The Physics of Tidal Tails

Computer simulations let us follow the evolution of merging galaxies in detail and focus on the physical mechanisms shaping tidal tails:
  • Tidal forces during the collision
  • Broad "resonance" between orbital/rotational motions
  • Binding energy of the outer disk
  • Orbital evolution of the interacting pair
  • Age of the interaction

    • (Holmberg 1943; Toomre & Toomre 1972; Wright 1972; Barnes 1988; Hibbard & Mihos 1995; Dubinski, Mihos, & Hernquist 1996; Springel & White 1999)


    Tail material is not in simple expansion. Most material remains bound to the remnant on very loosely bound orbits (details depend on DM halo). Tails fade rapidly in surface brightness due to dynamical evolution, but do not "go away".
    Energy
    		 Radial Velocity



    Mixing is partial -- the outermost tidal material comes from the loosely bound material in the outer disk. But significant "dredge-up" from the galactic disk can occur.
    Energy
    		 Radius
    ("final"at 1/2 Gyr)

    Where did tidal material come from?

    • Material in the tails is most loosely bound, coming largely from outside the solar circle
    • Material in the loops is more bound, comes from a broader range of radii

    Pre-encounter
    		 T = 1/2 Gyr


    Metallicity of tidal debris.

    Assume a simple model for the initial galactic stellar metallicity distribution:
    d[Fe/H]/dR = -0.05 kpc-1,
    normalized to solar at Rsun.

    Apply to model, view at T = 1 Gyr.



    For gas, we are skewed towards lower metallicity (for the same initial gradient), for two reasons:
    • more gas at initially large radius (and hence initially lower metallicity)
    • gas from inner regions does not survive dynamical expulsion, and is driven into the inner regions.