The Origin and Kinematics of Cold Gas in Galactic Winds: Insight from Numerical Simulations - AstrophysicsReport as inadecuate

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Abstract: We study the origin of Na I absorbing gas in ultraluminous infrared galaxiesmotivated by the recent observations by Martin of extremely superthermallinewidths in this cool gas. We model the effects of repeated supernovaexplosions driving supershells in the central regions of molecular disks withM d=10^10 M \sun, using cylindrically symmetric gas dynamical simulations runwith ZEUS-3D. The shocked swept-up shells quickly cool and fragment byRayleigh-Taylor instability as they accelerate out of the dense, stratifieddisks. The numerical resolution of the cooling and compression at the shockfronts determines the peak shell density, and so the speed of Rayleigh-Taylorfragmentation. We identify cooled shells and shell fragments as Na I absorbinggas and study its kinematics. We find that simulations with a numericalresolution of \le 0.2 pc produce multiple Rayleigh-Taylor fragmented shells ina given line of sight. We suggest that the observed wide Na I absorption lines, = 320 \pm 120 km s^-1 are produced by these multiple fragmented shellstraveling at different velocities. We also suggest that some shell fragmentscan be accelerated above the observed average terminal velocity of 750 km s^-1by the same energy-driven wind with an instantaneous starburst of \sim 10^9M \sun. The bulk of mass is traveling with the observed average shell velocity330 \pm 100 km s^-1. Our results show that an energy-driven bubble causingRayleigh-Taylor instabilities can explain the kinematics of cool gas seen inthe Na I observations without invoking additional physics relying primarily onmomentum conservation, such as entrainment of gas by Kelvin-Helmholtzinstabilities, ram pressure driving of cold clouds by a hot wind, or radiationpressure acting on dust. abridged

Author: Akimi Fujita 1,2, Crystal L. Martin 1, Mordecai-Mark Mac Low 3,2,4, Kimberly C. B. New 5, Robert Weaver 5 1 Dept. of Physics, UC


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