GALACTIC BULGE M-GIANTS .3. NEAR-INFRARED SPECTRA AND IMPLICATIONS FOR THE STELLAR CONTENT OF E-GALAXY AND S0-GALAXIES

We have obtained 0.6-0.9 μm spectra of 320 K and M giants in six low-absorption fields along the minor axis of the galactic bulge and of an additional 60 late-type giants and dwarfs in the solar neighborhood. Indices of the strength of molecular absorption are combined with previous infrared and optical photometry to estimate the mean metal abundance of the bulge M giants and to constrain the relative contribution of late giants and dwarfs to the integrated light of E and S0 galaxies. The M giants in all the bulge fields have consistently stronger TiO absorption at a given J - K than do local stars, with those in the inner bulge having the strongest molecular bands. The same effect is also seen in J - H and is present but weaker in H - K. This confirms earlier evidence for a metallicity gradient in the inner kiloparsec of the bulge, the effect of which is to make the M giants bluer in J - K or J - H with increasing metallicity at constant M class. We find from a comparison of the infrared color/TiO strength relation to recent theoretical models that the mean abundance of the bulge M giants declines from [M/H] ∼ 0.2 to ∼ -0.2 along the minor axis from b = -3° to -12°. With decreasing metallicity, the distribution of M giants shifts rapidly to earlier types, with the luminosity function remaining unchanged. Photometry and molecular band indices of the bulge giants are gathered from this paper and our previous ones into an empirical model for the stellar population of early-type galaxies. This bulge-based model successfully reproduces the observed TiO strengths and VJHK colors of these galaxies by a simple integration of the observed giant-dominated luminosity function and optical-infrared colors. The model also shows that the contribution of late-type dwarfs to the integrated light of E and S0 galaxies at 8000 A cannot be larger than 30%.