  One of the key questions in observational cosmology is the identification of
the sources responsible for ionisation of the Universe after the cosmic Dark
Ages, when the baryonic matter was neutral. The currently identified distant
galaxies are insufficient to fully reionise the Universe by redshift z~6, but
low-mass star-forming galaxies are thought to be responsible for the bulk of
the ionising radiation. Since direct observations at high redshift are
difficult for a variety of reasons, one solution is to identify local proxies
of this galaxy population. However, starburst galaxies at low redshifts are
generally opaque to their ionising radiation. This radiation with small escape
fractions of 1-3% is directly detected only in three low-redshift galaxies.
Here we present far-ultraviolet observations of a nearby low-mass star-forming
galaxy, J0925+1403, selected for its compactness and high excitation. The
galaxy is leaking ionising radiation, with an escape fraction of ~8%. The total
number of photons emitted during the starburst phase is sufficient to ionize
intergalactic medium material, which is about 40 times more massive than the
stellar mass of the galaxy.
