Using the XMM-Newton Slew Survey, we construct a hard-band selected sample of
low-luminosity Galactic X-ray sources. Two source populations are represented,
namely coronally-active stars and binaries (ASBs) and cataclysmic variables
(CVs), with X-ray luminosities collectively spanning the range 10^(28-34) erg/s
(2-10 keV). We derive the 2-10 keV X-ray luminosity function (XLF) and volume
emissivity of each population. Scaled to the local stellar mass density, the
latter is found to be 1.08 +/- 0.16 x 10^28 erg/s/M and 2.5 +/- 0.6 x 10^27
erg/s/M, for the ASBs and CVs respectively, which in total is a factor 2 higher
than previous estimates. We employ the new XLFs to predict the X-ray source
counts on the Galactic plane at l = 28.5 deg and show that the result is
consistent with current observational constraints. The X-ray emission of faint,
unresolved ASBs and CVs can account for a substantial fraction of the Galactic
ridge X-ray emission (GRXE). We discuss a model in which roughly 80 per cent of
the 6-10 keV GRXE intensity is produced in this way, with the remainder
attributable to X-ray scattering in the interstellar medium and/or young
Galactic source populations. Much of the hard X-ray emission attributed to the
ASBs is likely to be produced during flaring episodes
