Infrared Observations of the Central Region of Our Galaxy: Investigation of the Stellar Population and the Integral Properties of Stars and Dust in the Galactic Center Region
Publications of the Astronomical Society of the Pacific
The Milky Way consists of the Galactic disk, the halo, and the Galactic bulge. The halo and the Galactic bulge contain an old stellar population. The population of the central (hereafter referred to as the "" nuclear *l ] *b D 1¡ .5 ] 0¡ .3 bulge ÏÏ) shows a great similarity to the young stellar population of the spiral arms. Its stellar population is investigated in this thesis applying methods of infrared astronomy. A K-band (j \ 2.2 km) mosaic of the central D30 pc of the nuclear bulge was
... nuclear bulge was constructed out of D500 individual source images that were obtained at the European Southern Observatory (ESO), La Silla, Chile, using the IRAC2B camera with the ESO-MPG 2.2 m telescope. The K-band mosaic consists of sources overlaid on a continuum background, which we hypothesize to consist of a very large number of stars too weak to be observed as individual sources. We were able to separate luminous, discrete stars from the unresolved background continuum by Ðtting Lorentzian distributions to the sources. These seeing-limited data allowed us to measure D6.1 ] 104 individual stars as weak as kJy or mag, yielding a surface S K @ D 100 m K D 17.0 density of D80 stars pc~2, most of which are giants, supergiants, and main-sequence (MS) stars earlier than O9. We estimate a completeness limit of our source Ðts at D2000 kJy or mag. We Ðnd that about one-half of an m K D 14.5 integrated, not dereddened K-band Ñux density within the mosaic of 752 Jy is contributed by these D6 ] 104 individual stars which we Ðtted with Ñux densities S K @ Z 100kJy, and the remainder is contributed by an extended continuum provided, as discussed later, by about 6 ] 108 stars too weak to be observed as individual sources. We Ðnd that these discrete stars are mainly giants, supergiants, and early O stars. The unresolved background should therefore consist of MS stars later than spectral type O9 (M * [ 19 and giants less luminous than spectral type M0 M _ ) (luminosity class III) whose progenitor stars had masses [4 M _ . The mosaic gives information about the stellar distribution in the intermediate distance range For 2@@ [ R [ 300@@. radii R \ 15@@, nearly 90% of the spatially integrated K-band Ñux density comes from resolved sources that represent a relatively young generation of luminous stars. Farther out, the contribution of the unresolved continuum (and hence of low-and medium-mass MS stars) to the integrated K-band Ñux density increases and reaches D40% at a distance R D 300@@. This indicates a deÐciency of low-mass stars within the central 30A (D1.25 pc) and high star formation activity during the past D107È108 yr, as noticed already by R. Genzel et al. (ApJ, 472, 153 ). Fitting King proÐles to the observed surface brightnesses, we derive core radii of D7A and D30A, respectively, for resolved and unresolved stars. Also, the investigation of the surface brightness showed clearly the underabundance of low-mass, low-luminosity stars within the central D20A. The mass-toluminosity ratio of low-mass stars in the central 30 pc is found to be D1 M _ /L _ . For the mosaic and subareas, K-band luminosity functions (KLFs) are constructed (see Fig. 1a ). These KLFs di †er signiÐcantly from each other. In the direction of Sgr A East, we look deep into the nuclear bulge and therefore sample a greater number of bright stars. In case of the subarea M-0.13-0.08, a dense cloud core blocks the emission from the (luminous) stars in the center of the nuclear bulge. We therefore should observe a KLF that relates predominantly to the stellar population of the Galactic bulge and disk. The KLF of the mosaic contains contributions from areas of high and low foreground extinction and therefore lies between the two extreme cases. To separate the KLF of the nuclear bulge from that of the Galactic bulge and disk we construct di †erence KLFs (see Fig. 1b ) using the KLFs of the di †erent subregions and the KLF of our reference Ðeld (Dark Cloud).