975 research outputs found
Constrained Simulations of the Magnetic Field in the Local Supercluster and the Propagation of UHECR
Magnetic fields (MF) in the Local Supercluster (LSC) of galaxies may have
profound consequences for the propagation of Ultra High Energy Cosmic Rays
(UHECR). Faraday rotations measurements provide some informations about MF in
compact clusters. However, very few is known about less dense regions and about
the global structure of MF in the LSC. In order to get a better knowledge of
these fields we are performing constrained magnetohydrodynamical simulations of
the LSC magnetic field. We will present the results of our simulation and
discuss their implications for the angular distribution of expected UHECR
deflections.Comment: 4 pages + 1 figure. Published on the Proceedings of the 28th
International Cosmic Ray Conference, Tsukuba, Japan (2003
Helium-ignited violent mergers as a unified model for normal and rapidly declining Type Ia Supernovae
The progenitors of Type Ia Supernovae (SNe Ia) are still unknown, despite
significant progress during the last years in theory and observations. Violent
mergers of two carbon--oxygen (CO) white dwarfs (WDs) are one candidate
suggested to be responsible for at least a significant fraction of normal SNe
Ia. Here, we simulate the merger of two CO WDs using a moving-mesh code that
allows for the inclusion of thin helium (He) shells (0.01\,\msun) on top of the
WDs, at an unprecedented numerical resolution. The accretion of He onto the
primary WD leads to the formation of a detonation in its He shell. This
detonation propagates around the CO WD and sends a converging shock wave into
its core, known to robustly trigger a second detonation, as in the well-known
double-detonation scenario for
He-accreting CO WDs. However, in contrast to that scenario where a massive He
shell is required to form a detonation through thermal instability, here the He
detonation is ignited dynamically. Accordingly the required He-shell mass is
significantly smaller, and hence its burning products are unlikely to affect
the optical display of the explosion. We show that this scenario, which works
for CO primary WDs with CO- as well as He-WD companions, has the potential to
explain the different brightness distributions, delay times and relative rates
of normal and fast declining SNe Ia. Finally, we discuss extensions to our
unified merger model needed to obtain a comprehensive picture of the full
observed diversity of SNe Ia.Comment: accepted for publication by ApJL, significant changes to first
version, including addition of merger simulatio
Galactic Centre stellar winds and Sgr A* accretion
(ABRIDGED) We present in detail our new 3D numerical models for the accretion
of stellar winds on to Sgr A*. In our most sophisticated models, we put stars
on realistic orbits around Sgr A*, include `slow' winds (300 km/s), and account
for radiative cooling. We first model only one phase `fast' stellar winds (1000
km/s). For wind sources fixed in space, the accretion rate is Mdot ~ 1e-5
Msun/yr, fluctuates by < 10%, and is in a good agreement with previous models.
In contrast, Mdot decreases by an order of magnitude for stars following
circular orbits, and fluctuates by ~ 50%. Then we allow a fraction of stars to
produce slow winds. Much of these winds cool radiatively, forming cold clumps
immersed into the X-ray emitting gas. We test two orbital configurations for
the stars in this scenario, an isotropic distribution and two rotating discs
with perpendicular orientation. The morphology of cold gas is quite sensitive
to the orbits. In both cases, however, most of the accreted gas is hot, with an
almost constant Mdot ~ 3e-6 Msun/yr, consistent with Chandra observations. The
cold gas accretes in intermittent, short but powerful episodes which may give
rise to large amplitude variability in the luminosity of Sgr A* on time scales
of 10s to 100s of years. The circularisation radii for the flows are ~ 1e3 and
1e4 Rsch, for the one and two-phase wind simulations, respectively, never
forming the quasi-spherical accretion flows suggested in some previous work.
Our work suggests that, averaged over time scales of 100s to 1000s of years,
the radiative and mechanical luminosity of Sgr A* may be substantially higher
than it is in its current state. Further improvements of the wind accretion
modelling of Sgr A* will rely on improved observational constraints for the
wind properties and stellar orbits.Comment: 16 pages, 18 colour figures. Accepted by MNRAS. Full resolution paper
and movies available at http://www.mpa-garching.mpg.de/~jcuadra/Winds/ . (v2:
minor changes
Back-in-time dynamics of the cluster IE 0657-56 (the Bullet System)
We present a simplified dynamical model of the ``Bullet'' system of two
colliding clusters. The model constrains the masses of the system by requiring
that the orbits of the main and sub components satisfy the cosmological initial
conditions of vanishing physical separation a Hubble time ago. This is also
known as the timing argument. The model considers a system embedded in an
over-dense region. We argue that a relative speed of between
the two components is consistent with cosmological conditions if the system is
of a total mass of is embedded in a region of
a (mild) over-density of 10 times the cosmological background density.
Combining this with the lensing measurements of the projected mass, the model
yields a ratio of 3:1 for the mass of the main relative to that of the
subcomponent. The effect of the background weakens as the relative speed
between the two components is decreased. For relative speeds lower than , the timing argument yields masses which are too low to be
consistent with lensing.Comment: 5 pages, 3 figures, submitted to MNRA
The cosmological co-evolution of supermassive black holes, AGN and galaxies
We model the cosmological co-evolution of galaxies and their central
supermassive black holes (BHs) within a semi-analytical framework developed on
the outputs of the Millennium Simulation (Croton et al., 2006; De Lucia &
Blaizot, 2007). In this work, we analyze the model BH scaling relations,
fundamental plane and mass function, and compare them with the most recent
observational data. Furthermore, we extend the original code developed by
Croton et al. (2006) to follow the evolution of the BH mass accretion and its
conversion into radiation, and compare the derived AGN bolometric luminosity
function with the observed one. We find, for the most part, a very good
agreement between predicted and observed BH properties. Moreover, the model is
in good agreement with the observed AGN number density in 0<z<5, provided it is
assumed that the cold gas fraction accreted by BHs at high redshifts is larger
than at low redshifts (Marulli et al., 2008).Comment: Proceedings of "The Central Kiloparsec: Active Galactic Nuclei and
Their Hosts", Ierapetra, Crete, 4-6 June, 2008. To appear in Volume 79 of the
Memorie della Societa' Astronomica Italiana. 5 pages, 4 figure
The phase-space structure of cold dark-matter halos: Insights into the Galactic halo
We study the formation of the Milky Way's halo in a CDM cosmology by
scaling down a high resolution simulation of the formation of a cluster of
galaxies. We determine how much phase-space substructure is left over from the
objects that merge to build up the present galaxy. We study the debris streams
originating from such objects and find that their evolution can be explained
simply in terms of the conservation of phase-space density. Analysing the mass
growth history of our halo we find that its inner regions have been in place
for more than 10 Gyr, but that the growth of the halo as a whole is more
gradual, in agreement with other high resolution simulations of dark-matter
halos. Recent accretion contributes to the inner 10 kpc of the halo only at the
10 level. Finally we determine the number of dark-matter streams as a
function of distance from the centre of the halo. In the equivalent of the
``Solar vicinity'', we find that the dark-matter is smoothly distributed in
space, and that the velocity ellipsoid is formed by hundreds of thousands of
streams, most of which have velocity dispersions of the order of 1 km/s or
less.Comment: 16 pages, 21 figures, MNRAS in press. Postscript version with high
resolution figures available from
http://www.mpa-garching.mpg.de/~ahelmi/research/lcdm_cl.html. Minor change
Probing Yukawian Gravitational Potential by Numerical Simulations. II. Elliptical Galaxies
Since the Newtonian gravitation is largely used to model with success the
structures of the universe, such as galaxies and clusters of galaxies, for
example, a way to probe and constrain alternative theories, in the weak field
limit, is to apply them to model the structures of the universe. We then
modified the well known Gadget-2 code to probe alternative theories of
gravitation through galactic dynamics. In particular, we modified the Gadget-2
code to probe alternatives theories whose weak field limits have a Yukawa-like
gravitational potential. As a first application of this modified Gadget-2 code
we simulate the evolution of elliptical galaxies. These simulations show that
galactic dynamics can be used to constrain the parameters associated with
alternative theories of gravitation.Comment: 6 pages, 5 figures - To appear in General Relativity and Gravitatio
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