  The extremely metal-poor (XMP) galaxies analyzed in a previous paper have
large star-forming regions with a metallicity lower than the rest of the
galaxy. Such a chemical inhomogeneity reveals the external origin of the
metal-poor gas fueling star formation, possibly indicating accretion from the
cosmic web. This paper studies the kinematic properties of the ionized gas in
these galaxies. Most XMPs have rotation velocity around a few tens of km/s. The
star-forming regions appear to move coherently. The velocity is constant within
each region, and the velocity dispersion sometimes increases within the
star-forming clump towards the galaxy midpoint, suggesting inspiral motion
toward the galaxy center. Other regions present a local maximum in velocity
dispersion at their center, suggesting a moderate global expansion. The Halpha
line wings show a number of faint emission features with amplitudes around a
few percent of the main Halpha component, and wavelength shifts between 100 and
400 km/s. The components are often paired, so that red and blue emission
features with similar amplitudes and shifts appear simultaneously. Assuming the
faint emission to be produced by expanding shell-like structures, the inferred
mass loading factor (mass loss rate divided by star formation rate) exceeds 10.
Since the expansion velocity exceeds by far the rotational and turbulent
velocities, the gas may eventually escape from the galaxy disk. The observed
motions involve energies consistent with the kinetic energy released by
individual core-collapse supernovae. Alternative explanations for the faint
emission have been considered and discarded.
