814 research outputs found
Towards a more realistic sink particle algorithm for the RAMSES code
We present a new sink particle algorithm developed for the Adaptive Mesh
Refinement code RAMSES. Our main addition is the use of a clump finder to
identify density peaks and their associated regions (the peak patches). This
allows us to unambiguously define a discrete set of dense molecular cores as
potential sites for sink particle formation. Furthermore, we develop a new
scheme to decide if the gas in which a sink could potentially form, is indeed
gravitationally bound and rapidly collapsing. This is achieved using a general
integral form of the virial theorem, where we use the curvature in the
gravitational potential to correctly account for the background potential. We
detail all the necessary steps to follow the evolution of sink particles in
turbulent molecular cloud simulations, such as sink production, their
trajectory integration, sink merging and finally the gas accretion rate onto an
existing sink. We compare our new recipe for sink formation to other popular
implementations. Statistical properties such as the sink mass function, the
average sink mass and the sink multiplicity function are used to evaluate the
impact that our new scheme has on accurately predicting fundamental quantities
such as the stellar initial mass function or the stellar multiplicity function.Comment: submitted to MNRAS, 24 pages, 19 figures, 5 table
Metal enrichment in galactic winds
Observations give evidences of the presence of metals in the intergalactic
medium (IGM). The stars responsible for transforming hydrogen and helium into
more complex atoms do not form outside the galaxies in the standard scenario of
galaxy formation. Supernovae-driven winds and their associated feedback was
proposed as a possible solution to explain such enrichment of the IGM. It
turned out that a proper modelling of supernovae explosions within a turbulent
interstellar medium (ISM) is a difficult task. Recent advances have been
obtained using a multiphase approach to solve for the thermal state of the ISM,
plus some additional recipes to account for the kinetic effect of supernovae on
the galactic gas. We briefly describe here our implementation of supernovae
feedback within the RAMSES code, and apply it to the formation and evolution of
isolated galaxies of various masses and angular momenta. We have explored under
what conditions a galactic wind can develop, if one considers only a quiescent
mode of star formation. We have also characterized the distribution and
evolution of metallicity in the gas outflow spreading in the IGM.Comment: 6 pages, 6 figures, To appear in the proceedings of the
CRAL-Conference Series I "Chemodynamics: from first stars to local galaxies",
Lyon 10-14 July 2006, France, Eds. Emsellem, Wozniak, Massacrier, Gonzalez,
Devriendt, Champavert, EAS Publications Serie
The combined effect of AGN and supernovae feedback in launching massive molecular outflows in high-redshift galaxies
We have recently improved our model of active galactic nucleus (AGN) by
attaching the supermassive black hole (SMBH) to a massive nuclear star cluster
(NSC). Here we study the effects of this new model in massive, gas-rich
galaxies with several simulations of different feedback recipes with the
hydrodynamics code RAMSES. These simulations are compared to a reference
simulation without any feedback, in which the cooling halo gas is quickly
consumed in a burst of star formation. In the presence of strong supernovae
(SN) feedback, we observe the formation of a galactic fountain that regulates
star formation over a longer period, but without halting it. If only AGN
feedback is considered, as soon as the SMBH reaches a critical mass, strong
outflows of hot gas are launched and prevent the cooling halo gas from reaching
the disk, thus efficiently halting star formation, leading to the so-called
"quenching". If both feedback mechanisms act in tandem, we observe a non-linear
coupling, in the sense that the dense gas in the supernovae-powered galactic
fountain is propelled by the hot outflow powered by the AGN at much larger
radii than without AGN. We argue that these particular outflows are able to
unbind dense gas from the galactic halo, thanks to the combined effect of SN
and AGN feedback. We speculate that this mechanism occurs at the end of the
fast growing phase of SMBH, and is at the origin of the dense molecular
outflows observed in many massive high-redshift galaxies.Comment: 16 pages, 13 figures, accepted to MNRA
A small-scale dynamo in feedback-dominated galaxies - III. Cosmological simulations
Magnetic fields are widely observed in the Universe in virtually all
astrophysical objects, from individual stars to entire galaxies, even in the
intergalactic medium, but their specific generation has long been debated. Due
to the development of more realistic models of galaxy formation, viable
scenarios are emerging to explain cosmic magnetism, thanks to both deeper
observations and more efficient and accurate computer simulations. We present
here a new cosmological high-resolution zoom-in magnetohydrodynamic (MHD)
simulation, using the adaptive mesh refinement (AMR) technique, of a dwarf
galaxy with an initially weak and uniform magnetic seed field that is amplified
by a small-scale dynamo driven by supernova-induced turbulence. As first
structures form from the gravitational collapse of small density fluctuations,
the frozen-in magnetic field separates from the cosmic expansion and grows
through compression. In a second step, star formation sets in and establishes a
strong galactic fountain, self-regulated by supernova explosions. Inside the
galaxy, the interstellar medium becomes highly turbulent, dominated by strong
supersonic shocks, as demonstrated by the spectral analysis of the gas kinetic
energy. In this turbulent environment, the magnetic field is quickly amplified
via a small-scale dynamo process and is finally carried out into the
circumgalactic medium by a galactic wind. This realistic cosmological
simulation explains how initially weak magnetic seed fields can be amplified
quickly in early, feedback-dominated galaxies, and predicts, as a consequence
of the small scale dynamo process, that high-redshift magnetic fields are
likely to be dominated by their small scale components.Comment: 6 pages, 6 figures, submitted to MNRA
Numerical simulations of galaxy evolution in cosmological context
Large volume cosmological simulations succeed in reproducing the large-scale
structure of the Universe. However, they lack resolution and may not take into
account all relevant physical processes to test if the detail properties of
galaxies can be explained by the CDM paradigm. On the other hand, galaxy-scale
simulations could resolve this in a robust way but do not usually include a
realistic cosmological context.
To study galaxy evolution in cosmological context, we use a new method that
consists in coupling cosmological simulations and galactic scale simulations.
For this, we record merger and gas accretion histories from cosmological
simulations and re-simulate at very high resolution the evolution of baryons
and dark matter within the virial radius of a target galaxy. This allows us for
example to better take into account gas evolution and associated star
formation, to finely study the internal evolution of galaxies and their disks
in a realistic cosmological context.
We aim at obtaining a statistical view on galaxy evolution from z = 2 to 0,
and we present here the first results of the study: we mainly stress the
importance of taking into account gas accretion along filaments to understand
galaxy evolution.Comment: 6 pages - Proceedings of IAU Symposium 254 "The Galaxy disk in
cosmological context", Copenhagen, June 2008 - Movies available at
http://aramis.obspm.fr/~bournaud/stargas35small.avi and
http://aramis.obspm.fr/~bournaud/stargasZ35_small.av
Baryonic and dark matter distribution in cosmological simulations of spiral galaxies
We study three cosmological hydrodynamical simulations of Milky Way(MW)-sized
halos including a comparison with the dark matter(DM)-only counterparts. We
find one of our simulated galaxies with interesting MW-like features. Thanks to
a consistently tuned star formation rate and supernovae feedback we obtain an
extended disk and a flat rotation curve with a satisfying circular velocity and
a reasonable DM density in the solar neighbourhood. Mimicking observational
methods, we re-derive the stellar mass and obtain stellar-to-halo mass ratios
reduced by more than 50\%. We show the interaction between the baryons and the
dark matter which is first contracted by star formation and then cored by
feedback processes. Indeed, we report an unprecedentedly observed effect in the
DM density profile consisting of a central core combined with an adiabatic
contraction at larger galactic radii. The cores obtained are typically 5
kpc large. Moreover, this also impacts the DM density at the solar radius. In
our simulation resembling most to the MW, the density is raised from 0.23
GeV/cm in the dark matter only run to 0.36 GeV/cm (spherical shell) or
0.54 GeV/cm (circular ring) in the hydrodynamical run. Studying the
subhalos, the dark matter within luminous satellites is also affected by
baryonic processes and exihibits cored profiles whereas dark satellites are
cuspy. We find a shift in mass compared to DM-only simulations and obtain, for
halos in the lower MW mass range, a distribution of luminous satellites
comparable to the MW spheroidal dwarf galaxies.Comment: matches version accepted in MNRAS, 17 pages, 15 figures, text
improved, satellite section extende
Galaxy Evolution: Modeling the Role of Non-thermal Pressure in the Interstellar medium
Galaxy evolution depends strongly on the physics of the interstellar medium
(ISM). Motivated by the need to incorporate the properties of the ISM in
cosmological simulations we construct a simple method to include the
contribution of non-thermal components in the calculation of pressure of
interstellar gas. In our method we treat three non-thermal components -
turbulence, magnetic fields and cosmic rays - and effectively parametrize their
amplitude. We assume that the three components settle into a quasi-steady-state
that is governed by the star formation rate, and calibrate their magnitude and
density dependence by the observed Radio-FIR correlation, relating synchrotron
radiation to star formation rates of galaxies. We implement our model in single
cell numerical simulation of a parcel of gas with constant pressure boundary
conditions and demonstrate its effect and potential. Then, the non-thermal
pressure model is incorporated into RAMSES and hydrodynamic simulations of
isolated galaxies with and without the non-thermal pressure model are presented
and studied. Specifically, we demonstrate that the inclusion of realistic
non-thermal pressure reduces the star formation rate by an order of magnitude
and increases the gas depletion time by as much. We conclude that the
non-thermal pressure can prolong the star formation epoch and achieve
consistency with observations without invoking artificially strong stellar
feedback.Comment: 18 pages, 14 figures, accepted to MNRAS. Updated to match final
versio
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