Tidal dwarf galaxies in the nearby Universe
  We present a statistical observational study of the tidal dwarf (TD)
population in the nearby Universe, by exploiting a large, homogeneous catalogue
of galaxy mergers compiled from the SDSS. 95% of TD-producing mergers involve
two spiral progenitors, while most remaining systems have at least one spiral
progenitor. The fraction of TD-producing mergers where both parents are
early-type galaxies is <2%, suggesting that TDs are unlikely to form in such
mergers. The bulk of TD-producing systems inhabit a field environment and have
mass ratios greater than 1:7 (the median value is 1:2.5). TDs forming at the
tidal-tail tips are ~4 times more massive than those forming at the base of the
tails. TDs have stellar masses that are less than 10% of the stellar masses of
their parents and typically lie within 15 optical half-light radii of their
parent galaxies. The TD population is typically bluer than the parents, with a
median offset of ~0.3 mag in the (g-r) colour and the TD colours are not
affected by the presence of AGN activity in their parents. An analysis of their
star formation histories indicates that TDs contain both newly formed stars
(with a median age of ~30 Myr) and old stars drawn from the parent disks, each
component probably contributing roughly equally to their stellar mass. Thus,
TDs are not formed purely through gas condensation in tidal tails but host a
significant component of old stars from the parent disks. Finally, an analysis
of the TD contribution to the local dwarf-to-massive galaxy ratio indicates
that ~6% of dwarfs in nearby clusters may have a tidal origin, if TD production
rates in nearby mergers are representative of those in the high-redshift
Universe. Even if TD production rates at high redshift were several factors
higher, it seems unlikely that the entire dwarf galaxy population today is a
result of merger activity over the lifetime of the Universe.
