arXiv subjects

Fazeel Mahmood Khan

Publications and source records attributed to Fazeel Mahmood Khan.

21 records · Page 2Linked to original sources

arXiv2015

Scattering experiments meet N-body I: a practical recipe for the evolution of massive black hole binaries in stellar environments

The N-independence observed in the evolution of massive black hole binaries (MBHBs) in recent simulation of merging stellar bulges suggests a simple interpretation beyond complex time-dependent relaxation processes. We conjecture that the MBHB hardening rate is equivalent to that of a binary immersed in a field of unbound stars with density $ρ$ and typical velocity $σ$, provided that $ρ$ and $σ$ are the stellar density and the velocity dispersion at the influence radius of the MBHB. By comparing direct N-body simulations to an hybrid model based on 3-body scattering experiments, we verify this hypothesis: when normalized to the stellar density and velocity dispersion at the binary influence radius, the N-body MBHB hardening rate approximately matches that predicted by 3-body scatterings in the investigated cases. The eccentricity evolution obtained with the two techniques is also in reasonable agreement. This result is particularly practical because it allows to estimate the lifetime of MBHBs forming in dry mergers based solely on the stellar density profile of the host galaxy. We briefly discuss some implications of our finding for the gravitational wave signal observable by pulsar timing arrays and for the expected population of MBHBs lurking in massive ellipticals.

astro-ph.GA↗

arXiv2012

Formation and Hardening of Supermassive Black Hole Binaries in Minor Mergers of Disk Galaxies

We model for the first time the complete orbital evolution of a pair of Supermassive Black Holes (SMBHs) in a 1:10 galaxy merger of two disk dominated gas-rich galaxies, from the stage prior to the formation of the binary up to the onset of gravitational wave emission when the binary separation has shrunk to 1 milli parsec. The high-resolution smoothed particle hydrodynamics (SPH) simulations used for the first phase of the evolution include star formation, accretion onto the SMBHs as well as feedback from supernovae explosions and radiative heating from the SMBHs themselves. Using the direct N-body code ϕ-GPU we evolve the system further without including the effect of gas, which has been mostly consumed by star formation in the meantime. We start at the time when the separation between two SMBHs is ~ 700 pc and the two black holes are still embedded in their galaxy cusps. We use 3 million particles to study the formation and evolution of the SMBH binary till it becomes hard. After a hard binary is formed, we reduce (reselect) the particles to 1.15 million and follow the subsequent shrinking of the SMBH binary due to 3-body encounters with the stars. We find approximately constant hardening rates and that the SMBH binary rapidly develops a high eccentricity. Similar hardening rates and eccentricity values are reported in earlier studies of SMBH binary evolution in the merging of dissipation-less spherical galaxy models. The estimated coalescence time is ~ 2.9 Gyr, significantly smaller than a Hubble time. We discuss why this timescale should be regarded as an upper limit. Since 1:10 mergers are among the most common interaction events for galaxies at all cosmic epochs, we argue that several SMBH binaries should be detected with currently planned space-borne gravitational wave interferometers, whose sensitivity will be especially high for SMBHs in the mass range considered here.

astro-ph.CO↗

arXiv2012

Mergers of Unequal Mass Galaxies: Supermassive Black Hole Binary Evolution and Structure of Merger Remnants

Galaxy centers are residing places for Super Massive Black Holes (SMBHs). Galaxy mergers bring SMBHs close together to form gravitationally bound binary systems which, if able to coalesce in less than a Hubble time, would be one of the most promising sources of gravitational waves for the Laser Interferometer Space Antenna (LISA). In spherical galaxy models, SMBH binaries stall at a separation of approximately one parsec, leading to the "final parsec problem" (FPP). On the other hand, it has been shown that merger-induced triaxiality of the remnant in equal-mass mergers is capable of supporting a constant supply of stars on so-called centrophilic orbits that interact with the binary and thus avoid the FPP. In this paper, using a set of direct N-body simulations of mergers of initially spherically symmetric galaxies with different mass ratios, we show that the merger-induced triaxiality is able to drive unequal-mass SMBH binaries to coalescence. The binary hardening rates are high and depend only weakly on the mass ratios of SMBHs for a wide range of mass ratios q. The hardening rates are significantly higher for galaxies having steep cusps in comparison with those having shallow cups at centers. The evolution of the binary SMBH leads to relatively shallower inner slopes at the centers of the merger remnants. The stellar mass displaced by the SMBH binary on its way to coalescence is ~ 1-5 times the combined mass of binary SMBHs. The coalescence times for SMBH binary with mass ~ million solar masses are less than 1 Gyr and for those at the upper end of SMBH masses (~ billion solar masses) are 1-2 Gyr for less eccentric binaries whereas less than 1 Gyr for highly eccentric binaries. SMBH binaries are thus expected to be promising sources of gravitational waves at low and high redshifts.

astro-ph.CO↗