83 research outputs found
Evolution of massive binary black holes
Since many or most galaxies have central massive black holes (BHs), mergers
of galaxies can form massive binary black holes (BBHs). In this paper, we study
the evolution of massive BBHs in realistic galaxy models, using a
generalization of techniques used to study tidal disruption rates around
massive BHs. The evolution of BBHs depends on BH mass ratio and host galaxy
type. BBHs with very low mass ratios (say, \la 0.001) are hardly ever formed
by mergers of galaxies because the dynamical friction timescale is too long for
the smaller BH to sink into the galactic center within a Hubble time. BBHs with
moderate mass ratios are most likely to form and survive in spherical or nearly
spherical galaxies and in high-luminosity or high-dispersion galaxies; they are
most likely to have merged in low-dispersion galaxies (line-of-sight velocity
dispersion \la 90 km/s) or in highly flattened or triaxial galaxies. The
semimajor axes and orbital periods of surviving BBHs are generally in the range
10^{-3}-10 pc and 10-10^5 yr; and they are larger in high-dispersion galaxies
than in low-dispersion galaxies, larger in nearly spherical galaxies than in
highly flattened or triaxial galaxies, and larger for BBHs with equal masses
than for BBHs with unequal masses. The orbital velocities of surviving BBHs are
generally in the range 10^2-10^4 km/s. The methods of detecting surviving BBHs
are also discussed. If no evidence of BBHs is found in AGNs, this may be either
because gas plays a major role in BBH orbital decay or because nuclear activity
switches on soon after a galaxy merger, and ends before the smaller BH has had
time to spiral to the center of the galaxy.Comment: 32 pages, including 14 figures, submitted to MNRA
Kinematics of hypervelocity stars in the triaxial halo of the Milky Way
Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic
center have unique kinematic properties compared to other halo stars. Their
trajectories will deviate from being exactly radial because of the asymmetry of
the Milky Way potential produced by the flattened disk and the triaxial dark
matter halo, causing a change of angular momentum that can be much larger than
the initial small value at injection. We study the kinematics of HVSs and
propose an estimator of dark halo triaxiality that is determined only by
instantaneous position and velocity vectors of HVSs at large Galactocentric
distances (r>~50kpc). We show that, in the case of a substantially triaxial
halo, the distribution of deflection angles (the angle between the stellar
position and velocity vector) for HVSs on bound orbits is spread uniformly over
the range 10--180deg. Future astrometric and deep wide-field surveys should
measure the positions and velocities of a significant number of HVSs, and
provide useful constraints on the shape of the Galactic dark matter halo.Comment: 10 pages,including 9 figure
Orbital orientation evolution of massive binary black holes at the centres of non-spherical galaxies
At the centre of a spherical and kinematically isotropic galaxy, the
orientation of a massive binary black hole (BBH) orbit (i.e., the direction of
the BBH orbital angular momentum) undergoes a random walk. If the stars in a
spherical system have a non-zero total angular momentum, the BBH orbital
orientation evolves towards aligning with the total stellar angular momentum
direction. In this paper, we show that a triaxial galaxy has an
alignment-erasing effect, that is, the alignment of the BBH orientations
towards the galaxy rotation axis can be decreased significantly or erased. We
also show that in a non-rotating axisymmetric galaxy, the BBH orbital
orientation evolves towards the axisymmetric axis and precesses about it in a
retrograde direction. Our results provide a step towards understanding the spin
orientations of the final merged BH (and hence probable orientation of any jet
produced) within its host galaxy, and may help to constrain the recoiling
velocity of the merged BH arose from gravitational wave radiation as well.Comment: 16 pages, 9 figures, MNRAS accepte
The dynamics of Plutinos
Plutinos are Kuiper-belt objects that share the 3:2 Neptune resonance with
Pluto. The long-term stability of Plutino orbits depends on their eccentricity.
Plutinos with eccentricities close to Pluto (fractional eccentricity difference
|e-e_p|/e_p<=0.1) can be stable because the longitude difference librates, in a
manner similar to the tadpole and horseshoe libration in coorbital satellites.
Plutinos with |e-e_p|/e_p>=0.3 can also be stable; the longitude difference
circulates and close encounters are possible, but the effects of Pluto are weak
because the encounter velocity is high. Orbits with intermediate eccentricity
differences are likely to be unstable over the age of the solar system, in the
sense that encounters with Pluto drive them out of the 3:2 Neptune resonance
and thus into close encounters with Neptune. This mechanism may be a source of
Jupiter-family comets.Comment: 14 pages, 4 gif figures, 9 ps figures, Latex. Submitted to A
Probing baryonic processes and gastrophysics in the formation of the Milky Way dwarf satellites: I. metallicity distribution properties
In this paper, we study the chemical properties of the stars in the dwarf
satellites around the MW-like host galaxies, and explore the possible effects
of several baryonic processes, including supernova (SN) feedback, the
reionization of the universe and H cooling, on them and how current and
future observations may put some constraints on these processes. We use a
semi-analytical model to generate MW-like galaxies, for which a fiducial model
can reproduce the luminosity function and the stellar metallicity--stellar mass
correlation of the MW dwarfs. Using the simulated MW-like galaxies, we focus on
investigating three metallicity properties of their dwarfs: the stellar
metallicity--stellar mass correlation of the dwarf population, and the
metal-poor and metal-rich tails of the stellar metallicity distribution in
individual dwarfs. We find that (1) the slope of the stellar
metallicity--stellar mass correlation is sensitive to the SN feedback strength
and the reionization epoch; (2) the extension of the metal-rich tails is mainly
sensitive to the SN feedback strength; (3) the extension of the metal-poor
tails is mainly sensitive to the reionization epoch; (4) none of the three
chemical properties are sensitive to the H cooling process; and (5)
comparison of our model results with the current observational slope of the
stellar metallicity--stellar mass relation suggests that the local universe is
reionized earlier than the cosmic average and local sources may have a
significant contribution to the reionization in the local region, and an
intermediate to strong SN feedback strength is preferred. Future observations
of metal-rich and metal-poor tails of stellar metallicity distributions will
put further constraints on the SN feedback and the reionization processes.Comment: 22 pages, 16 figures, accepted for publication in the Astrophysical
Journa
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