5.1. Star formation
As mentioned in the previous section, tails contain all the necessary ingredients for the onset of star-formation, in particular molecular gas and dust, and indeed young stars are often observed in collisional debris.
Census of star-forming regions in tidal tails has been carried out using a variety of tracers, such as the ultraviolet (Boquien et al. 2009, Smith et al. 2010), H (Bournaud et al. 2004, Torres-Flores et al. 2009) or mid-infrared emission (Smith et al. 2007, Boquien et al. 2010). These tracers may be combined to further constrain the star formation history (see composite image on Figure 10). Star-forming regions in collisional debris may consist of extremely compact and tiny knots with star formation rate (SFR) as low as 0.001 M / yr (see examples at the tip of the tails of systems 3, 6 and 8 in Figure 10) or giant complexes with SFR reaching 0.1 M / yr (see systems 5, 7 on Figure 10).
Figure 10. Star formation in a sequence of merging galaxies. The displayed systems are the same as in Figure 7. The contours of the HI 21cm emission are superimposed on a composite image combining light emission from three tracers of on-going or recent star-formation: the ultraviolet (blue), the H line emission (green) and the mid-infrared (red). The most active star-forming regions belong to the so-called tidal dwarf galaxies.
A few studies have detailed the star-formation process in tidal tails, from the observational and theoretical point of view (e.g. Elmegreen et al. 1993). Tidal objects are a priori a special environment simultaneously characterized by (a) the same local chemical conditions as in spiral galaxies (ISM composition, metallicity) (b) the lack of an underlying massive stellar disk, like dwarf irregular and low surface brightness galaxies (c) the kinematical conditions typical of mergers, i.e. an enhanced gas turbulence and possibly shocks. Does then star-formation in collisional debris obey the rules that prevail (a) in regular massive disks (b) in low-metallicity dwarfs, characterized by a low star-formation efficiency (SFE, the ratio between the star-formation rate and molecular gas content) (c) in the central regions of mergers where deviations from the so-called Kennicut-Schmidt relation (a correlation between the star-formation rate per unit area and the gas surface density) have been measured (Daddi et al. 2010)? The SFE estimated in several tidal objects favors the first hypothesis: its value is close to that usually measured in galactic disks (Braine et al. 2001, Boquien et al. 2011).
Therefore, with respect to star-formation, tidal tails do not appear as exotic objects. The properties of the pre-enriched interstellar medium inherited from their parent galaxies govern their star-formation capabilities rather than the violent episode at their origin or the large-scale (intergalactic) environment in which they now evolve.
5.2. Star cluster formation
Galaxy mergers do not only enhance star-formation. The increase of the gas pressure during mergers triggers the formation of star clusters as well. The Hubble Space Telescope has revealed the presence of a large population of young Super Star Clusters (SSCs) in nearby merging systems, including along tidal tails (see Schweizer 2006 for a review). The most massive of them are believed to evolve into globular clusters (GCs), thus making mergers a possible origin of GCs. Numerical simulations at high resolution support this hypothesis (Bournaud et al. 2008b). Figure 11 presents two different models that were able to form SSCs. Globally, the cluster formation rate follows the star-formation rate. The infant mortality of SSCs less than 10 Myr after their formation appears however to be very high. SSCs in particular suffer from sudden gas loss due to feedback effects that alter their dynamical stability. There are special locations in merging systems, where local compressive tidal modes might contribute to (at least partially) protect them and increase their life-time (Renaud et al. 2009). Large volumes (up to 10 kpc wide) of compressive modes have been located in the tidal tails of major mergers, with an intensity comparable to that found in the central regions. But the lower gas density and turbulence in such an environment do not seem to particularly favor the formation of SSCs in tails.
Figure 11. Formation of stellar structures in high-resolution numerical simulations of major mergers. Left: after the first pericentric passage, with the hydrodynamical AMR code RAMSES (Teyssier et al. 2010). Right: at the merger stage, with a sticky-particle code (Bournaud et al. 2008b). On the electronic version of this figure, gas is rendered in green, young stars in blue and old stars in magenta/brown. Both models show the formation of stellar objects (rendered in yellow/white): compact knots with properties similar as Super Star Clusters, or more massive, extended structures resembling Tidal Dwarf Galaxies.
5.3. Formation of Tidal Dwarf Galaxies
Tidal tails host the most massive structures that may be born during galaxy mergers: the Tidal Dwarf Galaxies (TDGs). As indicated by their name, TDGs have the mass of classical dwarf galaxies, i.e. above 108 M. They have originally been detected on optical images as prominent and generally blue (thus star-forming) condensations at the end of tidal tails. Follow-up radio observations revealed that they were associated with massive HI clouds (see Duc 2011 for a recent review on TDGs). Detailed kinematical studies of the ionized, HI or molecular gas indicate that TDGs are gravitational bound entities that are kinematically decoupled from their parent galaxies. They exhibit velocity curves that are typical of rotating objects. In practice, the kinematical study of tidal tails suffers from strong projection effects: tidal tails are highly curved filaments; when seen edge-on, several components of the tail may be projected along the same line of sight. This creates an artificial velocity gradient that may be mistaken with a genuine rotation curve. Projection effects are especially critical near the end of tidal tails where most TDGs are precisely located (Bournaud et al. 2004).
Numerical simulations have provided clues on the formation mechanism of tidal dwarf galaxies (see examples on Figure 11). Several scenarios have been proposed:
In the context of this Review, we detail here the latter scenario as it grants to the shape of tidal forces a key role in the formation of TDGs. In the potential well of disk galaxies, constrained by extended massive dark matter halo (see Section 6.3), the tidal field carries away the outer material, while keeping its high column density - the radial excursions are constant, as illustrated in Figure 12. Gas may pile up at the tip of tidal tails before self-gravity takes over and the clouds fragment and collapse. Toy models show that the local shape of the tidal field plays the key role in structuring tidal tails and enabling the formation of TDGs.
Figure 12. The effect of tidal forces on the potential well corresponding to an extended dark matter halo. Amplitude of the radial excursions of matter as a function of the initial radius in a numerical model made of concentric annuli. Above a certain distance, it becomes constant, enabling an accumulation of gas in tidal tails, the seed of tidal dwarf galaxies. Adapted from Duc et al. (2004).
The presentation of the long term evolution and survival of TDGs is behind the scope of this Review. Details on the predictions of numerical simulations and observations of old TDGs may be found in Duc (2011).
6 Wetzstein et al. (2007) claimed that the clumps formed in N-body models that do not include gas are numerical artifacts. Back.