389 research outputs found
Magnetic Fields in Clusters of Galaxies
A brief overview about our knowledge on galaxy cluster magnetic fields is
provided. Emphasize is given to the mutual dependence of our knowledge on
relativistic particles in galaxy clusters and the magnetic field strength.
Furthermore, we describe efforts to measure magnetic field strengths,
characteristic length-scales, and power-spectra with reliable accuracy. An
interpretation of these results in terms of non-helical dynamo theory is given.
If this interpretation turns out to be correct, the understanding of cluster
magnetic fields is directly connected to our understanding of intra-cluster
turbulence.Comment: to appear in "Magnetized plasma in galaxy evolution", 8 pages, 2
figure
Cosmic ray feedback in hydrodynamical simulations of galaxy and galaxy cluster formation
It is well known that cosmic rays (CRs) contribute significantly to the
pressure of the interstellar medium in our own Galaxy, suggesting that they may
play an important role in regulating star formation during the formation and
evolution of galaxies. We will present a novel numerical treatment of the
physics of CRs and its implementation in the parallel smoothed particle
hydrodynamics (SPH) code GADGET-2. In our methodology, the non-thermal CR
population is treated self-consistently in order to assess its dynamical impact
on the thermal gas as well as other implications on cosmological observables.
In simulations of galaxy formation, we find that CRs can significantly reduce
the star formation efficiencies of small galaxies. This effect becomes
progressively stronger towards low mass scales. In cosmological simulations of
the formation of dwarf galaxies at high redshift, we find that the total
mass-to-light ratio of small halos and the faint-end of the luminosity function
are affected. In high resolution simulations of galaxy clusters, we find lower
contributions of CR pressure, due to the smaller CR injection efficiencies at
low Mach number flow shocks inside halos, and the softer adiabatic index of
CRs, which disfavours them when a composite of thermal gas and CRs is
adiabatically compressed. Within cool core regions, the CR pressure reaches
equipartition with the thermal pressure leading to an enhanced compressibility
of the central intra-cluster medium, an effect that increases the central
density and pressure of the gas. While the X-ray luminosity in low mass cool
core clusters is boosted, the integrated Sunyaev-Zel'dovich effect is only
slightly changed. The resolved Sunyaev-Zel'dovich maps, however, show a larger
variation with an increased central flux decrement.Comment: 4 pages, 2 figures, to appear in the Proceedings of "Cosmic
Frontiers", August 2006, Durham (UK), full resolution version available at
http://www.cita.utoronto.ca/~pfrommer/Proceedings/Durham.pd
Detecting shock waves in cosmological smoothed particle hydrodynamics simulations
We develop a formalism for the identification and accurate estimation of the
strength of structure formation shocks during cosmological smoothed particle
hydrodynamics simulations. Shocks not only play a decisive role for the
thermalization of gas in virialising structures but also for the acceleration
of relativistic cosmic rays (CRs) through diffusive shock acceleration. Our
formalism is applicable both to ordinary non-relativistic thermal gas, and to
plasmas composed of CRs and thermal gas. To this end, we derive an analytical
solution to the one-dimensional Riemann shock tube problem for a composite
plasma of CRs and thermal gas. We apply our methods to study the properties of
structure formation shocks in high-resolution hydrodynamic simulations of the
LCDM model. We find that most of the energy is dissipated in weak internal
shocks with Mach numbers M~2 which are predominantly central flow shocks or
merger shock waves traversing halo centres. Collapsed cosmological structures
are surrounded by external shocks with much higher Mach numbers up to M~1000,
but they play only a minor role in the energy balance of thermalization. We
show that after the epoch of cosmic reionisation the Mach number distribution
is significantly modified by an efficient suppression of strong external shock
waves due to the associated increase of the sound speed of the diffuse gas.
Invoking a model for CR acceleration in shock waves, we find that the average
strength of shock waves responsible for CR energy injection is higher than that
for shocks that dominate the thermalization of the gas. When combined with
radiative dissipation and star formation, our formalism can also be used to
study CR injection by supernova shocks, or to construct models for
shock-induced star formation in the interstellar medium. (abridged)Comment: 20 pages, 7 figures, just appeared in MNRAS, full resolution version
available at
http://www.cita.utoronto.ca/~pfrommer/Publications/MNRAS.367.113.pd
Cosmological structure formation shocks and cosmic rays in hydrodynamical simulations
Cosmological shock waves during structure formation not only play a decisive
role for the thermalization of gas in virializing structures but also for the
acceleration of relativistic cosmic rays (CRs) through diffusive shock
acceleration. We discuss a novel numerical treatment of the physics of cosmic
rays in combination with a formalism for identifying and measuring the shock
strength on-the-fly during a smoothed particle hydrodynamics simulation. In our
methodology, the non-thermal CR population is treated self-consistently in
order to assess its dynamical impact on the thermal gas as well as other
implications on cosmological observables. Using this formalism, we study the
history of the thermalization process in high-resolution hydrodynamic
simulations of the Lambda cold dark matter model. Collapsed cosmological
structures are surrounded by shocks with high Mach numbers up to 1000, but they
play only a minor role in the energy balance of thermalization. However, this
finding has important consequences for our understanding of the spatial
distribution of CRs in the large-scale structure. In high resolution
simulations of galaxy clusters, we find a low contribution of the averaged CR
pressure, due to the small acceleration efficiency of lower Mach numbers of
flow shocks inside halos and the softer adiabatic index of CRs. However, within
cool core regions, the CR pressure reaches equipartition with the thermal
pressure leading there to a lower effective adiabatic index and thus to an
enhanced compressibility of the central intracluster medium. This effect
increases the central density and pressure of the cluster and thus the
resulting X-ray emission and the central Sunyaev-Zel'dovich flux decrement. The
integrated Sunyaev-Zel'dovich effect, however, is only slightly changed.Comment: 6 pages, 3 figures, to appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany), full resolution version available at
http://www.cita.utoronto.ca/~pfrommer/Proceedings/Garching.pd
Simulating cosmic rays in clusters of galaxies - I. Effects on the Sunyaev-Zel'dovich effect and the X-ray emission
We performed high-resolution simulations of a sample of 14 galaxy clusters
that span a mass range from 5 x 10^13 M_solar/h to 2 x 10^15 M_solar/h to study
the effects of cosmic rays (CRs) on thermal cluster observables such as X-ray
emission and the Sunyaev-Zel'dovich effect. We analyse the CR effects on the
intra-cluster medium while simultaneously taking into account the cluster's
dynamical state as well as the mass of the cluster. The modelling of the cosmic
ray physics includes adiabatic CR transport processes, injection by supernovae
and cosmological structure formation shocks, as well as CR thermalization by
Coulomb interaction and catastrophic losses by hadronic interactions. While the
relative pressure contained in CRs within the virial radius is of the order of
2 per cent in our non-radiative simulations, their contribution rises to 32 per
cent in our simulations with dissipative gas physics including radiative
cooling, star formation, and supernova feedback. Interestingly, in the
radiative simulations the relative CR pressure reaches high values of the order
of equipartition with the thermal gas in each cluster galaxy due to the fast
thermal cooling of gas which diminishes the thermal pressure support relative
to that in CRs. This also leads to a lower effective adiabatic index of the
composite gas that increases the compressibility of the intra-cluster medium.
This effect slightly increases the central density, thermal pressure and the
gas fraction. While the X-ray luminosity in low mass cool core clusters is
boosted by up to 40 per cent, the integrated Sunyaev-Zel'dovich effect appears
to be remarkably robust and the total flux decrement only slightly reduced by
typically 2 per cent. The resolved Sunyaev-Zel'dovich maps, however, show a
larger variation with an increased central flux decrement. [abridged]Comment: 25 pages, 15 figures, accepted by MNRAS, full resolution version
available at
http://www.cita.utoronto.ca/~pfrommer/Publications/CRs_clusters.pd
Impact of tangled magnetic fields on AGN-blown bubbles
There is growing consensus that feedback from AGN is the main mechanism
responsible for stopping cooling flows in clusters of galaxies. AGN are known
to inflate buoyant bubbles that supply mechanical power to the intracluster gas
(ICM). High Reynolds number hydrodynamical simulations show that such bubbles
get entirely disrupted within 100 Myr, as they rise in cluster atmospheres,
which is contrary to observations. This artificial mixing has consequences for
models trying to quantify the amount of heating and star formation in cool core
clusters of galaxies. It has been suggested that magnetic fields can stabilize
bubbles against disruption. We perform MHD simulations of fossil bubbles in the
presence of tangled magnetic fields using the high order PENCIL code. We focus
on the physically-motivated case where thermal pressure dominates over magnetic
pressure and consider randomly oriented fields with and without maximum
helicity and a case where large scale external fields drape the bubble.We find
that helicity has some stabilizing effect. However, unless the coherence length
of magnetic fields exceeds the bubble size, the bubbles are quickly shredded.
As observations of Hydra A suggest that lengthscale of magnetic fields may be
smaller then typical bubble size, this may suggest that other mechanisms, such
as viscosity, may be responsible for stabilizing the bubbles. However, since
Faraday rotation observations of radio lobes do not constrain large scale ICM
fields well if they are aligned with the bubble surface, the draping case may
be a viable alternative solution to the problem. A generic feature found in our
simulations is the formation of magnetic wakes where fields are ordered and
amplified. We suggest that this effect could prevent evaporation by thermal
conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures,
color printing recommended
The quest for cosmic ray protons in galaxy clusters
There have been many speculations about the presence of cosmic ray protons
(CRps) in galaxy clusters over the past two decades. However, no direct
evidence such as the characteristic gamma-ray signature of decaying pions has
been found so far. These pions would be a direct tracer of hadronic CRp
interactions with the ambient thermal gas also yielding observable synchrotron
and inverse Compton emission by additionally produced secondary electrons. The
obvious question concerns the type of galaxy clusters most likely to yield a
signal: Particularly suited sites should be cluster cooling cores due to their
high gas and magnetic energy densities. We studied a nearby sample of clusters
evincing cooling cores in order to place stringent limits on the cluster CRp
population by using non-detections of EGRET. In this context, we examined the
possibility of a hadronic origin of Coma-sized radio halos as well as radio
mini-halos. Especially for mini-halos, strong clues are provided by the very
plausible small amount of required CRp energy density and a matching radio
profile. Introducing the hadronic minimum energy criterion, we show that the
energetically favored CRp energy density is constrained to 2% +/- 1% of the
thermal energy density in Perseus. We also studied the CRp population within
the cooling core region of Virgo using the TeV gamma-ray detection of M 87 by
HEGRA. Both the expected radial gamma-ray profile and the required amount of
CRp support this hadronic scenario.Comment: 5 pages, 8 figures, to appear in a dedicated issue of the Journal of
the Korean Astronomical Society (JKAS). Proceedings of the "International
Conference on Cosmic Rays and Magnetic Fields in Large Scale Structure",
Busan, Korea, 200
Unveiling the composition of radio plasma bubbles in galaxy clusters with the Sunyaev-Zel'dovich effect
The Chandra X-ray Observatory is finding a large number of cavities in the
X-ray emitting intra-cluster medium which often coincide with the lobes of the
central radio galaxy. We propose high-resolution Sunyaev-Zel'dovich (SZ)
observations in order to infer the still unknown dynamically dominating
component of the radio plasma bubbles. This work calculates the thermal and
relativistic SZ emission of different compositions of these plasma bubbles
while simultaneously allowing for the cluster's kinetic SZ effect. As examples,
we present simulations of an Atacama Large Millimeter Array (ALMA) observation
and of a Green Bank Telescope (GBT) observation of the cores of the Perseus
cluster and Abell 2052. We predict a 5 sigma detection of the southern radio
bubble of Perseus in a few hours with the GBT and ALMA while assuming a
relativistic electron population within the bubble. In Abell 2052, a similar
detection would require a few ten hours with either telescope, the longer
exposures mainly being the result of the higher redshift and the lower central
temperature of this cluster. Future high-sensitivity multi-frequency SZ
observations will be able to infer the energy spectrum of the dynamically
dominating electron population in order to measure its temperature or spectral
characteristics. This knowledge can yield indirect indications for an
underlying radio jet model.Comment: 12 pages, 7 figures, accepted for publication in A&
Cosmic ray feedback in hydrodynamical simulations of galaxy formation
It is well known that cosmic rays (CRs) contribute significantly to the
pressure of the interstellar medium in our own Galaxy, suggesting that they may
play an important role in regulating star formation during the formation and
evolution of galaxies. We here discuss a novel numerical treatment of the
physics of CRs and its implementation in the parallel smoothed particle
hydrodynamics code GADGET-2. In our methodology, the non-thermal CR population
of each gaseous fluid element is approximated by a simple power law spectrum in
particle momentum, characterized by an amplitude, a cut-off, and a fixed slope.
Adiabatic compression, and a number of physical source and sink terms are
modelled which modify the CR pressure of each particle. The most important
sources considered are injection by supernovae and diffusive shock
acceleration, while the primary sinks are thermalization by Coulomb
interactions, and catastrophic losses by hadronic interactions. We also include
diffusion of CRs. Our scheme allows us to carry out the first cosmological
structure formation simulations that self-consistently account for CR physics.
In simulations of isolated galaxies, we find that CRs can significantly reduce
the star formation efficiencies of small galaxies, with virial velocities below
\~80 km/s, an effect that becomes progressively stronger towards low mass
scales. In cosmological simulations at high redshift, the total mass-to-light
ratio of small halos and the faint-end of the luminosity function are strongly
affected. When CR acceleration in shocks is followed as well, up to ~40% of the
energy dissipated at structure formation shocks can appear as CR pressure at
z~3-6, but this fraction drops to ~10% at low redshifts when the shock
distribution becomes increasingly dominated by lower Mach numbers. (abridged)Comment: submitted to A&A, 36 pages, 27 figures (partially in reduced
resolution
- …