======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 10, No. 17 Sep 16, 1999 Recently submitted papers: -------------------------- The Nuclear Bulge I & II (Philipp et al., Mezger et al., A&A) Email : sphilipp@mpifr-bonn.mpg.de Title : The Nuclear Bulge I: K Band Observations of the Central 30 pc Author(s): S. Philipp(1,2), R. Zylka(2), P.G. Mezger(1), W.J. Duschl(2,1), T. Herbst(3), R.J. Tuffs(4) Institute: (1) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn, Germany (2) Institut fuer Theoretische Astrophysik, Tiergartenstrasse 15, D-69121 Heidelberg, Germany (3) Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg, Germany (4) Max-Planck-Institut fuer Kernphysik, Saupfercheckweg, D-69117 Heidelberg, Germany Paper : A&A, 348, 768 Weblink : http://www.aoc.nrao.edu/staff/sphilipp/ Abstract: Out of ~ 500 individual source images we have constructed a mosaic map of the K band surface brightness in an area \Delta alpha *\Delta delta ~ 650''*710'' (R_equiv ~ 15.8 pc for R_0 = 8.5 kpc) centered approximately on Sgr A*. An observing technique was used which allows us to recover an extended background emission. To separate sources from an unresolved background continuum we fitted Lorentzian distributions to the sources and find that about one half of an integrated, not dereddened K band flux density of 752 Jy is contributed by ~ 6*10^4 stars with flux densities S_K' >~ 100 mu Jy and the remainder is contributed by an extended continuum provided by about 6*10^8 stars too weak to be observed as individual sources. We estimate that >~ 80% of the integrated flux density of the mosaic is contributed by stars in the Nuclear Bulge (NB; R <~ 300 pc); the remaining <~ 20% come from stars located along the line of sight in the Galactic Bulge (GB; 0.3 <~ R/kpc <~ 3) and Galactic Disk (GD; R > 3 kpc). We determine the K band luminosity functions (KLF) of the mosaic and of subareas dominated by Nuclear Bulge, Galactic Bulge and Disk stars, respectively, and construct difference KLFs which relate to the specific stellar populations of these regions. The detection limit is S_K' ~ 100 mu Jy, for the completeness limit we estimate S_K' ~ 2 000 mu Jy. We find that the stellar population of the Nuclear Bulge contains considerably more bright stars (i.e. with reddened K band flux densities S_K' >~ 5*10^3 mu Jy), most of which are probably early O stars, Giants and Supergiants. The stellar population of the Galactic Bulge on the other hand is dominated by stars which appear to be lower mass (<~ 6 M_o) Main Sequence (MS) stars. A model KLF constructed with a Salpeter Initial Mass Function (IMF) for stars of spectral type O9 or later (S_K' <~ 2 000 mu Jy) explains the observations satisfactorily and connects well with the observed KLF of more luminous stars. About 6*10^8 stars with masses ranging from 0.06 to 6 M_o account for the unresolved continuum. Combining observed and model KLF we obtain a mosaic KLF which increases \propto S_K^' - 1 for 10^6 >~ S_K'/ mu Jy >~ 10^3 and \propto S_K^' - 0.6 for 10^3 >~ S_K'/ mu Jy >~ 3*10^-3. For radii R < 15'' nearly 90% of the spatially integrated K band flux density comes from resolved sources which represent a relatively young generation of luminous stars. Farther out the contribution of the the unresolved continuum (and hence of low and medium mass MS stars) to the integrated K band flux density increases and attains ~ 40% at a distance R ~ 300''. This indicates a deficiency of low-mass stars within the central 30'' (~ 1.25 pc) and a high star formation activity during the past ~ 10^7 - 10^8 yr. Fitting King profiles to the observed surface brightnesses we derive core radii of ~ 7'' and ~ 30'' respectively for resolved and unresolved stars. The mass-to-luminosity ratio of low mass stars in the central 30 pc is found to be ~ 1 M_o/L_o. We present and discuss the radio/IR spectrum of the central 30'' (~ 1.25 pc) and derive dust and Lyman continuum (Lyc) luminosities of 7.5 * 10^7 L_o and 1.2 * 10^50 s^-1, respectively. ------------------------------------------------------------------------ Email : sphilipp@mpifr-bonn.mpg.de Title : The Nuclear Bulge of the Galaxy II. The K band Luminosity Function of the Central 30 pc Author(s): P.G. Mezger(1), R. Zylka(2,1), S. Philipp(1), R. Launhardt(1) Institute: (1) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn, Germany (2) Institut fuer Theoretische Astrophysik, Tiergartenstrasse 15, D-69121 Heidelberg, Germany Paper : A&A, 348, 457 Weblink : http://www.aoc.nrao.edu/staff/sphilipp/ Abstract: 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 R_0 = 8.5 kpc). For the ~ 6*10^4 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*10^6. 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. The slope of sector II of the KLF, \gamma_1 + 1 ~ -0.8 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 (~ 1 M_o) MS stars at ~ 56 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 Guesten \& 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 ~ 10^8 - 10^9 yrs ago. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================