SIMULATIONS OF DISK GALAXIES WITH COSMIC RAY DRIVEN GALACTIC WINDS

Abstract

We present results from high-resolution hydrodynamic simulations of isolated\nSMC- and Milky Way-sized galaxies that include a model for feedback from\ngalactic cosmic rays (CRs). We find that CRs are naturally able to drive winds\nwith mass loading factors of up to ~10 in dwarf systems. The scaling of the\nmass loading factor with circular velocity between the two simulated systems is\nconsistent with \\propto v_c^{1-2} required to reproduce the faint end of the\ngalaxy luminosity function. In addition, simulations with CR feedback reproduce\nboth the normalization and the slope of the observed trend of wind velocity\nwith galaxy circular velocity. We find that winds in simulations with CR\nfeedback exhibit qualitatively different properties compared to SN driven\nwinds, where most of the acceleration happens violently in situ near star\nforming sites. In contrast, the CR-driven winds are accelerated gently by the\nlarge-scale pressure gradient established by CRs diffusing from the\nstar-forming galaxy disk out into the halo. The CR-driven winds also exhibit\nmuch cooler temperatures and, in the SMC-sized system, warm (T~10^4 K) gas\ndominates the outflow. The prevalence of warm gas in such outflows may provide\na clue as to the origin of ubiquitous warm gas in the gaseous halos of galaxies\ndetected via absorption lines in quasar spectra.\n