs show vertical mass segregation by which the heavy stars sink to lower inclinations than light stars. These two effects cause heavy stars to be more susceptible to tidal disruption, which can be seen in the higher fraction of heavy stars that are disrupted compared to light stars.

astro-ph.GA↗