Issue |
A&A
Volume 481, Number 1, April I 2008
Science with Hinode
|
|
---|---|---|
Page(s) | 33 - 63 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20065295 | |
Published online | 12 December 2007 |
Cosmic ray feedback in hydrodynamical simulations of galaxy formation
1
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85740 Garching bei München, Germany e-mail: [jubelgas;vspringel;ensslin]@mpa-garching.mpg.de
2
Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 3H8, Canada e-mail: pfrommer@cita.utoronto.ca
Received:
28
March
2006
Accepted:
16
November
2007
It is well known that cosmic rays 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 cosmic rays
and its implementation in the parallel smoothed particle
hydrodynamics code GADGET-2. In our methodology, the
non-thermal cosmic ray 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 cosmic ray 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 cosmic rays.
Using a number of test problems, we show that our scheme is
numerically robust and efficient, allowing us to carry out the first
cosmological structure formation simulations that account for cosmic
ray physics, together with radiative cooling and star formation. In
simulations of isolated galaxies, we find that cosmic rays can
significantly reduce the star formation efficiencies of small
galaxies, with virial velocities below ~,
an effect that 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. The latter becomes slightly flatter. When cosmic ray
acceleration in shock waves is followed as well, we find that up to
of the energy dissipated at structure formation shocks can
appear as cosmic ray pressure at redshifts around
, but
this fraction drops to ~
at low redshifts when the shock
distribution becomes increasingly dominated by lower Mach numbers.
Despite this large cosmic ray energy content in the high-redshift
intergalactic medium, the flux power spectrum of the Lyman-α forest is only affected on very small scales of
, and at a weak level of
. Within virialized
objects, we find lower contributions of CR-pressure, due to the
increased efficiency of loss processes at higher densities, the
lower Mach numbers of shocks inside halos, and the softer adiabatic
index of CRs, which disadvantages them when a composite of thermal
gas and cosmic rays is adiabatically compressed. The total energy in
cosmic rays relative to the thermal energy within the virial radius
drops from 20% for
halos to 5% for rich galaxy clusters
of mass
in non-radiative simulations. Interestingly, the
lower effective adiabatic index also increases the compressibility
of the intrahalo medium, an effect that slightly increases the
central concentration of the gas and the baryon fraction within the
virial radius. We find that this can enhance the cooling rate onto
central cluster galaxies, even though the galaxies in the cluster
periphery become slightly less luminous as a result of cosmic ray
feedback.
Key words: methods: numerical / acceleration of particles / ISM: general / galaxies: structure / galaxies: clusters: general / intergalactic medium
© ESO, 2008
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