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), R. Launhardt(1)

Paper: A&A, 348, 457

Weblink: http://www.aoc.nrao.edu/staff/sphilipp/

Philipp et al. (1999, Paper I) investigated the K band emission from a mosaic of size DELTA alpha * DELTA delta ~ 650''*710'' centered approximately on Sgr A* (R_equiv ~ 15.8 pc for R₀ = 8.5 kpc). For the ~ 6*10⁴ stars above the detection limit (S_K' ~ 100 mu Jy) an observed K-band luminosity function (KLF) has been obtained. Below the completeness limit (S_K' ~ 2 000 mu 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 3*10^-3 >~ S_K' / mu Jy >~ 2*10⁶. The overall KLF consists of four sectors obeying power laws of the form dN(S_K') / dlog S_K' \propto S_K' ^{gamma ₁ + 1}, where gamma ₁ + 1 decreases from -0.6 to -1.75. Sector I corresponds to a Salpeter Initial Mass Function (IMF) and represents Main Sequence (MS) stars with M_* <~ 1 M_o, which account for ~ 90 % of the dynamical mass but only ~ 6 % of the K band flux density. Sector II represents MS stars with M_* >~ 1 M_o and red giants. These stars account for only ~ 6 % of the dynamical mass and a similar percentage of the integrated K-band surface brightness but represent ~ 80 % of the bolometric stellar luminosity in the mosaic. The Mass Function (MF) of MS stars is dN / dM_* \propto M_*^-2.35 (i.e., the Salpeter IMF) for M_* <~ 1 M_o and dN / dM_* \propto M_*^-4.5 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. ... shortened .. 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 M_* >~ M_c ~ 1 - 5 M_o 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 ~ 10⁸ - 10⁹ yrs ago.