Complex spectra of the galactic ridge X-rays observed with ASCA

X-ray spectra of the Galactic ridge emission in the Scutum arm region have been obtained with ASCA GIS and SIS in the energy range 0.7-10 keV. The observed spectra are basically of thermal emission from thin hot plasmas, and individual K emission lines from helium-like Mg, Si, S, and Fe ions are confirmed in both the GIS and SIS spectra. This means that the Galactic ridge X-ray emission cannot be explained by a single-temperature ionization-equilibrium plasma model. It cannot, however, be reproduced even if a nonequilibrium ionization model is introduced; thus multiple plasma components are required. The GIS spectra are fairly well fitted by a double-temperature nonequilibrium ionization plasma model with temperatures of kT similar to 0.8 keV and kT similar to 7 keV. The softer component is found to be in an extremely low ionization state, with n(e)t similar to 10(9) cm(-3) s, while the harder component is in a relatively high ionization state, though not yet in a full equilibrium, The GRXE properties obtained with the GIS are carefully reexamined by the highly resolved spectral-line features with the SIS. The soft and hard components are absorbed by equivalent hydrogen columns of 0.7 x 10(22) cm(-2) and 4.6 x 10(22) cm(-2), respectively. The surface brightness of the soft and hard components at b similar to 0 degrees are estimated to be 1.9 x 10(-6) and 5.3 x 10(-7) ergs cm(-2) s(-1) sr(-1) respectively, both in the 0.5-10 keV band. The surface brightness of the softer component extends toward significantly higher (similar to 2 degrees) Galactic latitudes than the harder component, although their actual scale heights may be similar at similar to 100 pc if the differences in their observable depths are taken into account. Spectral properties of the two components are seen to depend on the latitude; the most noticeable effect is a rapid decrease in the Fe K line equivalent width seen in the hard component. Attempts are made to interpret the two components in terms of diffuse hot plasmas filling the interstellar space.