214 research outputs found
Consequences of Mechanical and Radiative Feedback from Black Holes in Disc Galaxy Mergers
We study the effect of AGN mechanical and radiation feedback on the formation
of bulge dominated galaxies via mergers of disc galaxies. The merging galaxies
have mass-ratios of 1:1 to 6:1 and include pre-existing hot gaseous halos to
properly account for the global impact of AGN feedback. Using smoothed particle
hydrodynamics simulation code (GADGET-3) we compare three models with different
AGN feedback models: (1) no black hole and no AGN feedback; (2) thermal AGN
feedback; and (3) mechanical and radiative AGN feedback. The last model is
motivated by observations of broad line quasars which show winds with initial
velocities of 10,000 km/s and also heating associated with the
central AGN X-ray radiation. The primary changes in gas properties due to
mechanical AGN feedback are lower thermal X-ray luminosity from the final
galaxy - in better agreement with observations - and galactic outflows with
higher velocity km/s similar to recent direct observations of
nearby merger remnants. The kinetic energy of the outflowing gas is a factor of
20 higher than in the thermal feedback case. All merger remnants with
momentum-based AGN feedback with km/s and , independent of their progenitor mass-ratios, reproduce the
observed relations between stellar velocity dispersion and black hole mass
() as well as X-ray luminosity () with
erg/s erg/s for
velocity dispersions in the range of 120 km/s 190
km/s. In addition, the mechanical feedback produces a much greater AGN
variability. We also show that gas is more rapidly and impulsively stripped
from the galactic centres driving a moderate increase in galaxy size and
decrease in central density with the mechanical AGN feedback model.Comment: 16 pages, 10 figures, resubmitted to MNRA
The Dark Halo - Spheroid Conspiracy and the Origin of Elliptical Galaxies
Dynamical modeling and strong lensing data indicate that the total density
profiles of early-type galaxies are close to isothermal, i.e., rho_tot ~
r^gamma with gamma approx -2. To understand the origin of this universal slope
we study a set of simulated spheroids formed in isolated binary mergers as well
as the formation within the cosmological framework. The total stellar plus dark
matter density profiles can always be described by a power law with an index of
gamma approx -2.1 with a tendency toward steeper slopes for more compact,
lower-mass ellipticals. In the binary mergers the amount of gas involved in the
merger determines the precise steepness of the slope. This agrees with results
from the cosmological simulations where ellipticals with steeper slopes have a
higher fraction of stars formed in situ. Each gas-poor merger event evolves the
slope toward gamma ~ -2, once this slope is reached further merger events do
not change it anymore. All our ellipticals have flat intrinsic combined stellar
and dark matter velocity dispersion profiles. We conclude that flat velocity
dispersion profiles and total density distributions with a slope of gamma ~ -2
for the combined system of stars and dark matter act as a natural attractor.
The variety of complex formation histories as present in cosmological
simulations, including major as well as minor merger events, is essential to
generate the full range of observed density slopes seen for present-day
elliptical galaxies.Comment: Accepted by the Astrophysical Journal, 17 pages, 12 figure
The Effects of X-Ray Feedback from AGN on Host Galaxy Evolution
Hydrodynamic simulations of galaxies with active galactic nuclei (AGN) have
typically employed feedback that is purely local: i.e., an injection of energy
to the immediate neighborhood of the black hole. We perform GADGET-2
simulations of massive elliptical galaxies with an additional feedback
component: an observationally calibrated X-ray radiation field which emanates
from the black hole and heats gas out to large radii from the galaxy center. We
find that including the heating and radiation pressure associated with this
X-ray flux in our simulations enhances the effects which are commonly reported
from AGN feedback. This new feedback model is twice as effective as traditional
feedback at suppressing star formation, produces 3 times less star formation in
the last 6 Gyr, and modestly lowers the final BH mass (30%). It is also
significantly more effective than an X-ray background in reducing the number of
satellite galaxies.Comment: 9 emulateapj pages, 8 figures; accepted to Ap
Reviving stochasticity: uncertainty in SMBH binary eccentricity is unavoidable
We study supermassive black hole (SMBH) binary eccentricity of equal-mass
galaxy mergers in -body simulations with the KETJU code, which combines the
GADGET-4 fast multipole gravity solver with accurate regularized integration
and Post-Newtonian corrections around SMBHs. In simulations with realistic,
high eccentricity galactic merger orbits, the binary eccentricity is found to
be a non-linear function of the deflection angle in the SMBH orbit during the
final, nearly radial close encounter between the SMBHs before they form a bound
binary. This mapping between the deflection angle and the binary eccentricity
has no apparent resolution dependence in our simulations spanning the
resolution range of particles per galaxy. The
mapping is also captured using a simple model with an analytic potential,
indicating that it is driven by the interplay between a smooth asymmetric
stellar background potential and dynamical friction acting on the SMBHs. Due to
the non-linearity of this mapping, in certain merger configurations small,
parsec-scale variations in the merger orbit can result in binary eccentricities
varying in nearly the full possible range between and . In idealized
simulations, such variations are caused by finite resolution effects, and
convergence of the binary eccentricity can be achieved with increasing
resolution. However, in real galaxies, other mechanisms such as nuclear gas and
substructure that perturb the merger orbit are likely to be significant enough
for the binary eccentricity to be effectively random. Our results indicate that
the distribution of these effectively random eccentricities can be studied
using even moderate resolution simulations.Comment: 9 pages, 5 figure
SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation
The intrinsic anisotropy delta and flattening epsilon of simulated merger
remnants is compared with elliptical galaxies that have been observed by the
SAURON collaboration, and that were analysed using axisymmetric Schwarzschild
models. Collisionless binary mergers of stellar disks and disk mergers with an
additional isothermal gas component, neglecting star formation cannot reproduce
the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit
of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is
however found for merger simulations of disks with gas fractions >= 20%,
including star formation and stellar energy feedback. Massive black hole
feedback does not strongly affect this result. Subsequent dry merging of merger
remnants however does not generate the slowly-rotating SAURON ellipticals which
are characterized by low ellipticities epsilon < 0.25 and low anisotropies.
This indicates that at least some ellipticals on the red galaxy sequence did
not form by binary mergers of disks or early-type galaxies. We show that
stellar spheroids resulting from multiple, hierarchical mergers of
star-bursting subunits in a cosmological context are in excellent agreement
with the low ellipticities and anisotropies of the slowly rotating SAURON
ellipticals and their observed trend of delta with epsilon. The numerical
simulations indicate that the SAURON relation might be a result of strong
violent relaxation and phase mixing of multiple, kinematically cold stellar
subunits with the angular momentum of the system determining its location on
the relation.Comment: 13 pages, 3 figures, submitted to Ap
The formation of extremely diffuse galaxy cores by merging supermassive black holes
Given its velocity dispersion, the early-type galaxy NGC 1600 has an
unusually massive () central
supermassive black hole (SMBH), surrounded by a large core ( kpc) with a tangentially biased stellar distribution. We present
high-resolution equal-mass merger simulations including SMBHs to study the
formation of such systems. The structural parameters of the progenitor
ellipticals were chosen to produce merger remnants resembling NGC 1600. We test
initial stellar density slopes of and and vary the initial SMBH masses from to . With increasing SMBH mass the merger remnants show a
systematic decrease in central surface brightness, an increasing core size, and
an increasingly tangentially biased central velocity anisotropy.
Two-dimensional kinematic maps reveal decoupled, rotating core regions for the
most massive SMBHs. The stellar cores form rapidly as the SMBHs become bound,
while the velocity anisotropy develops more slowly after the SMBH binaries
become hard. The simulated merger remnants follow distinct relations between
the core radius and the sphere-of-influence, and the SMBH mass, similar to
observed systems. We find a systematic change in the relations as a function of
the progenitor density slope, and present a simple scouring model reproducing
this behavior. Finally, we find the best agreement with NGC 1600 using SMBH
masses totaling the observed value of .
In general, density slopes of for the progenitor
galaxies are strongly favored for the equal-mass merger scenario.Comment: Accepted for publication in Ap
MSTAR - a fast parallelized algorithmically regularized integrator with minimum spanning tree coordinates
We present the novel algorithmically regularized integration method MSTAR for high-accuracy (vertical bar Delta E/E vertical bar greater than or similar to 10(-14)) integrations of N-body systems using minimum spanning tree coordinates. The twofold parallelization of the O(N-part(2)) force loops and the substep divisions of the extrapolation method allow for a parallel scaling up to N-CPU = 0.2 x N-part. The efficient parallel scaling of MSTAR makes the accurate integration of much larger particle numbers possible compared to the traditional algorithmic regularization chain (AR-CHAIN) methods, e.g. N-part = 5000 particles on 400 CPUs for 1 Gyr in a few weeks of wall-clock time. We present applications of MSTAR on few particle systems, studying the Kozai mechanism and N-body systems like star clusters with up to N-part = 10(4) particles. Combined with a tree or fast multipole-based integrator, the high performance of MSTAR removes a major computational bottleneck in simulations with regularized subsystems. It will enable the next-generation galactic-scale simulations with up to 109 stellar particles (e.g. m(star) = 100 M-circle dot) for an M-star = 10(11) M-circle dot galaxy), including accurate collisional dynamics in the vicinity of nuclear supermassive black holes.Peer reviewe
Resolving the Complex Evolution of a Supermassive Black Hole Triplet in a Cosmological Simulation
We present here a self-consistent cosmological zoom-in simulation of a triple supermassive black hole (SMBH) system forming in a complex multiple galaxy merger. The simulation is run with an updated version of our code KETJU, which is able to follow the motion of SMBHs down to separations of tens of Schwarzschild radii while simultaneously modeling the large-scale astrophysical processes in the surrounding galaxies, such as gas cooling, star formation, and stellar and AGN feedback. Our simulation produces initially an SMBH binary system for which the hardening process is interrupted by the late arrival of a third SMBH. The KETJU code is able to accurately model the complex behavior occurring in such a triple SMBH system, including the ejection of one SMBH to a kiloparsec-scale orbit in the galaxy due to strong three-body interactions as well as Lidov-Kozai oscillations suppressed by relativistic precession when the SMBHs are in a hierarchical configuration. One pair of SMBHs merges similar to 3 Gyr after the initial galaxy merger, while the remaining binary is at a parsec-scale separation when the simulation ends at redshift z = 0. We also show that KETJU can capture the effects of the SMBH binaries and triplets on the surrounding stellar population, which can affect the binary merger timescales as the stellar density in the system evolves. Our results demonstrate the importance of dynamically resolving the complex behavior of multiple SMBHs in galactic mergers, as such systems cannot be readily modeled using simple orbit-averaged semianalytic models.Peer reviewe
The fate of the Antennae galaxies
27 pages, 18 figures, submitted to MNRASWe present a high-resolution smoothed particle hydrodynamics simulation of the Antennae galaxies (NGC 4038/4039) and follow the evolution Gyrs beyond the final coalescence. The simulation includes metallicity dependent cooling, star formation, and both stellar feedback and chemical enrichment. The simulated best-match Antennae reproduces well both the observed morphology and the off-nuclear starburst. We also produce for the first time a simulated two-dimensional metallicity map of the Antennae and find good agreement with the observed metallicity of off-nuclear stellar clusters, however the nuclear metallicities are overproduced by dex. Using the radiative transfer code SKIRT we produce multi-wavelength observations of both the Antennae and the merger remnant. The Gyr old remnant is well fitted with a S\'ersic profile of , and with an -band effective radius of kpc and velocity dispersion of kms the remnant is located on the fundamental plane of early-type galaxies (ETGs). The initially blue Antennae remnant evolves onto the red sequence after Gyr of secular evolution. The remnant would be classified as a fast rotator, as the specific angular momentum evolves from to during its evolution. The remnant shows ordered rotation and a double peaked maximum in the mean 2D line-of-sight velocity. These kinematical features are relatively common among local ETGs and we specifically identify three local ETGs (NGC 3226, NGC 3379 and NGC 4494) in the ATLAS sample, whose photometric and kinematic properties most resemble the Antennae remnant.We present a high-resolution smoothed particle hydrodynamic simulation of the Antennae galaxies (NGC 4038/4039) and follow the evolution 3 Gyr beyond the final coalescence. The simulation includes metallicity-dependent cooling, star formation, and both stellar feed-back and chemical enrichment. The simulated best-match Antennae reproduce well both the observed morphology and the off-nuclear starburst. We also produce for the first time a simulated two-dimensional (2D) metallicity map of the Antennae and find good agreement with the observed metallicity of off-nuclear stellar clusters; however, the nuclear metallicities are overproduced by similar to 0.5 dex. Using the radiative transfer code SKIRT, we produce multiwavelength observations of both the Antennae and the merger remnant. The 1-Gyr-old remnant is well fitted with a Sersic profile of n = 7.07, and with an r-band effective radius of r(e) = 1.6 kpc and velocity dispersion of sigma(e) = 180 km s(-1) the remnant is located on the Fundamental Plane of early-type galaxies (ETGs). The initially blue Antennae remnant evolves on to the red sequence after similar to 2.5 Gyr of secular evolution. The remnant would be classified as a fast rotator, as the specific angular momentum evolves from lambda(Re) approximate to 0.11 to 0.14 during its evolution. The remnant shows ordered rotation and a double peaked maximum in the mean 2D line-of-sight velocity. These kinematical features are relatively common amongst local ETGs and we specifically identify three local ETGs (NGC 3226, NGC 3379, and NGC 4494) in the ATLAS(3D) sample, whose photometric and kinematic properties most resemble the Antennae remnant.Peer reviewe
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