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Astron. Astrophys. 348, 457-465 (1999)

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The nuclear bulge of the Galaxy

II. The K band luminosity function of the central 30 pc

P.G. Mezger 1, R. Zylka 2,1, S. Philipp 1 and R. Launhardt 1

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2 Institut für Theoretische Astrophysik, Tiergartenstrasse 15, D-69121 Heidelberg, Germany

Received 19 January 1999 / Accepted 4 June 1999


Philipp et al. (1999, Paper I) investigated the K band emission from a mosaic of size [FORMULA] centered approximately on Sgr A* ([FORMULA] pc for [FORMULA] kpc). For the [FORMULA] stars above the detection limit ([FORMULA]Jy 1) an observed K-band luminosity function (KLF) has been obtained. Below the completeness limit ([FORMULA]Jy), an ever increasing fraction of stars merges into the background continuum. In this paper we combine the observed with model KLFs and thus obtain a complete KLF for the flux density range [FORMULA]. The overall KLF consists of four sectors obeying power laws of the form [FORMULA], where [FORMULA] decreases from -0.6 to -1.75. Sector I corresponds to a Salpeter Initial Mass Function (IMF) and represents Main Sequence (MS) stars with [FORMULA], which account for [FORMULA] of the dynamical mass but only [FORMULA] of the K band flux density. Sector II represents MS stars with [FORMULA] and red giants. These stars account for only [FORMULA] of the dynamical mass and a similar percentage of the integrated K-band surface brightness but represent [FORMULA] of the bolometric stellar luminosity in the mosaic. The Mass Function (MF) of MS stars is [FORMULA] [FORMULA] (i.e., the Salpeter IMF) for [FORMULA] and [FORMULA] [FORMULA] for more massive stars, which is similar to the Present Day MF in the solar vicinity. Part of sector II of the KLF, as well as sectors III and IV , represent giants and supergiants which, though they account for only a small fraction of the mass, dominate the integrated K-band surface brightness.

The slope of sector II of the KLF, [FORMULA] has been inferred from the KLF in the NGC 6522 Baade's Window (BW). To make this sector join smoothly to the neighboring KLF sections we have to set the surface density of low-mass ([FORMULA]) MS stars at [FORMULA] times that in BW.

Paper I shows, in agreement with earlier observations, that massive stars are preferentially formed in the central parsec. A preliminary discussion of star formation rates suggests that bimodal star formation (introduced by Güsten & Mezger [1983] for the spiral arm region of the Galactic Disk) may also apply to the central 30 pc. Preferential formation of stars with masses [FORMULA] would make conversion of matter into radiation by star formation much more efficient and could be the process which powers star burst galaxies. There is an overabundance of evolved stars which can be explained by a strongly increased star formation rate [FORMULA] yrs ago.

Key words: stars: luminosity function, mass function – ISM: dust, extinction – Galaxy: center – Galaxy: evolution – Galaxy: stellar content – infrared: stars

Send offprint requests to: P.G. Mezger, Bonn

Correspondence to: sphilipp@mpifr-bonn.mpg.de

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999