201 research outputs found
Clustered Supernovae Drive Powerful Galactic Winds After Super-Bubble Breakout
We use three-dimensional hydrodynamic simulations of vertically stratified
patches of galactic discs to study how the spatio-temporal clustering of
supernovae (SNe) enhances the power of galactic winds. SNe that are randomly
distributed throughout a galactic disc drive inefficient galactic winds because
most supernova remnants lose their energy radiatively before breaking out of
the disc. Accounting for the fact that most star formation is clustered
alleviates this problem. Super-bubbles driven by the combined effects of
clustered SNe propagate rapidly enough to break out of galactic discs well
before the clusters' SNe stop going off. The radiative losses post-breakout are
reduced dramatically and a large fraction () of the energy
released by SNe vents into the halo powering a strong galactic wind. These
energetic winds are capable of providing strong preventative feedback and eject
substantial mass from the galaxy with outflow rates on the order of the star
formation rate. The momentum flux in the wind is only of order that injected by
the SNe, because the hot gas vents before doing significant work on the
surroundings. We show that our conclusions hold for a range of galaxy
properties, both in the local Universe (e.g., M82) and at high redshift (e.g.,
star forming galaxies). We further show that if the efficiency of
forming star clusters increases with increasing gas surface density, as
suggested by theoretical arguments, the condition for star cluster-driven
super-bubbles to break out of galactic discs corresponds to a threshold star
formation rate surface density for the onset of galactic winds
M yr kpc, of order that observed.Comment: 19 pages, 12 figures, and 3 page appendix with 6 figures. Movies
available at http://w.astro.berkeley.edu/~dfielding/#SNeDrivenWinds
The biasing of baryons on the cluster mass function and cosmological parameter estimation
We study the effect of baryonic processes on the halo mass function in the
galaxy cluster mass range using a catalogue of 153 high resolution cosmological
hydrodynamical simulations performed with the AMR code ramses. We use the
results of our simulations within a simple analytical model to gauge the
effects of baryon physics on the halo mass function. Neglect of AGN feedback
leads to a significant boost in the cluster mass function similar to that
reported by other authors. However, including AGN feedback not only gives rise
to systems that are similar to observed galaxy clusters, but they also reverse
the global baryonic effects on the clusters. The resulting mass function is
closer to the unmodified dark matter halo mass function but still contains a
mass dependent bias at the 5-10% level. These effects bias measurements of the
cosmological parameters, such as and . For current cluster
surveys baryonic effects are within the noise for current survey volumes, but
forthcoming and planned large SZ, X-ray and multi-wavelength surveys will be
biased at the percent level by these processes. The predictions for the halo
mass function including baryonic effects need to be carefully studied with
larger and improved simulations. However, simulations of full cosmological
boxes with the resolution we achieve and including AGN feedback are still
computationally challenging.Comment: 12 pages, 3 tables, 6 figures, accepted for publication in MNRA
Brightest Cluster Galaxies in Cosmological Simulations with Adaptive Mesh Refinement: Successes and Failures
A large sample of cosmological hydrodynamical zoom-in simulations with
Adaptive Mesh Refinement (AMR) is analysed to study the properties of simulated
Brightest Cluster Galaxies (BCGs). Following the formation and evolution of
BCGs requires modeling an entire galaxy cluster, because the BCG properties are
largely influenced by the state of the gas in the cluster and by interactions
and mergers with satellites. BCG evolution is also deeply influenced by the
presence of gas heating sources such as Active Galactic Nuclei (AGNs) that
prevent catastrophic cooling of large amounts of gas. We show that AGN feedback
is one of the most important mechanisms in shaping the properties of BCGs at
low redshift by analysing our statistical sample of simulations with and
without AGN feedback. When AGN feedback is included BCG masses, sizes, star
formation rates and kinematic properties are closer to those of the observed
systems. Some small discrepancies are observed only for the most massive BCGs
and in the fraction of star-forming BCGs, effects that might be due to physical
processes that are not included in our model.Comment: 11 pages, 2 tables, 8 figures. Accepted for publication in MNRA
Simulations of Jet Heating in Galaxy Clusters: Successes and Challenges
We study how jets driven by active galactic nuclei influence the cooling flow
in Perseus-like galaxy cluster cores with idealised, non-relativistic,
hydrodynamical simulations performed with the Eulerian code ATHENA using
high-resolution Godunov methods with low numerical diffusion. We use novel
analysis methods to measure the cooling rate, the heating rate associated to
multiple mechanisms, and the power associated with adiabatic
compression/expansion. A significant reduction of the cooling rate and cooling
flow within 20 kpc from the centre can be achieved with kinetic jets. However,
at larger scales and away from the jet axis, the system relaxes to a cooling
flow configuration. Jet feedback is anisotropic and is mostly distributed along
the jet axis, where the cooling rate is reduced and a significant fraction of
the jet power is converted into kinetic power of heated outflowing gas. Away
from the jet axis weak shock heating represents the dominant heating source.
Turbulent heating is significant only near the cluster centre, but it becomes
inefficient at 50 kpc scales where it only represents a few percent of the
total heating rate. Several details of the simulations depend on the choice
made for the hydro solver, a consequence of the difficulty of achieving proper
numerical convergence for this problem: current physics implementations and
resolutions do not properly capture multi-phase gas that develops as a
consequence of thermal instability. These processes happen at the grid scale
and leave numerical solutions sensitive to the properties of the chosen hydro
solver.Comment: Accepted for publication on MNRA
Infall near clusters of galaxies: comparing gas and dark matter velocity profiles
We consider the dynamics in and near galaxy clusters. Gas, dark matter and
galaxies are presently falling into the clusters between approximately 1 and 5
virial radii. At very large distances, beyond 10 virial radii, all matter is
following the Hubble flow, and inside the virial radius the matter particles
have on average zero radial velocity. The cosmological parameters are imprinted
on the infall profile of the gas, however, no method exists, which allows a
measurement of it. We consider the results of two cosmological simulations
(using the numerical codes RAMSES and Gadget) and find that the gas and dark
matter radial velocities are very similar. We derive the relevant dynamical
equations, in particular the generalized hydrostatic equilibrium equation,
including both the expansion of the Universe and the cosmological background.
This generalized gas equation is the main new contribution of this paper. We
combine these generalized equations with the results of the numerical
simulations to estimate the contribution to the measured cluster masses from
the radial velocity: inside the virial radius it is negligible, and inside two
virial radii the effect is below 40%, in agreement the earlier analyses for DM.
We point out how the infall velocity in principle may be observable, by
measuring the gas properties to distance of about two virial radii, however,
this is practically not possible today.Comment: 7 pages, 3 figures, to appear in MNRA
The formation of the brightest cluster galaxies in cosmological simulations: the case for active galactic nucleus feedback
We use 500 pc resolution cosmological simulations of a Virgo-like galaxy cluster to study the properties of the brightest cluster galaxy (BCG) that forms at the centre of the halo. We compared two simulations; one incorporating only supernova feedback and a second that also includes prescriptions for black hole growth and the resulting active galactic nucleus (AGN) feedback from gas accretion. As previous work has shown, with supernova feedback alone we are unable to reproduce any of the observed properties of massive cluster ellipticals. The resulting BCG rotates quickly, has a high Sérsic index, a strong mass excess in the centre and a total central density profile falling more steeply than isothermal. Furthermore, it is far too efficient at converting most of the available baryons into stars which is strongly constrained by abundance matching. With a treatment of black hole dynamics and AGN feedback the BCG properties are in good agreement with data: they rotate slowly, have a cored surface density profile, a flat or rising velocity dispersion profile and a low stellar mass fraction. The AGN provides a new mechanism to create cores in luminous elliptical galaxies; the core expands due to the combined effects of heating from dynamical friction of sinking massive black holes and AGN feedback that ejects gaseous material from the central region
Rhapsody-G simulations: galaxy clusters as baryonic closed boxes and the covariance between hot gas and galaxies
Within a sufficiently large cosmic volume, conservation of baryons implies a
simple `closed box' view in which the sum of the baryonic components must equal
a constant fraction of the total enclosed mass. We present evidence from
Rhapsody-G hydrodynamic simulations of massive galaxy clusters that the
closed-box expectation may hold to a surprising degree within the interior,
non-linear regions of haloes. At a fixed halo mass, we find a significant
anti-correlation between hot gas mass fraction and galaxy mass fraction (cold
gas + stars), with a rank correlation coefficient of -0.69 within .
Because of this anti-correlation, the total baryon mass serves as a low-scatter
proxy for total cluster mass. The fractional scatter of total baryon fraction
scales approximately as , while the scatter of
either gas mass or stellar mass is larger in magnitude and declines more slowly
with increasing radius. We discuss potential observational tests using cluster
samples selected by optical and hot gas properties; the simulations suggest
that joint selection on stellar and hot gas has potential to achieve 5% scatter
in total halo mass.Comment: 10 pages, 6 figures, 3 tables; replaced to match published versio
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