  I Zw 18 is a prototype Blue Compact Dwarf (BCD), characterized by a strong
starburst and extremely low metallicity (Z ~ 0.02 Zsun). It has long been
considered a candidate young galaxy in the Local Universe, but recent studies
indicate the presence of old stars. We analysed archival VLA observations of
the 21 cm line and found that the HI associated to the starburst region forms a
compact fast-rotating disk. The HI column densities are very high, up to
~50-100 Msun/pc^2 (~0.6-1.2 x 10^22 atoms/cm^2). The rotation curve is flat
with a steep rise in the inner parts, indicating the presence of a strong
central concentration of mass. Mass models with a dark matter halo show that
baryons may dominate the gravitational potential in the inner regions. A radial
inflow/outflow motion of ~15 km/s is also present. I Zw 18 appears structurally
different from typical dwarf irregulars in terms of gas distribution, stellar
distribution and dynamics. It may be considered as a "miniature"
high-surface-brightness disk galaxy. These dynamical properties must be tightly
related to the starburst. They also shed new light on the question of the
descendants of BCDs. There is also extended HI emission towards the outlying
stellar complex I Zw 18 C and a ~13.5 kpc HI tail. An interaction/merger
between gas-rich dwarfs is the most likely explanation for the starburst.

Cosmology and Nongalactic Astrophysics