27 research outputs found
Cosmological Hydrodynamics: Thermal Conduction and Cosmic Rays
Hydrodynamische Simulationen haben sich in den letzten Jahren zu einem wichtigen Werkzeug in der Kosmologie entwickelt. Es ist Ziel dieser Arbeit, einen bestehenden Simulationscode durch weitere physikalische Effekte zu erweitern, um deren Auswirkungen in selbstkonsistenter Art und Weise untersuchen zu können. Es wird ein Formalismus vorgestellt, der die Wärmeleitung in einem heißen, diffusen Plasma nachgebildet. Ferner präsentiere ich eine neuartige Methode, kosmische Teilchenstrahlung durch ein als einfach parametrisiert angenommenes Impulssprektrum der Strahlungsteilchen in hydrodynamischen Simulationen mitsamt ihren dynamischen Effekten zu berücksichtigen und untersuchen.
Es zeigt sich in durchgeführten Simulationen, daß die Wärmeleitung, obwohl sie unter bestimmten Umständen die Kühleffekte ausgleichen kann, in den durchgeführten kosmologischen Simulationen nicht zu einer Reduzierung der Akkretionsrate in Galaxienhaufen führte. Es zeigen sich dennoch in Temperatur- und Strahlungsprofilen der simulierten Objekte starke Auswirkungen der Wärmeleitung.
Die kosmische Teilchenstrahlung zeigt in weiteren Simulationen deutliche Auswirkungen auf die Evolution von Strukturen, insbesondere bei der Regulierung von Sternentstehung in kleinen Galaxien (solchen mit Virialgeschwindigkeiten von unter ∼ 80km/s). Hier führt sie zu einer staken Unterdrückung der Sternenbildung, in zunehmendem Maße für kleinere Galaxien mit einer geringeren Gesamtmasse. Durch diese Unterdrückung wird bei statistischer Betrachtung auch die Steigung der Leuchtkrafts-Verteilungfunktion von Galaxien an ihrem leuchtschwachen Ende stark beeinflußt; letztere wird deutlich flacher und bringt Simulationsergebnisse somit merklich näher an beobachtete Werte
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
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
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
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
Galactic winds driven by cosmic-ray streaming
Galactic winds are observed in many spiral galaxies with sizes from dwarfs up
to the Milky Way, and they sometimes carry a mass in excess of that of newly
formed stars by up to a factor of ten. Multiple driving processes of such winds
have been proposed, including thermal pressure due to supernova-heating, UV
radiation pressure on dust grains, or cosmic ray (CR) pressure. We here study
wind formation due to CR physics using a numerical model that accounts for CR
acceleration by supernovae, CR thermalization, and advective CR transport. In
addition, we introduce a novel implementation of CR streaming relative to the
rest frame of the gas. We find that CR streaming drives powerful and sustained
winds in galaxies with virial masses M_200 < 10^{11} Msun. In dwarf galaxies
(M_200 ~ 10^9 Msun) the winds reach a mass loading factor of ~5, expel ~60 per
cent of the initial baryonic mass contained inside the halo's virial radius and
suppress the star formation rate by a factor of ~5. In dwarfs, the winds are
spherically symmetric while in larger galaxies the outflows transition to
bi-conical morphologies that are aligned with the disc's angular momentum axis.
We show that damping of Alfven waves excited by streaming CRs provides a means
of heating the outflows to temperatures that scale with the square of the
escape speed. In larger haloes (M_200 > 10^{11} Msun), CR streaming is able to
drive fountain flows that excite turbulence. For halo masses M_200 > 10^{10}
Msun, we predict an observable level of H-alpha and X-ray emission from the
heated halo gas. We conclude that CR-driven winds should be crucial in
suppressing and regulating the first epoch of galaxy formation, expelling a
large fraction of baryons, and - by extension - aid in shaping the faint end of
the galaxy luminosity function. They should then also be responsible for much
of the metal enrichment of the intergalactic medium.Comment: 25 pages, 14 figures, accepted by MNRA
Cosmic ray driven outflows from high redshift galaxies
We study winds in high redshift galaxies driven by a relativistic cosmic ray
(proton) component in addition to the hot thermal gas component. Cosmic rays
(CRs) are likely to be efficiently generated in supernova shocks inside
galaxies. We obtain solutions of such CR driven free winds in a gravitational
potential of the NFW form, relevant to galaxies. Cosmic rays naturally provide
the extra energy and/or momentum input to the system, needed for a transonic
wind solution in a gas with adiabatic index . We show that CRs can
effectively drive winds even when the thermal energy of the gas is lost due to
radiative cooling. These wind solutions predict an asymptotic wind speed
closely related to the circular velocity of the galaxy. Furthermore, the mass
outflow rate per unit star formation rate (eta_w) is predicted to be ~ 0.2-0.5
for massive galaxies, with masses . We show
eta_w to be inversely proportional to the square of the circular velocity.
Magnetic fields at the G levels are also required in these galaxies to
have a significant mass loss. A large eta_w for small mass galaxies implies
that CR driven outflows could provide a strong negative feedback to the star
formation in dwarf galaxies. Further, our results will also have important
implications to the metal enrichment of the IGM. These conclusions are
applicable to the class of free wind models where the source region is confined
to be within the sonic point.Comment: Now accepted for publication in MNRAS, Change in the "Discussion and
conclusions" sectio
Satellite galaxies in hydrodynamical simulations of Milky Way sized galaxies
Collisionless simulations of the CDM cosmology predict a plethora of dark
matter substructures in the halos of Milky Way sized galaxies, yet the number
of known luminous satellites galaxies is very much smaller, a discrepancy that
has become known as the `missing satellite problem'. The most massive
substructures have been shown to be plausibly the hosts of the brightest
satellites, but it remains unclear which processes prevent star formation in
the many other, purely dark substructures. We use high-resolution hydrodynamic
simulations of the formation of Milky Way sized galaxies in order to test how
well such self-consistent models of structure formation match the observed
properties of the Galaxy's satellite population. For the first time, we include
in such calculations feedback from cosmic rays injected into the star forming
gas by supernovae as well as the energy input from supermassive black holes
growing at the Milky Way's centre and its progenitor systems. We find that
non-thermal particle populations quite strongly suppress the star formation
efficiency of the smallest galaxies. In fact, our cosmic ray model is able to
reproduce the observed faint-end of the satellite luminosity function, while
models that include only the effects of cosmic reionization, or galactic winds,
do significantly worse. Our simulated satellite population approximately
matches available kinematic data on the satellites and their observed spatial
distribution. We conclude that a proper resolution of the missing satellite
problem likely requires the inclusion of non-standard physics for regulating
star formation in the smallest halos, and that cosmic reionization alone may
not be sufficient.Comment: 20 pages, 17 figure
Thermal conduction in cosmological SPH simulations
Thermal conduction in the intracluster medium has been proposed as a possible
heating mechanism for offsetting central cooling losses in rich clusters of
galaxies. In this study, we introduce a new formalism to model conduction in a
diffuse ionised plasma using smoothed particle hydrodynamics (SPH), and we
implement it in the parallel TreePM/SPH-code GADGET-2. We consider only
isotropic conduction and assume that magnetic suppression can be described in
terms of an effective conductivity, taken as a fixed fraction of the
temperature-dependent Spitzer rate. We also account for saturation effects in
low-density gas. Our formulation manifestly conserves thermal energy even for
individual and adaptive timesteps, and is stable in the presence of small-scale
temperature noise. This allows us to evolve the thermal diffusion equation with
an explicit time integration scheme along with the ordinary hydrodynamics. We
use a series of simple test problems to demonstrate the robustness and accuracy
of our method. We then apply our code to spherically symmetric realizations of
clusters, constructed under the assumptions of hydrostatic equilibrium and a
local balance between conduction and radiative cooling. While we confirm that
conduction can efficiently suppress cooling flows for an extended period of
time in these isolated systems, we do not find a similarly strong effect in a
first set of clusters formed in self-consistent cosmological simulations.
However, their temperature profiles are significantly altered by conduction, as
is the X-ray luminosity.Comment: 14 pages, 7 figures, accepted by MNRAS, high resolution version
available at http://www.mpa-garching.mpg.de/~jubelgas/conduction.pdf. Fixed
typos in eq. 20,22,2