We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in
the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that
the GRXE intensity closely follows the Galactic near-infrared surface
brightness and thus traces the Galactic stellar mass distribution. The GRXE
consists of two spatial components which can be identified with the bulge/bar
and the disk of the Galaxy. The parameters of these components determined from
X-ray data are compatible with those derived from near-infrared data. The
inferred ratio of X-ray to near-infrared surface brightness I(3-20 keV) (1e-11
erg/s/cm2/deg2)/I_(3.5micron)(MJy/sr)=0.26+/-0.05, and the ratio of X-ray to
near-infrared luminosity L_(3-20 keV)/L_(3-4 micron)=(4.1+/-0.3)e-5. The
corresponding ratio of the 3-20 keV luminosity to the stellar mass is
L_x/M_Sun=
(3.5\pm0.5) 10^{27} erg/s, which agrees within the uncertainties with the
cumulative emissivity per unit stellar mass of point X-ray sources in the Solar
neighborhood, determined in an accompanying paper (Sazonov et al.). This
suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly
cataclysmic variables and coronally active binaries. The fractional
contributions of these classes of sources to the total X-ray emissivity
determined from the Solar neighborhood data can also explain the GRXE energy
spectrum. Based on the luminosity function of local X-ray sources we predict
that in order to resolve 90% of the GRXE into discrete sources a sensitivity
limit of ~10^{-16} erg/s/cm2 (2--10 keV) will need to be reached in future
observations.Comment: 12 pages, 9 figures. Submitted to A&