  The Galactic plane is a strong hard x-ray emitter and the emission forms a
narrow continuous ridge. The currently known hard x-ray sources are far too few
to explain the ridge x-ray emission, and the fundamental question as to whether
the ridge emission is ultimately resolved into numerous dimmer discrete sources
or truly diffuse emission has not yet been settled. In order to obtain a
decisive answer, using the Chandra x-ray observatory, we have carried out the
deepest hard x-ray survey of a Galactic plane region which is devoid of known
x-ray point sources. We have detected at least 36 new hard x-ray point sources
in addition to strong diffuse emission within a 17' x 17' field of view. The
surface density of the point sources is comparable to that at high Galactic
latitudes after the effects of Galactic absorption are considered. Therefore,
most of these point sources are probably extragalactic, presumably active
galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission
is diffuse, which indicates omnipresence of the hot plasma along the Galactic
plane whose energy density is more than one order of magnitude higher than any
other substances in the interstellar space.
