505 research outputs found
Modeling Dynamical Dark Energy
Cosmological models with different types of Dark Energy are becoming viable
alternatives for standard models with the cosmological constant. Yet, such
models are more difficult to analyze and to simulate. We present analytical
approximations and discuss ways of making simulations for two families of
models, which cover a wide range of possibilities and include models with both
slow and fast changing ratio w=p\rho. More specifically, we give analytical
expressions for the evolution of the matter density parameter Omega_m(z) and
the virial density contrast Delta_c at any redshift z. The latter is used to
identify halos and to find their virial masses. We also provide an
approximation for the linear growth factor of linear fluctuations between
redshift z=40 and z=0. This is needed to set the normalization of the spectrum
of fluctuations. Finally, we discuss the expected behavior of the halo mass
function and its time evolution.Comment: 10 pages, 10 figures ApJ submitte
Galaxy alignment on large and small scales
Galaxies are not randomly distributed across the universe but showing
different kinds of alignment on different scales. On small scales satellite
galaxies have a tendency to distribute along the major axis of the central
galaxy, with dependence on galaxy properties that both red satellites and
centrals have stronger alignment than their blue counterparts. On large scales,
it is found that the major axes of Luminous Red Galaxies (LRGs) have
correlation up to 30Mpc/h. Using hydro-dynamical simulation with star
formation, we investigate the origin of galaxy alignment on different scales.
It is found that most red satellite galaxies stay in the inner region of dark
matter halo inside which the shape of central galaxy is well aligned with the
dark matter distribution. Red centrals have stronger alignment than blue ones
as they live in massive haloes and the central galaxy-halo alignment increases
with halo mass. On large scales, the alignment of LRGs is also from the
galaxy-halo shape correlation, but with some extent of mis-alignment. The
massive haloes have stronger alignment than haloes in filament which connect
massive haloes. This is contrary to the naive expectation that cosmic filament
is the cause of halo alignment.Comment: 4 pages, 3 figures, To appear in the proceedings of the IAU Symposium
308 "The Zeldovich Universe: Genesis and Growth of the Cosmic Web
3D simulations of the early stages of AGN jets: geometry, thermodynamics and backflow
We investigate the interplay between jets from Active Galactic Nuclei (AGNs)
and the surrounding InterStellar Medium (ISM) through full 3D, high resolution,
Adaptive Mesh Refinement simulations performed with the FLASH code. We follow
the jet- ISM system for several Myr in its transition from an early, compact
source to an extended one including a large cocoon. During the jet evolution,
we identify three major evolutionary stages and we find that, contrary to the
prediction of popular theoretical models, none of the simulations shows a
self-similar behavior. We also follow the evolution of the energy budget, and
find that the fraction of input power deposited into the ISM (the AGN coupling
constant) is of order of a few percent during the first few Myr. This is in
broad agreement with galaxy formation models employing AGN feedback. However,
we find that in these early stages, this energy is deposited only in a small
fraction (< 1%) of the total ISM volume. Finally we demonstrate the relevance
of backflows arising within the extended cocoon generated by a relativistic AGN
jet within the ISM of its host galaxy, previously proposed as a mechanism for
self-regulating the gas accretion onto the central object. These backflows tend
later to be destabilized by the 3D dynamics, rather than by hydrodynamic
(Kelvin- Helmholtz) instabilities. Yet, in the first few hundred thousand
years, backflows may create a central accretion region of significant extent,
and convey there as much as a few millions of solar masses.Comment: Accepted in MNRAS - 16 pages, 12 figures - Multimedia available on
the author's webpage: http://www.mpia.de/~ciel
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