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تسجيل مستخدم جديدThe Launching of Cosmic Ray Driven Outflows
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Cosmic rays (CRs) are thought to be an important feedback mechanism in star-forming galaxies. They can provide an important source of pressure support and possibly drive outflows. We perform multidimensional CR-magnetohydrodynamic simulations including transport by streaming and diffusion to investigate wind launching from an initially hydrostatic atmosphere by CRs. We estimate a characteristic Eddington limit on the CR flux for which the CR force exceeds gravity and compare it to simulated systems. Scaling our results to conditions in star-forming galaxies, we find that CRs are likely to contribute to driving outflows for a broad range of star formation environments. We quantify the momentum and energy transfer between CRs and gas, along with the associated mass outflow rates under different assumptions about the relative importance of streaming and diffusion for transport. In simulations with streaming, we observe the growth and saturation of the CR acoustic instability, but the CRs and gas remain well coupled, with CR momentum transferred efficiently to the gas even when this instability is present. Higher CR fluxes transferr more energy to the gas and drive stronger outflows. When streaming is present, most of the transferred energy takes the form of Alfv{e}n wave heating of the gas, raising its pressure and internal energy, with a lower fractional contribution to the kinetic energy of the outflow. We also consider runs with radiative cooling, which modifies gas temperature and pressure profiles but does not seem to have a large impact on the mass outflow for super-Eddington CR fluxes.
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Cosmic rays (CRs) are a plausible mechanism for launching winds of cool material from the discs of star-forming galaxies. However, there is no consensus on what types of galaxies likely host CR-driven winds, or what role these winds might play in reg
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We study the effects of cosmic rays (CRs) on outflows from star-forming galaxies in the circum and inter-galactic medium (CGM/IGM), in high-resolution, fully-cosmological FIRE-2 simulations (accounting for mechanical and radiative stellar feedback, m
agnetic fields, anisotropic conduction/viscosity/CR diffusion and streaming, and CR losses). We showed previously that massive ($M_{rm halo}gtrsim 10^{11},M_{odot}$), low-redshift ($zlesssim 1-2$) halos can have CR pressure dominate over thermal CGM pressure and balance gravity, giving rise to a cooler CGM with an equilibrium density profile. This dramatically alters outflows. Absent CRs, high gas thermal pressure in massive halos traps galactic outflows near the disk, so they recycle. With CRs injected in supernovae as modeled here, the low-pressure halo allows escape and CR pressure gradients continuously accelerate this material well into the IGM in fast outflows, while lower-density gas at large radii is accelerated in-situ into slow outflows that extend to $>$Mpc scales. CGM/IGM outflow morphologies are radically altered: they become mostly volume-filling (with inflow in a thin mid-plane layer) and coherently biconical from the disk to $>$Mpc. The CR-driven outflows are primarily cool ($Tsim10^{5},$K) and low-velocity. All of these effects weaken and eventually vanish at lower halo masses ($lesssim 10^{11},M_{odot}$) or higher redshifts ($zgtrsim 1-2$), reflecting the ratio of CR to thermal+gravitational pressure in the outer halo. We present a simple analytic model which explains all of the above phenomena.
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