The nature of the mid-infrared background radiation in the galactic bulge from the infrared telescope in space observations

Using the Mid-Infrared Spectrometer on board the Infrared Telescope in Spac e, we obtained the 4.5-11.7 mu m spectra of the stellar populations and dif fuse interstellar medium in the Galactic bulge (l = 8.degrees 7, b approxim ate to 2.degrees 9, 4.degrees 0, 4.degrees 7, and 5.degrees 7). Below Galac tic latitudes of 4.degrees 0, the mid-infrared background spectra in the bu lge are similar to the spectra of M and K giants. The unidentified infrared emission bands (6.2, 7.7, 8.6, and 11.3 mu m) are also detected in these r egions and likely arise from the diffuse interstellar medium in the disk. A bove Galactic latitudes of 4.degrees 0, the mid-infrared background spectra are similar to the spectra of those oxygen-rich evolved stars with high ma ss-loss rates detected by IRAS. One likely interpretation is that this back ground emission arises predominantly from those stars with very low luminos ities 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 evolv ed stars with high mass loss rates in the bulge has a significant impact on our understanding of the stellar content in the Galactic bulge.