Dwarfs walking in a row. The filamentary nature of the NGC3109
  association
  We re-consider the association of dwarf galaxies around NGC3109, whose known
members were NGC3109, Antlia, Sextans A and Sextans B, based on a new updated
list of nearby galaxies and the most recent data. We find that the original
members of the NGC3109 association, together with the recently discovered and
adjacent dwarf irregular Leo P, form a very tight and elongated configuration
in space. All these galaxies lie within ~100 kpc of a line that is ~1070 kpc
long, from one extreme (NGC3109) to the other (Leo P), and they show a gradient
in the Local Group standard of rest velocity with a total amplitude of 43 km/s
Mpc and a r.m.s. scatter of just 16.8 km/s. It is shown that the reported
configuration is exceptional given the known dwarf galaxies in the Local Group
and its surroundings. We conclude that (a) Leo P is likely an additional member
of the NGC3109 association, and (b) the association is highly ordered in space
and velocity, and it is very elongated, suggesting that it has been originated
by a tidal interaction or it was accreted as a filamentary sub-structure.
