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Astron. Astrophys. 348, 768-782 (1999)

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The nuclear bulge

I. K band observations of the central 30 pc

S. Philipp 1,2, R. Zylka 2, P.G. Mezger 1, W.J. Duschl 2,1, T. Herbst 3 and R.J. Tuffs 4

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
3 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
4 Max-Planck-Institut für Kernphysik, Saupfercheckweg, D-69117 Heidelberg, Germany

Received 8 May 1998 / Accepted 22 January 1999


Out of [FORMULA] individual source images we have constructed a mosaic map 1 of the K band surface brightness in an area [FORMULA] ([FORMULA] pc for [FORMULA] 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 [FORMULA] stars with flux densities 2 [FORMULA]Jy and the remainder is contributed by an extended continuum provided by about [FORMULA] stars too weak to be observed as individual sources. We estimate that [FORMULA] of the integrated flux density of the mosaic is contributed by stars in the Nuclear Bulge (NB; [FORMULA] pc); the remaining [FORMULA] come from stars located along the line of sight in the Galactic Bulge (GB; [FORMULA]kpc[FORMULA]) and Galactic Disk (GD; [FORMULA] 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 [FORMULA]Jy, for the completeness limit we estimate [FORMULA]Jy. We find that the stellar population of the Nuclear Bulge contains considerably more bright stars (i.e. with reddened K band flux densities [FORMULA]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 ([FORMULA]) Main Sequence (MS) stars.

A model KLF constructed with a Salpeter Initial Mass Function (IMF) for stars of spectral type O9 or later ([FORMULA]Jy) explains the observations satisfactorily and connects well with the observed KLF of more luminous stars. About [FORMULA] stars with masses ranging from 0.06 to 6 [FORMULA] account for the unresolved continuum. Combining observed and model KLF we obtain a mosaic KLF which increases [FORMULA] for [FORMULA]Jy[FORMULA] and [FORMULA] for [FORMULA]Jy[FORMULA].

For radii [FORMULA] nearly 90[FORMULA] 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 [FORMULA] at a distance [FORMULA]. This indicates a deficiency of low-mass stars within the central [FORMULA] ([FORMULA] pc) and a high star formation activity during the past [FORMULA] yr. Fitting King profiles to the observed surface brightnesses we derive core radii of [FORMULA] and [FORMULA] respectively for resolved and unresolved stars. The mass-to-luminosity ratio of low mass stars in the central 30 pc is found to be [FORMULA].

We present and discuss the radio/IR spectrum of the central 30" ([FORMULA] 1.25 pc) and derive dust and Lyman continuum (Lyc) luminosities of [FORMULA] [FORMULA] and [FORMULA] s-1, respectively.

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: wjd@ita.uni-heidelberg.de

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199