The large-scale dynamical evolution of tidal debris is governed largely by simple gravitational physics. As first elegantly shown in simulations by Toomre & Toomre (1972) and Wright (1972), the tidal forces acting on spiral galaxies during a close encounter, coupled with the galaxies' rotational motion, draw out long slender "tidal tails" of gas and stars. An example of this process is shown in Figure 1. As the galaxies pass by each other on the first passage, tidal forces give disk material sufficient energy to escape the inner potential well. The symmetric nature of tidal forces means that streams are torn off both the near side and far side (with respect to pericenter) of the disks; the near side material forms a tidal "bridge" between the disks (which typically does not physically connect, depending on orbital geometry) while the far side material forms the tidal tails. The formation of tidal tails is a strong function of the orbital geometry - tidal tails are strongest in prograde encounters where the spin and orbital angular momentum vectors are (even moderately) aligned, while retrograde encounters yield weak tails at best. The length of the tidal tails is further pronounced due to the orbital decay of the merging pair (e.g., Barnes 1988), which causes the galaxies to "fall away" from their tails as they merge together.
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Figure 1. Evolution of the tidal debris in an equal-mass merger of two disk galaxies. Each frame is approximately 0.9 Mpc on a side. |
Once launched, tidal tails are not in simple expansion. Figure 2 shows the kinematic structure of the tidal tails shown in Figure 1, observed 1/2 Gyr after the merger is complete. Most of the material remains bound to the remnant on loosely bound elliptical orbits, with only the relatively small fraction at the tip of the tails being unbound. The radial velocity curve shows this orbital structure well: the loosely bound outer portion of the tails are still expanding, while material at the base of the tails has already reach apocenter and has started falling back in towards the merger remnant. This velocity structure results in a continual stretching of the tidal tails - they are long-lived and do not simply expand away, although their surface brightness drops rapidly due to this dynamical evolution (Mihos 1995). One important caveat to this description is the depth of the galaxies' potential well: a deep potential well provided by extended dark matter halos will result in less unbound material and a more rapid fall-back of the tidal debris to the parent galaxy (Dubinski et al. 1996, 1999; Springel & White 1999).
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Figure 2. The structure of the tidal debris from Figure 1, shown 0.5 Gyr after the merger is complete. Left: energy structure, middle: velocity structure, right: metallicity distribution (see text). |
The material forming the tidal tails comes from a wide range of initial radii in the progenitor disks. During close passages, tidal forces are effective at dredging up material from the inner disk and expelling it into the tidal debris. In the simulation shown in Figure 1, scaled to Milky Way sized progenitors, the extended tidal tails are formed from material originally outside the solar circle, while the loops and shells which fall back in the first Gyr after the merger include a significant amount of material from the solar circle and inwards. This "tidal dredge-up" means that tidal debris will be moderately metal rich, since it is not simply the outer parts of the disks involved. To demonstrate this effect, we imprint a metallicity distribution on the stellar disk model of d[Fe/H]/dR = -0.05 kpc-1, normalized to solar metallicity at the solar circle. Observed 1/2 Gyr after the merger is complete, we see that a significant amount of the debris in the outer (stellar) tidal tails has metallicities above 1/3 solar. A similar exercise for the gas skews the results towards lower metallicities, for a number of reasons. Gas disks are typically more extended than stellar disks; for a similar radial gradient there will be more low metallicity material in the gas than the stars. Additionally, the gas from the inner regions, which would have provided higher metallicity gas in the tails, does not survive the tidal expulsion process; instead shocks and gravitational torques drive the gas inwards to the center of the remnant where it fuels the merger-induced starburst instead (Mihos & Hernquist 1996; Barnes & Hernquist 1996).