633 research outputs found
Hydrodynamical simulations of cluster formation with central AGN heating
We analyse a hydrodynamical simulation model for the recurrent heating of the
central intracluster medium (ICM) by active galactic nuclei (AGN). Besides the
self-gravity of the dark matter and gas components, our approach includes the
radiative cooling and photoheating of the gas, as well as a subresolution
multiphase model for star formation and supernova feedback. Additionally, we
incorporate a periodic heating mechanism in the form of hot, buoyant bubbles,
injected into the intragalactic medium (IGM) during the active phases of the
accreting central AGN. We use simulations of isolated cluster halos of
different masses to study the bubble dynamics and the heat transport into the
IGM. We also apply our model to self-consistent cosmological simulations of the
formation of galaxy clusters with a range of masses. Our numerical schemes
explore a variety of different assumptions for the spatial configuration of
AGN-driven bubbles, for their duty cycles and for the energy injection
mechanism, in order to obtain better constraints on the underlying physical
picture. We argue that AGN heating can substantially affect the properties of
both the stellar and gaseous components of clusters of galaxies. Most
importantly, it alters the properties of the central dominant (cD) galaxy by
reducing the mass deposition rate of freshly cooled gas out of the ICM, thereby
offering an energetically plausible solution to the cooling flow problem. At
the same time, this leads to reduced or eliminated star formation in the
central cD galaxy, giving it red stellar colours as observed.Comment: 22 pages, 15 figures, minor revisions, MNRAS accepte
The unorthodox evolution of major merger remnants into star-forming spiral galaxies
Galaxy mergers are believed to play a key role in transforming star-forming
disk galaxies into quenched ellipticals. Most of our theoretical knowledge
about such morphological transformations does, however, rely on idealised
simulations where processes such as cooling of hot halo gas into the disk and
gas accretion in the post-merger phase are not treated in a self-consistent
cosmological fashion. In this paper we study the morphological evolution of the
stellar components of four major mergers occurring at z=0.5 in cosmological
hydrodynamical zoom-simulations. In all simulations the merger reduces the disk
mass-fraction, but all galaxies simulated at our highest resolution regrow a
significant disk by z=0 (with a disk fraction larger than 24%). For runs with
our default physics model, which includes galactic winds from star formation
and black hole feedback, none of the merger remnants are quenched, but in a set
of simulations with stronger black hole feedback we find that major mergers can
indeed quench galaxies. We conclude that major merger remnants commonly evolve
into star-forming disk galaxies, unless sufficiently strong AGN feedback
assists in the quenching of the remnant.Comment: 15 pages, 9 figures, Accepted for publication in MNRA
Shock finding on a moving-mesh: I. Shock statistics in non-radiative cosmological simulations
Cosmological shock waves play an important role in hierarchical structure
formation by dissipating and thermalizing kinetic energy of gas flows, thereby
heating the universe. Furthermore, identifying shocks in hydrodynamical
simulations and measuring their Mach number accurately is critical for
calculating the production of non-thermal particle components through diffusive
shock acceleration. However, shocks are often significantly broadened in
numerical simulations, making it challenging to implement an accurate shock
finder. We here introduce a refined methodology for detecting shocks in the
moving-mesh code AREPO, and show that results for shock statistics can be
sensitive to implementation details. We put special emphasis on filtering
against spurious shock detections due to tangential discontinuities and
contacts. Both of them are omnipresent in cosmological simulations, for example
in the form of shear-induced Kelvin-Helmholtz instabilities and cold fronts. As
an initial application of our new implementation, we analyse shock statistics
in non-radiative cosmological simulations of dark matter and baryons. We find
that the bulk of energy dissipation at redshift zero occurs in shocks with Mach
numbers around . Furthermore, almost of the
thermalization is contributed by shocks in the warm hot intergalactic medium
(WHIM), whereas occurs in clusters, groups and smaller halos.
Compared to previous studies, these findings revise the characterization of the
most important shocks towards higher Mach numbers and lower density structures.
Our results also suggest that regions with densities above and below
should be roughly equally important for the energetics of cosmic
ray acceleration through large-scale structure shocks.Comment: 16 pages, 13 figures, published in MNRAS, January 201
Zooming in on major mergers: dense, starbursting gas in cosmological simulations
We introduce the `Illustris zoom simulation project', which allows the study
of selected galaxies forming in the CDM cosmology with a 40 times
better mass resolution than in the parent large-scale hydrodynamical Illustris
simulation. We here focus on the starburst properties of the gas in four
cosmological simulations of major mergers. The galaxies in our high-resolution
zoom runs exhibit a bursty mode of star formation with gas consumption
timescales 10 times shorter than for the normal star formation mode. The strong
bursts are only present in the simulations with the highest resolution, hinting
that a too low resolution is the reason why the original Illustris simulation
showed a dearth of starburst galaxies. Very pronounced bursts of star formation
occur in two out of four major mergers we study. The high star formation rates,
the short gas consumption timescales and the morphology of these systems
strongly resemble observed nuclear starbursts. This is the first time that a
sample of major mergers is studied through self-consistent cosmological
hydrodynamical simulations instead of using isolated galaxy models setup on a
collision course. We also study the orbits of the colliding galaxies and find
that the starbursting gas preferentially appears in head-on mergers with very
high collision velocities. Encounters with large impact parameters do typically
not lead to the formation of starbursting gas.Comment: 13 pages, 7 figures, Accepted for publication in MNRA
The history of star formation in a LCDM universe
Employing hydrodynamic simulations of structure formation in a LCDM
cosmology, we study the history of cosmic star formation from the "dark ages"
at redshift z~20 to the present. In addition to gravity and ordinary
hydrodynamics, our model includes radiative heating and cooling of gas, star
formation, supernova feedback, and galactic winds. By making use of a
comprehensive set of simulations on interlocking scales and epochs, we
demonstrate numerical convergence of our results on all relevant halo mass
scales, ranging from 10^8 to 10^15 Msun/h. The predicted density of cosmic star
formation is broadly consistent with measurements, given observational
uncertainty. From the present epoch, it gradually rises by about a factor of
ten to a peak at z~5-6, which is beyond the redshift range where it has been
estimated observationally. 50% of the stars are predicted to have formed by
redshift z~2.1, and are thus older than 10.4 Gyr, while only 25% form at
redshifts lower than z~1. The mean age of all stars at the present is about 9
Gyr. Our model predicts a total stellar density at z=0 of Omega_*=0.004,
corresponding to about 10% of all baryons being locked up in long-lived stars,
in agreement with recent determinations of the luminosity density of the
Universe. We determine the "multiplicity function of cosmic star formation" as
a function of redshift; i.e. the distribution of star formation with respect to
halo mass. We also briefly examine possible implications of our predicted star
formation history for reionisation of hydrogen in the Universe. We find that
the star formation rate predicted by the simulations is sufficient to account
for hydrogen reionisation by z~6, but only if a high escape fraction close to
unity is assumed. (abridged)Comment: updated to match published version, minor plotting error in Fig.12
corrected, 25 pages, version with high-resolution figures available at
http://www.mpa-garching.mpg.de/~volker/paper_sfr
On the operation of the chemothermal instability in primordial star-forming clouds
We investigate the operation of the chemothermal instability in primordial
star-forming clouds with a suite of three-dimensional, moving-mesh simulations.
In line with previous studies, we find that the gas at the centre of
high-redshift minihaloes becomes chemothermally unstable as three-body
reactions convert the atomic hydrogen into a fully molecular gas. The
competition between the increasing rate at which the gas cools and the
increasing optical depth to H2 line emission creates a characteristic dip in
the cooling time over the free-fall time on a scale of 100 au. As a result, the
free-fall time decreases to below the sound-crossing time, and the cloud may
become gravitationally unstable and fragment on a scale of a few tens of au
during the initial free-fall phase. In three of the nine haloes investigated,
secondary clumps condense out of the parent cloud, which will likely collapse
in their own right before they are accreted by the primary clump. In the other
haloes, fragmentation at such an early stage is less likely. However, given
that previous simulations have shown that the infall velocity decreases
substantially once the gas becomes rotationally supported, the amount of time
available for perturbations to develop may be much greater than is evident from
the limited period of time simulated here.Comment: 17 pages, 12 figures, accepted for publication in MNRAS, simulation
movie available at http://www.cfa.harvard.edu/~tgrei
Simulating a metallicity-dependent initial mass function: Consequences for feedback and chemical abundances
Observational and theoretical arguments increasingly suggest that the initial
mass function (IMF) of stars may depend systematically on environment, yet most
galaxy formation models to date assume a universal IMF. Here we investigate
simulations of the formation of Milky Way analogues run with an empirically
derived metallicity-dependent IMF and the moving-mesh code AREPO in order to
characterize the associated uncertainties. In particular, we compare a constant
Chabrier and a varying metallicity-dependent IMF in cosmological,
magneto-hydrodynamical zoom-in simulations of Milky Way-sized halos. We find
that the non-linear effects due to IMF variations typically have a limited
impact on the morphology and the star formation histories of the formed
galaxies. Our results support the view that constraints on stellar-to-halo mass
ratios, feedback strength, metallicity evolution and metallicity distributions
are in part degenerate with the effects of a non-universal,
metallicity-dependent IMF. Interestingly, the empirical relation we use between
metallicity and the high mass slope of the IMF does not aid in the quenching
process. It actually produces up to a factor of 2-3 more stellar mass if
feedback is kept constant. Additionally, the enrichment history and the z = 0
metallicity distribution are significantly affected. In particular, the alpha
enhancement pattern shows a steeper dependence on iron abundance in the
metallicity-dependent model, in better agreement with observational
constraints.Comment: 9 pages, published in MNRA
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