Cosmic rays can drive strong outflows from gas-rich high-redshift disk galaxies

We present simulations of the magnetized interstellar medium (ISM) in models of massive star forming (40 Msun / yr) disk galaxies with high gas surface densities (~100 Msun / pc^2) similar to observed star forming high-redshift disks. We assume that type II supernovae deposit 10 per cent of their energy into the ISM as cosmic rays and neglect the additional deposition of thermal energy or momentum. With a typical Galactic diffusion coefficient for CRs (3e28 cm^2 / s) we demonstrate that this process alone can trigger the local formation of a strong low density galactic wind maintaining vertically open field lines. Driven by the additional pressure gradient of the relativistic fluid the wind speed can exceed 1000 km/s, much higher than the escape velocity of the galaxy. The global mass loading, i.e. the ratio of the gas mass leaving the galactic disk in a wind to the star formation rate becomes of order unity once the system has settled into an equilibrium. We conclude that relativistic particles accelerated in supernova remnants alone provide a natural and efficient mechanism to trigger winds similar to observed mass-loaded galactic winds in high-redshift galaxies. These winds also help explaining the low efficiencies for the conversion of gas into stars in galaxies as well as the early enrichment of the intergalactic medium with metals. This mechanism can be at least of similar importance than the traditionally considered momentum feedback from massive stars and thermal and kinetic feedback from supernova explosions.Comment: 5 pages, 5 figures, accepted in ApJL; corrected titl

Influence of internal energy on the stability of relativistic flows

A set of simulations concerning the influence of internal energy on the stability of relativistic jets is presented. Results show that perturbations saturate when the amplitude of the velocity perturbation approaches the speed of light limit. Also, contrary to what predicted by linear stability theory, jets with higher specific internal energy appear to be more stable.Comment: 4 pages, 2 figures. To be published in proceedings of the V Scientific Meeting of the SEA (Highlights of Spanish Astrophysics III), Toledo, Spain, September 9-13, 2002. Ed. J. Gallego, J. Zamorano and N. Cardiel, ASSL, Kluwer. Uploaded version of figure 3 is low resolution due to size problems: available high resolution version on request to Manuel Perucho ([email protected]

Resonant Kelvin-Helmholtz modes in sheared relativistic flows

Qualitatively new aspects of the (linear and non-linear) stability of sheared relativistic (slab) jets are analyzed. The linear problem has been solved for a wide range of jet models well inside the ultrarelativistic domain (flow Lorentz factors up to 20; specific internal energies $\approx 60c^2$). As a distinct feature of our work, we have combined the analytical linear approach with high-resolution relativistic hydrodynamical simulations, which has allowed us i) to identify, in the linear regime, resonant modes specific to the relativistic shear layer ii) to confirm the result of the linear analysis with numerical simulations and, iii) more interestingly, to follow the instability development through the non-linear regime. We find that very high-order reflection modes with dominant growth rates can modify the global, long-term stability of the relativistic flow. We discuss the dependence of these resonant modes on the jet flow Lorentz factor and specific internal energy, and on the shear layer thickness. The results could have potential applications in the field of extragalactic relativistic jets.Comment: Accepted for publication in Physical Review E. For better quality images, please check http://www.mpifr-bonn.mpg.de/staff/mperucho/Research.htm

Cooler and smoother - the impact of cosmic rays on the phase structure of galactic outflows

We investigate the impact of cosmic rays (CRs) on galactic outflows from a multiphase interstellar medium with solar neighbourhood conditions. The three-dimensional magneto-hydrodynamical simulations include CRs as a relativistic fluid in the advection-diffusion approximation. The thermal and chemical state of the interstellar medium is computed with a non-equilibrium chemical network. We find that CRs [injected with 10 per cent of the supernova (SN) energy] efficiently support the launching of outflows and strongly affect their phase structure. Outflows leaving the midplane are denser (rho similar to 10(-26) g cm(-3)), colder (similar to 10(4) K) and slower (similar to 30 km s(-1)) if CRs are considered in addition to thermal SNe. The CR-supported outflows are also smoother, in particular at larger heights (>1 kpc above the midplane) without the direct impact of SN explosions. Approximately, 5 per cent-25 per cent of the injected CR energy is lost via hadronic cooling. Smaller diffusion coefficients lead to slightly larger hadronic losses but allow for steeper CR pressure gradients, stronger outflows and larger accelerations. Up to a height of z similar to 1 kpc, there are large volumes in approximate pressure equilibrium between the thermal and the CR component. At larger altitudes, the CR pressure is 10-100 times as large as the thermal counterpart. More than similar to 1 kpc away from the midplane, CRs provide the dominant gas acceleration mechanism

LAUNCHING COSMIC-RAY-DRIVEN OUTFLOWS FROM THE MAGNETIZED INTERSTELLAR MEDIUM

We present a hydrodynamical simulation of the turbulent, magnetized, supernova (SN)-driven interstellar medium (ISM) in a stratified box that dynamically couples the injection and evolution of cosmic rays (CRs) and a self-consistent evolution of the chemical composition. CRs are treated as a relativistic fluid in the advection-diffusion approximation. The thermodynamic evolution of the gas is computed using a chemical network that follows the abundances of H+, H, H-2, CO, C+, and free electrons and includes (self-) shielding of the gas and dust. We find that CRs perceptibly thicken the disk with the heights of 90% (70%) enclosed mass reaching greater than or similar to 1.5 kpc (greater than or similar to 0.2 kpc). The simulations indicate that CRs alone can launch and sustain strong outflows of atomic and ionized gas with mass loading factors of order unity, even in solar neighborhood conditions and with a CR energy injection per SN of 10(50) erg, 10% of the fiducial thermal energy of an SN. The CR-driven outflows have moderate launching velocities close to the midplane (less than or similar to 100 km s(-1)) and are denser (rho similar to 10(-24)-10(-26) g cm(-3)), smoother, and colder than the (thermal) SN-driven winds. The simulations support the importance of CRs for setting the vertical structure of the disk as well as the driving of winds