arXiv subjects

Nader Khonji

Publications and source records attributed to Nader Khonji.

4 recordsLinked to original sources

arXiv2026

A Kiloparsec-Scale Stellar Cavity in the Center of Abell402-BCG May be Caused by Dynamic Interactions with an Ultramassive Black Hole

We present new observations from JWST NIRCam that reveal a striking kpc-wide cavity in the stellar distribution of the central galaxy in the cluster Abell402. Supporting data from HST allow us to rule out extinction due to dust as an explanation and, instead, suggest that this is a localized depression in the stellar density field corresponding to ~2x10^9 Msun in missing stars within a volume of 0.5kpc^3. On larger scales, both the JWST and HST data show evidence for a 2.2kpc flattened core in the stellar distribution (on which the smaller-scale cavity is superimposed), which implies the presence of a central ultra-massive black hole with M_BH = 6 +/- 4 x10^10 Msun. We report evidence for a mid-IR-bright point source at one edge of the cavity, suggesting that this black hole is actively accreting. MUSE spectroscopy reveal that this source is a LINER AGN and that there is a second candidate AGN on the opposite side of the cavity with a relative velocity of 370km/s -- if real, this implies the presence of a kpc-separation dual AGN with a total binary mass of 6 +/- 2 x10^10 Msun, which would make this the most massive binary black hole system discovered to date. We propose that this unique stellar cavity is the result of a short-lived dynamical interaction between at least one supermassive black hole and the background stellar density field, caused either by three-body scattering during binary hardening or the induction of a dipole instability in the stellar density field.

astro-ph.GA↗

arXiv2026

Converging on the eccentricity of massive black hole binaries in galactic mergers

Binaries of supermassive black holes (MBHBs) represent the primary sources of the gravitational wave background (GWB) detectable by Pulsar Timing Arrays (PTAs). The eccentricity with which binaries form in galactic mergers is the key parameter determining their evolutionary timescale from pairing to coalescence. However, accurately determining the binary eccentricity at formation is difficult in simulations due to stochastic effects. We present a numerical study of the formation and evolution of MBHBs that are potential PTA sources. We simulate mergers of equal-mass galaxies on different initial orbits and follow the dynamics of the MBHBs through the hardening phase. We find that low-resolution simulations are affected by stochasticity due to torques from the stellar distribution acting at pericentre passages. The dispersion in binary eccentricity decreases with increasing central resolution, as expected for a Poisson process. We provide a fitting formula for the resolution requirement of an N-body simulation of MBHB formation and evolution as a function of the initial eccentricity of the merger, e0, and the required accuracy in the binary eccentricity, eb. We find that binaries experience a torque at first pericentre that is approximately independent of initial eccentricity, producing a general trend in which the binary eccentricity decreases above sufficiently large initial orbital eccentricities. While this behaviour is generic, the precise cross-over eccentricity (e0~0.97 in our models) and the sharpness of the drop-off depend on the galaxy initial conditions. We provide a fitting formula for eb(e0) that can be used in semi-analytical models to determine the merger timescales of MBHBs as well as the amplitude and slope of the GWB.

astro-ph.GA↗

arXiv2025

Black hole dragging: a new mechanism for forming nuclear star clusters in giant elliptical galaxies

It has long been thought that nuclear star clusters (NSCs) cannot co-exist with the most massive supermassive black holes (SMBHs), since SMBH mergers - unavoidable for the most massive systems - would scatter away NSC stars. However, central concentrations of light have now been reported in up to a third of all massive ellipticals. We present a new mechanism for forming NSCs in giant elliptical galaxies, arising naturally from SMBH mergers, which could explain these observations. We call this ''black hole dragging''. After a major merger of two galaxies and their SMBHs, the newly-merged SMBH can receive a gravitational wave recoil kick. We show that recoiling SMBHs induce two competing effects on the galaxy's background stars. Firstly, some stars become bound to the SMBH and co-move with it, an effect strongest at low recoil velocities. Secondly, background stars are ejected as the recoiling SMBH falls back due to dynamical friction, an effect strongest at high recoil velocities. At intermediate recoil velocities (500-1000 km/s), both effects become important, and the density of bound stars can exceed that of the background stellar core. This yields a central dense NSC that is clearly visible in the galaxy's surface brightness profile. We show that NSCs formed in this way have realistic sizes, masses and velocity dispersions when measured similarly to observed systems. This provides a route for even giant ellipticals containing SMBHs to host an NSC. We predict such NSCs should have indistinguishable colors, ages and chemistry from non-NSC central stars, combined with low ellipticities.

astro-ph.GA↗

arXiv2024

Core formation by binary scouring and gravitational wave recoil in massive elliptical galaxies

Scouring by supermassive black hole (SMBH) binaries is the most accepted mechanism for the formation of the cores seen in giant elliptical galaxies. However, an additional mechanism is required to explain the largest observed cores. Gravitational wave (GW) recoil is expected to trigger further growth of the core, as subsequent heating from dynamical friction of the merged SMBH removes stars from the central regions. We model core formation in massive elliptical galaxies from both binary scouring and heating by GW recoil and examine their unique signatures. We aim to determine if the nature of cores in 3D space density can be attributed uniquely to either process and if the magnitude of the kick can be inferred. We perform $N$-body simulations of galactic mergers of multicomponent galaxies, based on the observed parameters of four massive elliptical galaxies with cores $> 0.5$ kpc. After binary scouring and hardening, the merged SMBH remnant is given a range of GW recoil kicks with $0.5$-$0.9$ of the escape speed of the galaxy. We find that binary scouring alone can form the cores of NGC 1600 and A2147-BCG, which are $< 1.3$ kpc in size. However, the $> 2$ kpc cores in NGC 6166 and A2261-BCG require heating from GW recoil kicks of $< 0.5$ of the galaxy escape speed. A unique feature of GW recoil heating is flatter cores in surface brightness, corresponding to truly flat cores in 3D space density. It also preferentially removes stars on low angular momentum orbits from the galactic nucleus.

astro-ph.GA↗