While on large scales the evolution of tidal debris is largely a gravitational phenomenon, on smaller scales a variety of mechanisms can drive structure formation within the tidal tails. Overdensities can form in the tidal tails either through gravitational collapse of small scale instabilities in the progenitor disks (Barnes & Hernquist 1992) or by cooling and fragmentation of structure in the tidal expelled gas (Elmegreen et al. 1993). This has led to the suggestion that dwarf galaxies may form within the tidal debris of merging galaxies. Observations have detected a number of discrete, often star-forming, sources in the tidal debris of interacting galaxies (e.g., Duc, these proceedings); whether or not these are truly bound objects destined to become dwarf galaxies remains to be seen.
We can use simulations of interacting galaxies to make predictions for the properties of any tidally spawned dwarfs. Coming from material stripped from their progenitor disks, they should have moderate metallicities and travel on loosely bound, highly eccentric orbits (Hibbard & Mihos 1995). They are unlikely to have significant amounts of dark matter, since the kinematically hot dark matter will not collapse into the shallow potential wells (Barnes & Hernquist 1992) formed from small-scale instabilities in the tails. Finally, these tidal dwarfs may well show different generations of stellar populations, as they arise in a mixed medium of old stellar disk material and young stars formed from the gaseous tidal debris.
The dynamical stretching of the tidal debris means that it should be hard for these condensations to grow continuously. On small scales, bound structures can form, but continual accretion onto these structures will be limited by shear in the surrounding material. In this context, it is important to make a cautionary note about claims that large, tidally spawned HI complexes are often found preferentially at the end of optical tidal tails. Dynamically it is unclear why this would be - HI tails often extend much further out than the optical tails do, and there is not clear reason why the "end of the optical tails" should be a dynamically important spot. It is more likely that many of these objects are the result of projection effects. Tidal tails are curved, and a sightline which passes along the tangent point to a curving tail will not only give the appearance of marking the end of the tail, but also will project along a large column of HI, artificially giving the impression that a massive HI complex lives at the end of a tidal tails (see e.g., Hibbard, these proceedings, but also Bournaud et al. 2003 for an alternative view).
The other context in which tidal debris is important in galactic recycling is the return of gas from the infalling tidal debris. As shown in Section 1, material in the tidal tails remains bound, and will continue to fall back to the remnant over many Gyr. The return is ordered (Hibbard & Mihos 1995); the first material to return is the most bound, lowest angular momentum material, which will fall back to small radius. As the remnant evolves, high angular momentum, loosely bound material will fall back to increasingly larger radius.
This long-lived "rain" of tidal debris on the merger remnant manifests itself in a number of ways. Diffuse loops and shells form as the stars fall back through and wrap around the remnant, while the infalling gas can dissipate energy and settle into a warped, rotating disk (Mihos & Hernquist 1996; Naab & Burkert 2001; Barnes 2002), such as those found in the nearby elliptical galaxies NGC 4753 (Steiman-Cameron et al. 1992) and Centaurus A (Nicholson et al. 1992). The most loosely bound tidal material forms less-well organized structures outside of a few effective radii as it falls back, and may be the source of the extended HI gas found in shells and broken rings around many elliptical galaxies (e.g., Schminovich & van Gorkom 1997). More speculatively, if the returning gas can efficiently form stars, this process provides a mechanism for rebuilding stellar disks. For example, the gaseous disk inside the merger remnant NGC 7252 is rapidly forming stars (Hibbard et al. 1994), and may ultimately result in a kiloparsec-scale stellar disk embedded in the r1/4 spheroid formed in the merger. If significant amount of tidal material exists to reform a stellar disk, it may even be possible for the remnant to eventually evolve towards a bulge-dominated S0 or Sa galaxy (e.g., Schweizer 1998).