Cusp Disruption in Minor Mergers (original) (raw)
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Galactic Mergers with Supermassive Black Holes
We present the results of N -body simulations of the accretion of high-density dwarf galaxies by low-density giant galaxies. Both galaxies contain power-law central density cusps and point masses representing supermassive black holes; the ratio of galaxy masses is 3 : 1. The cusp of the dwarf galaxy is always disrupted during the merger, leading to a remnant with a weak power law in the intrinsic density and a "core" in the projected density. Removing both black holes from the giant and dwarf galaxies allows the dwarf galaxy to remain intact and leads to a remnant with a high central density, contrary to what is observed. Our results support the hypothesis than the persistence of low-density cores in giant galaxies following mergers is a consequence of the existence of supermassive central black holes.
Swift Coalescence of Supermassive Black Holes in Cosmological Mergers of Massive Galaxies
The Astrophysical Journal, 2016
Supermassive black holes (SMBHs) are ubiquitous in galaxies with a sizable mass. It is expected that a pair of SMBHs originally in the nuclei of two merging galaxies would form a binary and eventually coalesce via a burst of gravitational waves. So far, theoretical models and simulations, focusing only on limited phases of the orbital decay of SMBHs under idealized conditions of the galaxy hosts, have been unable to directly predict the SMBH merger timescale from ab-initio galaxy formation theory. The predicted SMBH merger timescales are long, of order Gyrs, which could be problematic for future gravitational wave (GW) searches. Here, we present the first multi-scale ΛCDM cosmological simulation that follows the orbital decay of a pair of SMBHs in a merger of two typical massive galaxies atz 3, all the way to the final coalescence driven by GW emission. The two SMBHs, with masses~10 8 M , settle quickly in the nucleus of the merger remnant. The remnant is triaxial and extremely dense due to the dissipative nature of the merger and the intrinsic compactness of galaxies at high redshift. Such properties naturally allow a very efficient hardening of the SMBH binary. The SMBH merger occurs in only ∼10 Myr after the galactic cores have merged, which is two orders of magnitude smaller than the Hubble time.
Black hole fueling and coalescence in galaxy mergers
Astronomische Nachrichten, 2020
We study the gas accretion and orbital decay of super-massive black hole (SMBH) pairs in the nuclear region of galaxy merger remnants, where a large amount of gas is expected to accumulate. For that purpose, we perform numerical simulations with both smooth particle hydro-dynamics (SPH) (Gadget) and adaptive mesh refinement (AMR) (Ramses) techniques. We study the places of the highest SMBH gas accretion, finding that peaks of mass accretion rates are near pericenter passages. We also study the conditions for fast migration and coalescence of SMBHs in galactic nuclei, finding that, in the typical conditions of relatively wet mergers, the SMBHs will experience a fast shrinking, down to the scales where their final coalescence will be driven by the emission of gravitational waves, unless other processes strongly affect the distribution of the gas near the SMBH binary, such as active galactic nuclei (AGN) feedback.
The Astrophysical Journal, 2012
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 also 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. There is, however, an abrupt transition in the hardening rates for mergers with mass ratios somewhere between q ∼ 0.05 and 0.1, resulting from the monotonic decrease of merger-induced triaxiality with mass ratio q, as the secondary galaxy becomes too small and light to significantly perturb the primary, i.e., the more massive one. 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 time scales for SMBH binary with mass ∼ 10 6 M ⊙ are less than 1 Gyr and for those at the upper end of SMBH masses 10 9 M ⊙ 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.
FORMATION AND HARDENING OF SUPERMASSIVE BLACK HOLE BINARIES IN MINOR MERGERS OF DISK GALAXIES
The Astrophysical Journal, 2012
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 (GW) emission when the binary separation has shrunk to 1 mpc. 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 in the mean time has been mostly consumed by star formation. 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 three million particles to study the formation and evolution of the SMBH binary until 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 three-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 were reported in earlier studies of SMBH binary evolution in the merging of dissipationless spherical galaxy models. The estimated coalescence time is ∼5.5 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 GW interferometers, whose sensitivity will be especially high for SMBHs in the mass range considered here.
Major mergers of galaxy haloes: cuspy or cored inner density profile?
Monthly Notices of the Royal …, 2004
We present the results from a series of collisionless N-body simulations of major mergers of galaxy dark matter haloes with density profiles having either inner cusps or cores. Our simulations range from 2 × 10 5 to 10 7 particles, allowing us to probe the phase-space distribution of dark ...
Inspiralling Supermassive Black Holes: A New Signpost for Galaxy Mergers
The Astrophysical Journal, 2009
We present a new technique for observationally identifying galaxy mergers spectroscopically rather than through host galaxy imaging. Our technique exploits the dynamics of supermassive black holes (SMBHs) powering active galactic nuclei (AGNs) in merger-remnant galaxies. Because structure in the universe is built up through galaxy mergers and nearly all galaxies host a central SMBH, some galaxies should possess two SMBHs near their centers as the result of a recent merger. These SMBHs spiral to the center of the resultant merger-remnant galaxy, and one or both of the SMBHs may power AGNs. Using the DEEP2 Galaxy Redshift Survey, we have examined 1881 red galaxies, of which 91 exhibit [O III] and Hβ emission lines indicative of Seyfert 2 activity. Of these, 32 AGNs have [O III] emission-line redshifts significantly different from the redshifts of the host galaxies' stars, corresponding to velocity offsets of ∼ 50 km s −1 to ∼ 300 km s −1. Two of these AGNs exhibit doublepeaked [O III] emission lines, while the remaining 30 AGNs each exhibit a single set of velocity-offset [O III] emission lines. After exploring a variety of physical models for these velocity offsets, we argue that the most likely explanation is inspiralling SMBHs in merger-remnant galaxies. Based on this interpretation, we find that roughly half of the red galaxies hosting AGNs are also merger remnants, which implies that mergers may trigger AGN activity in red galaxies. The AGN velocity offsets we find imply a merger fraction of ∼ 30% and a merger rate of ∼ 3 mergers Gyr −1 for red galaxies at redshifts 0.34 < z < 0.82.
2008
Binary supermassive black holes form naturally in galaxy mergers, but their long-term evolution is uncertain. In spherical galaxies, N-body simulations show that binary evolution stalls at separations much too large for significant emission of gravitational waves (the “final parsec problem”). Here, we follow the long-term evolution of a massive binary in more realistic, triaxial and rotating galaxy models. We find that the binary does not stall. The binary hardening rates that we observe are sufficient to allow complete coalescence of binary SBHs in 10 Gyr or less, even in the absence of collisional loss-cone refilling or gas-dynamical torques, thus providing a potential solution to the final parsec problem. Subject headings: 1.
Evolution of the mass, size, and star formation rate in high redshift merging galaxies
Astronomy & Astrophysics, 2014
Context. In Λ-CDM models, galaxies are thought to grow both through continuous cold gas accretion coming from the cosmic web and episodic merger events. The relative importance of these different mechanisms at different cosmic epochs is nevertheless not yet well understood. Aims. We aim at addressing the questions related to galaxy mass assembly through major and minor wet merging processes in the redshift range 1 < z < 2, an epoch corresponding to the peak of the cosmic star formation history. A significant fraction of Milky Way-like galaxies are thought to have undergone an unstable clumpy phase at this early stage. We focus on the behavior of the young clumpy disks when galaxies are undergoing gas-rich galaxy mergers. Methods. Using the adaptive mesh refinement code RAMSES, we build the Merging and Isolated high-Redshift Adaptive mesh refinement Galaxies (MIRAGE) sample. It is composed of 20 mergers and 3 isolated idealized disks simulations, which sample disk orientations and merger masses. Our simulations can reach a physical resolution of 7 parsecs, and include: star formation, metal line cooling, metallicity advection, and a recent physically-motivated implementation of stellar feedback which encompasses OB-type stars radiative pressure, photo-ionization heating, and supernovae. Results. The star formation history of isolated disks shows stochastic star formation rate, which proceeds from the complex behavior of the giant clumps. Our minor and major gas-rich merger simulations do not trigger starbursts, suggesting a saturation of the star formation due to the detailed accounting of stellar feedback processes in a turbulent and clumpy interstellar medium fed by substantial accretion from the circum-galactic medium. Our simulations are globally close to the normal regime of the disk-like star formation on a Schmidt-Kennicutt diagram. The mass-size relation and its rate of evolution in the redshift range 1 < z < 2 matches observations, suggesting that the inside-out growth mechanisms of the stellar disk do not necessarily require to be achieved through a cold accretion.
Efficient Merger of Binary Supermassive Black Holes in Nonaxisymmetric Galaxies
The Astrophysical Journal, 2006
Binary supermassive black holes form naturally in galaxy mergers, but their long-term evolution is uncertain. In spherical galaxies, N-body simulations show that binary evolution stalls at separations much too large for significant emission of gravitational waves (the "final parsec problem"). Here, we follow the long-term evolution of a massive binary in more realistic, triaxial and rotating galaxy models. We find that the binary does not stall. The binary hardening rates that we observe are sufficient to allow complete coalescence of binary SBHs in 10 Gyr or less, even in the absence of collisional loss-cone refilling or gas-dynamical torques, thus providing a potential solution to the final parsec problem.