Properties of the Galactic Nucleus in the Direction of NGC 6522.

The region of the galactic center which is least obscured lies in the direction of the globular cluster NGC 6522 at I" - 1.0 and b" = - 3 9 . Forty-six photoelectric measures on thirty-one stars in the vicinity of the cluster were made with the 200-inch telescope Analysis of the UB V colors of those which were nearby field stars shows the B absorption increases at a rate of about 1 0 mag/kpc until the line of sight emerges from the absorbing layer at 140 pc above the plane, having suffered a total reddening of = 049 mag. The color-magnitude diagram of NGC 6522 identifies it as a relatively metal-rich globular cluster. Comparison to 47 Tuc yields a distance of 7.5 kpc (2.4 kpc in front of the center) and a total reddening of = 044 mag. A comparison of the integrated color and spectrum of the cluster by Kron and Mayall gives = 046 mag. The final adopted reddening for the cluster and the region between the sun and the nucleus in this direction is = 0.46 mag. + 003. In the field around NGC 6522 the color-magnitude diagram for over 1300 stars was obtained from measures on photographic plates. Analysis of the contribution of nearby stars to this diagram shows that bluer than apparent color B - V = 1.0 mag , most of the stars are within 5 kpc. The great majority of stars, however, are redder than B - V = 1 0 mag and are predominantly contributed by the nucleus of the galaxy, which acts roughly like a cluster of stars of about 1 kpc in diameter at a distance of about 10 kpc. This nuclear population turns out to have negligible numbers of very low metal-content globular- cluster-type stars of the classical Type II halo population. On the contrary, the nuclear population appears to be composed of either (1) all old stars with metal content ranging from moderately metal rich (47 Tuc type) to very metal rich (NGC 188 type), or (2) all very metal-rich stars with ages ranging from 10' to 10i0 years. The first interpretation is favored in the present paper. It is shown that the RR Lyrae Cepheids in the center have fairly closely the same absolute magnitude as RR Lyrae Cepheids in halo globular clusters The areal density of RR Lyrae Cepheids in the center region is high, however (0.1 per sq sec of arc), and it is concluded that (1) nuclear population with giants even less luminous than 47 Tuc giants contributes appreciable numbers of RR Lyrae variables-that 50-80 per cent of the nuclear RR Lyrae stars belong to a metal-rich RR Lyrae population group, or (2) the ratio of RR Lyrae stars to red giants is two to five times higher than for 47 Tuc-that is roughly one to three or one to six for the nucleus instead of the ratio of one to twelve which applies to the average globular cluster. The first alternative is favored A luminosity function is derived for the nuclear population and it is shown that 75 per cent of the luminosity of the nucleus comes from stars between Mv = -2 and +1.3 mag. This result confirms Morgan's spectroscopic statement that the light of the nucleus is dominated by G and K giants. The light of individual stars in the central region is integrated and compared to photometry in various sized areas in this area by Baum and and Haug. An apparent surface brightness for the nucleus of V = 20.3 mag/sq sec of arc and B - V = 1.1 mag and unreddened surface brightness of V, = 18.9 mag/sq sec of arc and (B - V), = 0 6 to 0.7 mag. is adopted. Comparison of our own Milky Way nucleus to the nuclear bulge of the Andromeda Nebula shows that, at equal distances from the center, our own bulge is fainter and bluer. It is estimated that our nuclear bulge is only about half the M31 size relative to the diameter of the disk. We conclude from this and other criteria that our own Galaxy would be more accurately classified as an Sc spiral galaxy