Long tidal tails in merging galaxies and their implications

Abstract

ABSTRACT We investigate the properties of long tidal tails using the largest to date sample of 461 merging galaxies with $\log (M_\ast /\rm M_\odot)\ge 9.5$ within 0.2 ≤ z ≤ 1 from the COSMOS survey in combination with Hubble Space Telescope imaging data. Long tidal tails can be briefly divided into three shape types: straight (41 per cent), curved (47 per cent), and plume (12 per cent). Their host galaxies are mostly at late stages of merging, although 31 per cent are galaxy pairs with projected separations d > 20 kpc. The high formation rate of straight tidal tails needs to be understood as the projection of curved tidal tails accounts for only a small fraction of the straight tails. We identify 165 tidal dwarf galaxies (TDGs), yielding a TDG production rate of 0.36 per merger. Combined with a galaxy merger fraction and a TDG survival rate from the literature, we estimate that ∼5 per cent of local dwarf galaxies (DGs) are of tidal origin, suggesting the tidal formation is not an important formation channel for the DGs. About half of TDGs are located at the tip of their host tails. These TDGs have stellar masses in the range of $7.5\le \log (M_\ast /\rm M_\odot)\le 9.5$ and appear compact with half-light radii following the M*–Re relation of low-mass elliptical galaxies. However, their surface brightness profiles are generally flatter than those of local disc galaxies. Only 10 out of 165 TDGs have effective radii larger than 1.5 kpc and would qualify as unusually bright ultradiffuse galaxies.