Dwarfs Emerge from the Tidal
Debris of Interacting Galaxies
Large galaxies are produced through the
merging or accretion of smaller galaxies. If
the merging galaxies contain enough gas-
eous material, a burst of star formation may
cause the stellar mass of the final galaxy to
be substantially larger than the combined
mass of the stars of the two original galaxies.
This is the basis of hierarchical structure for-
mation, the standard paradigm in the field
of galaxy evolution for many decades.
If two gas-rich galaxies exchange a glanc-
ing blow, rather than a head-on collision,
the encounter may give birth to one or more
smaller galaxies known as tidal dwarfs, rath-
er than a large merger remnant. Dwarf gal-
axies of this type are predicted to form when
gaseous material that is tidally stripped
from one of the larger galaxies condenses to
form a gravitationally bound stellar system.
The stripped gas may be highly enriched, in
which case the resulting dwarf will have an
unusually high metal content for its stellar
mass. Galaxies formed in this way are also
expected to have very little dark matter.
However, it is difficult to ascertain the past
history of any particular dwarf, and identify-
ing tidal dwarfs in the process of formation
has been quite tricky in practice.
A team of astronomers from Australia, Can-
ada, Argentina, Italy, and the United States
have used GMOS at Gemini North to obtain