2,594 research outputs found
Dynamics of gas and dust clouds in active galactic nuclei
We analyse the motion of single optically thick clouds in the potential of a
central mass under the influence of an anisotropic radiation field
~|cos(\theta)|, a model applicable to the inner region of active galactic
nuclei. Resulting orbits are analytically soluble for constant cloud column
densities. All stable orbits are closed, although they have non-trivial shapes.
Furthermore, there exists a stability criterion in the form of a critical
inclination, which depends on the luminosity of the central source and the
column density of the cloud.Comment: 4 pages, 3 figures; language corrections, minor formatting change
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
Numerical simulations of the possible origin of the two sub-parsec scale and counter-rotating stellar disks around SgrA*
We present a high resolution simulation of an idealized model to explain the
origin of the two young, counter-rotating, sub-parsec scale stellar disks
around the supermassive black hole SgrA* at the Center of the Milky Way. In our
model, the collision of a single molecular cloud with a circum-nuclear gas disk
(similar to the one observed presently) leads to multiple streams of gas
flowing towards the black hole and creating accretion disks with angular
momentum depending on the ratio of cloud and circum-nuclear disk material. The
infalling gas creates two inclined, counter-rotating sub-parsec scale accretion
disks around the supermassive black hole with the first disk forming roughly 1
Myr earlier, allowing it to fragment into stars and get dispersed before the
second, counter-rotating disk forms. Fragmentation of the second disk would
lead to the two inclined, counter-rotating stellar disks which are observed at
the Galactic Center. A similar event might be happening again right now at the
Milky Way Galactic Center. Our model predicts that the collision event
generates spiral-like filaments of gas, feeding the Galactic Center prior to
disk formation with a geometry and inflow pattern that is in agreement with the
structure of the so called mini-spiral that has been detected in the Galactic
Center.Comment: 14 pages, 12 figures, submitted to Ap
Bulgeless Galaxies and their Angular Momentum Problem
The specific angular momentum of Cold Dark Matter (CDM) halos in a
CDM universe is investigated. Their dimensionless specific angular
momentum with and
the virial velocity and virial radius, respectively depends strongly
on their merging histories. We investigate a set of CDM simulations
and explore the specific angular momentum content of halos formed through
various merging histories. Halos with a quiet merging history, dominated by
minor mergers and accretion until the present epoch, acquire by tidal torques
on average only 2% to 3% of the angular momentum required for their rotational
support (). This is in conflict with observational data for a
sample of late-type bulgeless galaxies which indicates that those galaxies
reside in dark halos with exceptionally high values of . Minor mergers and accretion preserve or slowly increase the
specific angular momentum of dark halos with time. This mechanism is however
not efficient enough in order to explain the observed spin values for late-type
dwarf galaxies. Energetic feedback processes have been invoked to solve the
problem that gas loses a large fraction of its specific angular momentum during
infall. Under the assumption that dark halos hosting bulgeless galaxies acquire
their mass via quiescent accretion, our results indicate yet another serious
problem: the specific angular momentum gained during the formation of these
objects is not large enough to explain their observed rotational
properties,even if no angular momentum would be lost during gas infall.Comment: 4 pages, 3 figures. To appear in September 1, 2004, issue of ApJ
Letter
A New Concept of Transonic Galactic Outflows in a Cold Dark Matter Halo with a Central Super-Massive Black Hole
We study fundamental properties of isothermal, steady and spherically
symmetric galactic outflow in the gravitational potential of a cold dark matter
halo and a central super-massive black hole. We find that there are two
transonic solutions having different properties: each solution is mainly
produced by the dark matter halo and the super-massive black hole,
respectively. Furthermore, we apply our model to the Sombrero galaxy. In this
galaxy, Chandra X-ray observatory detected the diffuse hot gas as the trace of
galactic outflows while the star-formation rate is low and the observed gas
density distribution presumably indicates the hydrostatic equilibrium. To solve
this discrepancy, we propose a solution that this galaxy has a transonic
outflow, however, the transonic point forms in a very distant region from the
galactic center (? 127 kpc). In this slowly accelerated transonic
outflow, the outflow velocity is less than the sound velocity for most of the
galactic halo. Since the gas density distribution in this subsonic region is
similar to the hydrostatic one, it is difficult to distinguish the wide
subsonic region from hydrostatic state. Such galactic outflows are dfferent
from the conventional supersonic outflows observed in star-forming galaxies.Comment: 7 pages, 3 figures, accepted for publication in JPS Conference
Proceedings. arXiv admin note: substantial text overlap with arXiv:1405.345
Electroexcitation of nucleon resonances at Q^2=0.65 GeV/c^2 from a combined analysis of single- and double-pion electroproduction data
Data on single- and double-charged pion electroproduction off protons are
successfully described in the second and third nucleon resonance regions with
common N* photocouplings. The analysis was carried out using separate isobar
models for both reactions. From the combined analysis of two exclusive
channels, the gamma* p --> N*+ helicity amplitudes are obtained for the
resonances P11(1440), D13(1520), S31(1620), S11(1650), F15(1680), D33(1700),
D13(1700), and P13(1720) at Q2=0.65 GeV/c^2.Comment: 12 pages, 12 figures (eps), Published in PHYSICAL REVIEW C 72, 045201
(2005
Time-resolved infrared emission from radiation-driven central obscuring structures in Active Galactic Nuclei
The central engines of Seyfert galaxies are thought to be enshrouded by
geometrically thick gas and dust structures. In this article, we derive
observable properties for a self-consistent model of such toroidal gas and dust
distributions, where the geometrical thickness is achieved and maintained with
the help of X-ray heating and radiation pressure due to the central engine.
Spectral energy distributions (SEDs) and images are obtained with the help of
dust continuum radiative transfer calculations with RADMC-3D. For the first
time, we are able to present time-resolved SEDs and images for a physical model
of the central obscurer. Temporal changes are mostly visible at shorter
wavelengths, close to the combined peak of the dust opacity as well as the
central source spectrum and are caused by variations in the column densities of
the generated outflow. Due to the three-component morphology of the
hydrodynamical models -- a thin disc with high density filaments, a surrounding
fluffy component (the obscurer) and a low density outflow along the rotation
axis -- we find dramatic differences depending on wavelength: whereas the
mid-infrared images are dominated by the elongated appearance of the outflow
cone, the long wavelength emission is mainly given by the cold and dense disc
component. Overall, we find good agreement with observed characteristics,
especially for those models, which show clear outflow cones in combination with
a geometrically thick distribution of gas and dust, as well as a geometrically
thin, but high column density disc in the equatorial plane.Comment: 16 pages, 12 figures, accepted for publication in MNRA
Three-dimensional Continuum Radiative Transfer Images of a Molecular Cloud Core Evolution
We analyze a three-dimensional smoothed particle hydrodynamics simulation of
an evolving and later collapsing pre-stellar core. Using a three-dimensional
continuum radiative transfer program, we generate images at 7 micron, 15
micron, 175 micron, and 1.3 mm for different evolutionary times and viewing
angles. We discuss the observability of the properties of pre-stellar cores for
the different wavelengths. For examples of non-symmetric fragments, it is shown
that, misleadingly, the density profiles derived from a one-dimensional
analysis of the corresponding images are consistent with one-dimensional core
evolution models. We conclude that one-dimensional modeling based on column
density interpretation of images does not produce reliable structural
information and that multidimensional modeling is required.Comment: accepted by ApJL, 4 pages, 4 figure
Highlights of Recent Results with Clas
Recent results on the study of the electromagnetic structure of nucleon
resonances, the spin structure of proton and neutrons at small and intermediate
photon virtualities, and the search for exotic pentaquark baryons are
presented.Comment: 12 pages, 8 figures, invited talk at MENU2004, Beijin
The life cycle of starbursting circumnuclear gas discs
High-resolution observations from the sub-mm to the optical wavelength regime
resolve the central few 100pc region of nearby galaxies in great detail. They
reveal a large diversity of features: thick gas and stellar discs, nuclear
starbursts, in- and outflows, central activity, jet interaction, etc.
Concentrating on the role circumnuclear discs play in the life cycles of
galactic nuclei, we employ 3D adaptive mesh refinement hydrodynamical
simulations with the RAMSES code to self-consistently trace the evolution from
a quasi-stable gas disc, undergoing gravitational (Toomre) instability, the
formation of clumps and stars and the disc's subsequent, partial dispersal via
stellar feedback. Our approach builds upon the observational finding that many
nearby Seyfert galaxies have undergone intense nuclear starbursts in their
recent past and in many nearby sources star formation is concentrated in a
handful of clumps on a few 100pc distant from the galactic centre. We show that
such observations can be understood as the result of gravitational
instabilities in dense circumnuclear discs. By comparing these simulations to
available integral field unit observations of a sample of nearby galactic
nuclei, we find consistent gas and stellar masses, kinematics, star formation
and outflow properties. Important ingredients in the simulations are the
self-consistent treatment of star formation and the dynamical evolution of the
stellar distribution as well as the modelling of a delay time distribution for
the supernova feedback. The knowledge of the resulting simulated density
structure and kinematics on pc scale is vital for understanding inflow and
feedback processes towards galactic scales.Comment: accepted by MNRA
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