1,101 research outputs found
A refined sub-grid model for black hole accretion and AGN feedback in large cosmological simulations
In large scale cosmological hydrodynamic simulations simplified sub-grid
models for gas accretion onto black holes and AGN feedback are commonly used.
Such models typically depend on various free parameters, which are not well
constrained. We present a new advanced model containing a more detailed
description of AGN feedback, where those parameters reflect the results of
recent observations. The model takes the dependency of these parameters on the
black hole properties into account and describes a continuous transition
between the feedback processes acting in the so-called radio-mode and
quasar-mode. In addition, we implement a more detailed description of the
accretion of gas onto black holes by distinguishing between hot and cold gas
accretion. Our new implementations prevent black holes from gaining too much
mass, particularly at low redshifts so that our simulations are now very
successful in reproducing the observed present-day black hole mass function.
Our new model also suppresses star formation in massive galaxies slightly more
efficiently than many state-of-the-art models. Therefore, the simulations that
include our new implementations produce a more realistic population of
quiescent and star-forming galaxies compared to recent observations, even if
some discrepancies remain. In addition, the baryon conversion efficiencies in
our simulation are - except for the high mass end - consistent with
observations presented in literature over the mass range resolved by our
simulations. Finally, we discuss the significant impact of the feedback model
on the low-luminous end of the AGN luminosity function.Comment: 25 pages, 19 figures. MNRAS accepted. Magneticum website:
http://www.magneticum.or
Origin and properties of dual and offset active galactic nuclei in a cosmological simulation at z=2
In the last few years, it became possible to observationally resolve galaxies
with two distinct nuclei in their centre. For separations smaller than 10kpc,
dual and offset active galactic nuclei (AGN) are distinguished: in dual AGN,
both nuclei are active, whereas in offset AGN only one nucleus is active. To
study the origin of such AGN pairs, we employ a cosmological, hydrodynamic
simulation with a large volume of (182Mpc)^3 from the set of Magneticum
Pathfinder Simulations. The simulation self-consistently produces 35 resolved
black hole (BH) pairs at redshift z=2, with a comoving distance smaller than
10kpc. 14 of them are offset AGN and nine are dual AGN, resulting in a fraction
of (1.2 \pm 0.3)% AGN pairs with respect to the total number of AGN. In this
paper, we discuss fundamental differences between the BH and galaxy properties
of dual AGN, offset AGN and inactive BH pairs and investigate their different
triggering mechanisms. We find that in dual AGN the BHs have similar masses and
the corresponding BH from the less massive progenitor galaxy always accretes
with a higher Eddington ratio. In contrast, in offset AGN the active BH is
typically more massive than its non-active counterpart. Furthermore, dual AGN
in general accrete more gas from the intergalactic medium than offset AGN and
non-active BH pairs. This highlights that merger events, particularly minor
mergers, do not necessarily lead to strong gas inflows and thus, do not always
drive strong nuclear activity.Comment: 17 pages, 18 figures, accepted for publication in MNRAS, website:
http://www.magneticum.or
QualitÀtssicherung interdisziplinÀrer Polytraumaversorgung: Möglichkeiten und Grenzen retrospektiver Standarderfassung
Zusammenfassung: Hintergrund: Inwieweit kann die Auswertung standardmĂ€Ăig erhobener Patienten- und Krankenhausdaten einen Behandlungsvergleich mit anderen Erhebungen gestatten? Material und Methoden: Es wurde eine retrospektive Analyse epidemiologischer und klinisch-technischer Parameter aller Mehrfachverletzten [Injury Severity Score (ISS)>15] einer Zentrumsklinik (n=172; Zeitraum: 01.01.1997-31.12.1999) bezĂŒglich der Ablauforganisation und des Outcome (p74Jahre, Hypotension, initial verminderte HĂ€moglobin- und Quick-Werte, verminderte Glasgow Coma Scale (GCS) sowie Anzahl erhaltener Blutkonzentrate. Eine GegenĂŒberstellung der erhobenen Daten mit der zeitgleichen prospektiven Multizenterstudie der Deutschen Gesellschaft fĂŒr Unfallchirurgie (DGU) bestĂ€tigte die Ergebnisse bezĂŒglich des Ablaufs und des Outcome. Schlussfolgerung: Die interdisziplinĂ€re retrospektive Datenauswertung ist unter Fokussierung auf prognoserelevante und routinemĂ€Ăig erhobene Parameter eine praktikable sowie aussagefĂ€hige Alternative zu prospektiven Erfassungen und ermöglicht eine erste qualitative Standortbestimmun
Energy and Momentum Distributions of the Magnetic Solution to (2+1) Einstein-Maxwell Gravity
We use Moeller's energy-momentum complex in order to explicitly evaluate the
energy and momentum density distributions associated with the three-dimensional
magnetic solution to the Einstein-Maxwell equations. The magnetic spacetime
under consideration is a one-parametric solution describing the distribution of
a radial magnetic field in a three-dimensional AdS background, and representing
the superposition of the magnetic field with a 2+1 Einstein static
gravitational field.Comment: LaTex, 13 pages; v2 clarifying comments and references added,
Conclusions improved, to appear in Mod. Phys. Lett.
Simulating binary neutron stars: dynamics and gravitational waves
We model two mergers of orbiting binary neutron stars, the first forming a
black hole and the second a differentially rotating neutron star. We extract
gravitational waveforms in the wave zone. Comparisons to a post-Newtonian
analysis allow us to compute the orbital kinematics, including trajectories and
orbital eccentricities. We verify our code by evolving single stars and
extracting radial perturbative modes, which compare very well to results from
perturbation theory. The Einstein equations are solved in a first order
reduction of the generalized harmonic formulation, and the fluid equations are
solved using a modified convex essentially non-oscillatory method. All
calculations are done in three spatial dimensions without symmetry assumptions.
We use the \had computational infrastructure for distributed adaptive mesh
refinement.Comment: 14 pages, 16 figures. Added one figure from previous version;
corrected typo
Magnetically charged solutions via an analog of the electric-magnetic duality in (2+1)-dimensional gravity theories
We find an analog of the electric-magnetic duality, which is a
transformation between magnetic and electric sectors of the static and
rotationally symmetric solutions in a class of (2+1)-dimensional
Einstein-Maxwell-Dilaton gravity theories. The theories in our consideration
include, in particular, one parameter class of theories continuously connecting
the Banados-Teitelboim-Zanelli (BTZ) gravity and the low energy string
effective theory. When there is no charge, we have or
symmetry, depending on a parameter that specifies each theory. Via the
transformation, we obtain exact magnetically charged solutions from the known
electrically charged solutions. We explain the relationship between the
transformation and symmetry, and comment on the -duality of the
string theory.Comment: 10 pages, RevTe
Critical Collapse of the Massless Scalar Field in Axisymmetry
We present results from a numerical study of critical gravitational collapse
of axisymmetric distributions of massless scalar field energy. We find
threshold behavior that can be described by the spherically symmetric critical
solution with axisymmetric perturbations. However, we see indications of a
growing, non-spherical mode about the spherically symmetric critical solution.
The effect of this instability is that the small asymmetry present in what
would otherwise be a spherically symmetric self-similar solution grows. This
growth continues until a bifurcation occurs and two distinct regions form on
the axis, each resembling the spherically symmetric self-similar solution. The
existence of a non-spherical unstable mode is in conflict with previous
perturbative results, and we therefore discuss whether such a mode exists in
the continuum limit, or whether we are instead seeing a marginally stable mode
that is rendered unstable by numerical approximation.Comment: 11 pages, 8 figure
Rough droplet model for spherical metal clusters
We study the thermally activated oscillations, or capillary waves, of a
neutral metal cluster within the liquid drop model. These deformations
correspond to a surface roughness which we characterize by a single parameter
. We derive a simple analytic approximate expression determining
as a function of temperature and cluster size. We then estimate the
induced effects on shell structure by means of a periodic orbit analysis and
compare with recent data for shell energy of sodium clusters in the size range
. A small surface roughness \AA~ is seen to
give a reasonable account of the decrease of amplitude of the shell structure
observed in experiment. Moreover -- contrary to usual Jahn-Teller type of
deformations -- roughness correctly reproduces the shape of the shell energy in
the domain of sizes considered in experiment.Comment: 20 pages, 4 figures, important modifications of the presentation, to
appear in Phys. Rev.
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