TIDAL DWARF GALAXIES
During the interaction of two disk galaxies, tidal forces eject stars and gas into intergalactic space. These tidal debris can eventually collapse under self-gravity and form new gravitationally bound structures: tidal dwarf galaxies (TDGs).
TDGs are very interesting objects for two key reasons: (1) they are expected to be free of non-baryonic dark matter because tidal forces effectively segregate baryons in the disk from dark matter in the halo of the parent galaxies, and (2) they probe the process of star formation under extreme conditions because they lack the large-scale gravitational field from an old stellar disk and (possibly) a dark matter halo.
There are many open questions regarding TDGs: Are they really free of dark matter? Can we use TDGs to distinguish dark matter from modified gravity? How many dwarf galaxies may have a tidal origin?
GAS DYNAMICS IN TIDAL DWARF GALAXIES: DISC FORMATION AT Z = 0
TDGs are associated with "young" rotating gas disks! In the figure, optical images are overlaid with HI emission tracing the cold gas distribution (blue) and UV images tracing the young stars (violet). Six TDGs are indicated with yellow arrows. From Lelli et al. (2015)
MOLECULAR GAS AND STAR FORMATION IN THE TIDAL DWARF GALAXY VCC 2062
Deep optical image of the merger remnant NGC 4694 and the tidal dwarf galaxy VCC 2062. In Lisenfeld et al. (2016), we find that molecular gas is very abundant in this TDGs but is forming stars in an unusually inefficient way.
MASSIVE STAR CLUSTER FORMATION AND EVOLUTION IN TIDAL DWARF GALAXIES
Tidal dwarf galaxies can form cospicuous amounts of massive star clusters! In the picture, the circles indicate young star clusters forming in the tidal dwarf galaxy NGC 5291-SW. From Fensch et al. (2019).
ALMA RESOLVES GIANT MOLECULAR CLOUDS IN A TIDAL DWARF GALAXY
TDG have giant molecular clouds with similar masses and sizes as those in spiral galaxies, suggesting that the star formation process is somewhat universal. In the picture, the atomic gas (blue), the molecular gas clouds (white), and the young star clusters (inset) of the TDG J1023+1952 are overlaid on top of a HST image of the parent galaxy NGC 3227, which lies in the background. From Querejeta, Lelli, Schinnerer et al. (2020).