|
Description
JHKL colors and CO and H2O indices are presented for an unbiased sample of about 250 M giants drawn from complete surveys for such stars along the minor axis of the Galaxy at latitudes between -3deg and -12deg. Magnitudes at 10 {mu}m and narrow-band near-infrared colors for some of these stars are also given. At constant J-K the mean CO index of the bulge M giants weakens monotonically with increasing latitude. From the relation between CO and [Fe/H] established for star clusters, this would correspond to a decrease of ~0.4 dex in the mean [Fe/H] between the -3deg and -12deg fields, from somewhat greater to somewhat less than solar. These values are consistent with abundances derived from optical color-magnitude diagrams. Despite these high [Fe/H] values, the ridgelines of the color-magnitude diagrams for all fields lie between those of 47 Tucanae and M3. There is a mild blueward shift with increasing latitude seen in the integrated mean J-K colors of the fields. A steady progression with latitude in the mean (J-H, H-K) relation for the bulge M giants is observed. For the -12deg field it is significantly displaced from the one for local field giants and overlaps that for globular cluster stars; for the -3deg field it is displaced by about the same amount, but on the opposite side of the field giant relation. Dispersions about this relation and the mean (CO, J-K) relation for the M giants in each field are comparable in size to the measuring uncertainties alone, but in a given field the displacements of individual stars from these two mean relations are statistically correlated. We attribute this correlation to a spread in metallicity within each field but note that this spread is smaller than the total metallicity range over all fields. In addition, metallicity-related effects result in J-H getting bluer with increasing metallicity in stars of similar temperature. The bolometric luminosity function of the nonvariable M giants is nearly independent of latitude. All show a sharp cutoff at Mbol<=-4.2; such a cutoff could be a good extragalactic distance indicator for bulgelike stellar populations. The reddest (J-K>=-1.35) and most luminous stars are found only in the low-latitude windows; all are M7-M9 giants, long-period variables (LPVs), or both. Except for these reddest stars, the ratio at a given color of M7-M9 giants to earlier type giants is independent of latitude. Only when stars of all colors in a spectral group are considered is the rapid falloff seen in the ratio of M7-M9 giants respect to earlier types. The bulge LPVs have <Mbol> = -4.2. Since this is nearly the same as for LPVs in globular clusters, it does not require an age for the bulge population significantly younger than that of globular clusters. Their spatial distribution is similar to that of the reddest late M giants. The LPVs with the reddest colors, attributed to the most extensive circumstellar dust shells, are found in the lowest latitude fields. Most of the bulge IRAS sources in the fields studied can be identified with these LPVs or with the brightest nonvariable M giants; many of the latter are probably foreground objects. The reddest IRAS sources are at the lowest latitudes and have the coolest [12-25]{mu}m colors. The bulge IRAS sources do not appear to represent a different or more luminous class of objects than the optically identified M giants. Five different estimators of the surface brightness and surface density between latitudes -3deg and -12deg have a power-law dependence on radius with exponent between -2.5 and -3.4. The value of the exponent depends on the metallicity of the estimator: the most metal-rich objects have the steepest falloff. However, even the smallest exponent, which characterizes the total light and mass distribution, places the bulge among the most spatially concentrated of those measured by Kent for a sample of 22 Sb-Sc galaxies. Thus the mass distribution within the inner 1.5 kpc of the Galaxy is quite sharply peaked.
|
