  Using the Mid-Infrared Spectrometer (MIRS) on board the Infrared Telescope in
Space (IRTS) we obtained the 4.5 to 11.7 micron spectra of the stellar
populations and diffuse interstellar medium in the Galactic bulge (l = 8.7
degree, b = 2.9, 4.0, 4.7, and 5.7 degree). Below galactic latitudes of 4.0
degree, the mid-infrared background spectra in the bulge are similar to the
spectra of M and K giants. The UIR emission bands (6.2, 7.7, 8.6, and 11.3
micron) are also detected in these regions and likely arise from the diffuse
interstellar medium in the disk. Above galactic latitudes of 4.0 degree, the
mid-infrared background spectra are similar to the spectra of those oxygen-rich
evolved stars with high mass-loss rates detected by IRAS. One likely
interpretation is that this background emission arises predominantly from those
stars with very low luminosities that have not been detected by IRAS. The age
for such low-luminosity evolved stars could be 15 Gyr, and the existence of a
large number of evolved stars with high mass-loss rates in the bulge has a
significant impact on our understanding of the stellar content in the Galactic
bulge.
