EDP Sciences
Free access
Issue
A&A
Volume 384, Number 1, March II 2002
Page(s) 112 - 139
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20020017


A&A 384, 112-139 (2002)
DOI: 10.1051/0004-6361:20020017

The nuclear bulge of the Galaxy

III. Large-scale physical characteristics of stars and interstellar matter
R. Launhardt1, 2, R. Zylka3, 4 and P. G. Mezger2

1  Division of Physics, Mathematics and Astronomy, California Institute of Technology, MS 105-24, Pasadena, CA 91125, USA
2  Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: mezger@mpifr-bonn.mpg.de
3  Institut für Theoretische Astrophysik (ITA), Tiergartenstraße 15, 69121 Heidelberg, Germany
4  I. Phys. Institut d. Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

(Received 31 August 2001 / Accepted 16 November 2001 )

Abstract
We analyse IRAS and COBE DIRBE data at wavelengths between 2.2 and 240 of the central 500 pc of the Galaxy and derive the large-scale distribution of stars and interstellar matter in the Nuclear Bulge. Models of the Galactic Disk and Bulge are developed in order to correctly decompose the total surface brightness maps of the inner Galaxy and to apply proper extinction corrections. The Nuclear Bulge appears as a distinct, massive disk-like complex of stars and molecular clouds which is, on a large scale, symmetric with respect to the Galactic Centre. It is distinguished from the Galactic Bulge by its flat disk-like morphology, very high density of stars and molecular gas, and ongoing star formation. The Nuclear Bulge consists of an R-2 Nuclear Stellar Cluster at the centre, a large Nuclear Stellar Disk with radius $230\pm20$ pc and scale height $45\pm 5$ pc, and the Nuclear Molecular Disk of same size. The total stellar mass and luminosity of the Nuclear Bulge are $1.4\pm0.6\times 10^9$ and $2.5\pm1\times 10^9$ , respectively. About 70% of the luminosity is due to optical and UV radiation from young massive Main-Sequence stars which are most abundant in the Nuclear Stellar Cluster. For the first time, we derive a photometric mass distribution for the central 500 pc of the Galaxy which is fully consistent with the kinematic mass distribution. We find that the often cited R-2 distribution holds only for the central ~30 pc and that at larger radii the mass distribution is dominated by the Nuclear Stellar Disk which has a flatter density profile. The total interstellar hydrogen mass in the Nuclear Bulge is $M_{\rm H}=2\pm0.3\times 10^7$ , distributed in a warm inner disk with $R=110\pm20$ pc and a massive, cold outer torus which contains more than 80% of this mass. Interstellar matter in the Nuclear Bulge is very clumpy with ~90% of the mass contained in dense and massive molecular clouds with a volume filling factor of only a few per cent. This extreme clumpiness, probably caused by the tidal stability limit in the gravitational potential of the Nuclear Bulge, enables the strong interstellar radiation field to penetrate the entire Nuclear Bulge and explains the relatively low average extinction towards the Galactic Centre. In addition, we find $3\times 10^7$ of cold and dense material outside the Nuclear Bulge at positive longitudes and $1\times 10^7$ at negative longitudes. This material is not heated by the stars in the Nuclear Bulge and gives rise to the observed asymmetry in the distribution of interstellar matter in the Central Molecular Zone.


Key words: dust, extinction -- ISM: structure -- Galaxy: centre -- Galaxy: structure -- infrared: ISM

Offprint request: R. Launhardt, rl@astro.caltech.edu

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