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There is yet no consensual definition of a Tidal Dwarf Galaxy (TDG). Let's however stick to the acronym to define it:

How does this definition of a TDG translate into observational properties?

Examples of observed Tidal Dwarf Galaxies are shown in Fig. 1. On these images of colliding systems, the TDGs appear as red stains on blue ribbons, i.e. star-forming objects within gas-rich tails. The most massive of them are usually located near their tip. Several papers have exploited the rich Ultraviolet/GALEX Infrared/Spitzer databases on interacting galaxies and investigated in details how star-formation proceeds in collisional debris (e.g. [27, 25, 2]). At this stage, it is worthwhile noting that the vast majority of the Tidal Dwarf Galaxies so far securely identified are young objects, formed in mergers that occurred less than one Gyr ago. They still exhibit the umbilical cord linking them to their parents... i.e. the tails and bridges in which they were formed have not had the time to evaporate. Once evolved, TDGs should become undistinguishable from regular satellite galaxies on optical images.

Figure 1

Figure 1. Sample of colliding systems exhibiting TDG candidates. The distribution of the gas is shown in blue and the star-forming regions in red.

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