A tidal tail is a thin, elongated region ofstars andinterstellar gas that extends into space from agalaxy. Tidal tails occur as a result ofgalactic tide forces betweeninteracting galaxies. Examples of galaxies with tidal tails include theTadpole Galaxy and theMice Galaxies. Tidal forces can eject a significant amount of a galaxy's gas into the tail; within theAntennae Galaxies, for example, nearly half of the observed gaseous matter is found within the tail structures.^[1] Within those galaxies which have tidal tails, approximately 10% of the galaxy'sstellar formation takes place in the tail.^[2] Overall, roughly 1% of all stellar formation in the known universe occurs within tidal tails.^[3]

Some interacting galaxy pairs have two distinct tails, as is the case for the Antennae Galaxies, while other systems have only one tail. Most tidal tails are slightly curved due to the rotation of the host galaxies. Those that are straight may actually be curved but still appear to be straight if they are being viewed edge-on.

The phenomena now referred to as tidal tails were first studied extensively byFritz Zwicky in 1953.^[4] Several astrophysicists expressed their doubts that these extensions could occur solely as the result of tidal forces,^[5][6] including Zwicky himself, who described his own views as "unorthodox".^[7]Boris Vorontsov-Velyaminov argued that the tails were too thin and too long to have been produced by gravity alone,^[8] as gravity should instead produce broad distortions. However, in 1972, renowned astronomerAlar Toomre proved that it was indeed tidal forces that were responsible for the tails.^[9]

When galaxies interact, the uneventidal forces between the galaxies can pull matter away from thegalactic disks forming tidal tails.^[9] Galaxy surveys have found the longest tidal tails can extend farther than 100 kpc from a galaxy, however, most tidal tails surveyed were 10-20 kpc.^[10] The shapes of tidal tails can be fit into three broad categories: straight, curved, and plume (formed by the fallback of tail materials to the galaxy).^[11]

The structure of tidal tails depends on the geometry of encountered galaxies.^[12] When the spin of a galactic disk is aligned with the direction of orbital motion, interactions usually generate long and prominent tidal tails. When the disk rotation opposes the orbital motion, tidal features tend to be weaker.^[13] The mass ratio between the interacting galaxies can influences how many tidal tails form and how symmetric they show.^[14] Gravitational torques transmitting during close passages transfer angular momentum outward, making material from the outer disk to move far from the center of the galaxy.^[15]

Tidal tails (and other tidal features) are found to have a gas content primarily composed of atomic hydrogen (HI),^[16] however, regions of molecular hydrogen (H₂), traced byCO, are sometimes found in areas of proportionally high densities of HI.^[17] Additionally, it is found that tidal tails tend to have lowmetallicity and dust content, which decrease exponentially at increasing distances from the galaxy.^[16]

Along with the stars pulled out or created by tidal effects, tidal tails have also been found to have active star forming regions with similar properties to those found in galactic disks.^[18] Star clusters, including active regions, can be generally found throughout the whole length of the tail.^[18]

Tidal tails tend to have low surface brightness. Therefore, they are often hard to find in shallow imaging surveys.

Tidal tails have been observed in clusters as well as galaxies. There have been tidal tails identified aroundNGC 5466.^[19] These tails appear to be roughly 4 degrees in the sky, or 1 kpc in length.^[19] There have been other clusters observed with tidal tails as well,^[20][21]

• NGC 2516, from tip of leading tail to tip of trailing tail, 380 pc.^[22]
• Theia 456 (COIN-Gaia 13), tidal tail spans 200 pc.^[22]
• NGC 752, estimates are less than 1.5 kpc to several kpc.^[22]
• ASCC 101, trailing tail is 100 pc, leading tail is hard to measure due to its position.^[23]
• Alessi 3, leading tail 120 pc, trailing tail 60 pc.^[23]
• Blanco 1, both tails extend approximately 50-60 pc.^[23]
• Collinder 350, both tails extend approximately 50pc.^[23]
• Melotte 111, both tails span 50 pc.^[23]
• Melotte 25, leading tail is 70 pc, trailing tail is 70 pc.^[23]
• Roslund 6, both tails extend approximately 100 pc.^[23]
• Theia 517, leading tail is 120 pc and trailing tail is 150 pc.^[23]

These tidal tails are difficult to measure and different studies show different results. We can most accurately measure tidal tails close to Earth and in the correct orientation.

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  Arc-shaped tidal tails in galaxy3C186.^[24]

• Interacting galaxies

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