P. G. Mezger

The nuclear bulge of the Galaxy III. Large-scale physical characteristics of stars and interstellar matter

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 ± 20 pc and scale height 45 ± 5 pc, and the Nuclear Molecular Disk of same size. The total stellar mass and luminosity of the Nuclear Bulge are 1.4 ± 0.6 x 10^9 and 2.5 ± 1 x 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_H = 2 ± 0.3 x 10^7, distributed in a warm inner disk with R = 110 ± 20 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 x 10^7 of cold and dense material outside the Nuclear Bulge at positive longitudes and 1 x 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.

Continuum observations of Sgr A at mm/submm wavelengths

We have mapped the source complex Sgr A (containing Sgr A West and East) at λ2.8mm, 1.3mm and 350/μm using the IRAM 30m MRT (Pico Veleta, Spain) and the 3m IRTF (Mauna Kea, Hawaii) telescopes. Detailed results have been published or will be published elsewhere (Mezger et al., 1986, Paper I; Zylka and Mezger, 1988, Paper II; Mezger et al., 1988, Paper III).

Three Principal Heating Sources of Dust in the Galactic Disk

The galactic infrared emission has been observed at various wavelengths from the near infrared 4 μm up to 900 μm. The spectrum of the central part of our galaxy is comparable to that observed towards external Sb/Sc galaxies: a broad emission peak centered around 100 μm and a distinct shoulder occuring shortwards of 20 μm. The presence of these features suggests that the infrared spectrum of normal spiral galaxies is a super position of spectra originating in different classes of sources (Fig. 1).

Star Formation in the Galactic Disk

Regarding the subject of my lecture it can be viewed under two different angles: A.) Global star formation and lockup rates in the galactic disk and B.) individual processes related to the formation of massive stars in molecular clouds. My lecture considers both aspects.

Dust Emission from Star Forming Clouds: A Progress Report

Model computations of protostellar evolution depend very strongly on the initial conditions: Fragmentation of massive cloud cores or coagulation of substellar condensations, the physical state of gas and dust (e.g. the formation of ice-mantles and grain coagulation), the presence of magnetic fields and its effect on gas and dust, and the formation of accretion disks as a consequence of an initial angular momentum of the protostellar condensation. The MPIfR bolometer group together with the molecular spectroscopists R. Mauersberger and T.L. Wilson have embarked on a program aimed at the exploration of the earliest evolutionary stages of high-and low-mass star formation. Here follows a brief progress report.

Submm Continuum Observations of Sources from the I R A S Point-Source Catalogue

More than 120 galactic and extragalactic objects, taken from the IRAS pointsource catalogue have been observed at 350 and 1300 µm. Supplementary near IR data and extensive model fits over this large spectral range allow the determination of the total luminosity, dust temperatures, the wavelength dependence of dust opacity, the total amount of extinction towards embedded stellar objects and the density distribution of dust around central heating sources within HII regions and dust clouds.

Sgr A* — A Starving Black Hole?

The Galactic Center was actually the first radio source identified after Carl Guthe Jansky’s discovery of cosmic radio radiation in 1933. At the long wavelength of λ 14.5 m Jansky nearly exclusively observed only synchrotron emission. Although the contribution of free-free emission becomes stronger at decimeter wavelengths, in later surveys with 25 m telescopes at λ 20 cm [1] and λ 11 cm (Altenhoff et al. [2]), the Sagittarius A (Sgr A) radio complex was still observed as a single compact but predominantly nonthermal radio source, well above the level of the diffuse emission from the Galactic Disk.

Low-Cost Enclosure For The Sub-Millimeter Telescope

The University of Arizona and the Max-Planck-Institut fur Radioastronomie are collaborating to construct a sub-millimeter wavelength radio telescope facility at the summit of Mt. Lemmon (2791 m above sea level) near Tucson, Arizona. We have designed a corotating building to protect the 10 m diameter Sub-Millimeter Telescope (SMT) against storm damage, to provide large instrumentation rooms at the Nasmyth foci, and to minimize degradation of the reflector profile accuracy and pointing errors caused by wind forces and solar radiation.

Molecules as Probes of the Interstellar Matter

26 interstellar molecules have now been observed. Some of the observational problems pertaining to detection of molecules by radio spectroscopy are discussed in section II. Our present knowledge concerning the interstellar matter, especially with regard to its cloud structure, is reviewed in section III. The observational results pertaining to the formation of stars from the interstellar gas are discussed in Section IV. Sections V and VI present some examples of how molecules can be used as probes of the physical state of dense and cool interstellar clouds, and, possibly, of the UV radiation within these clouds. Use of molecules to study the large-scale structure of the Galaxy, especially the inner nuclear disk, is discussed in Section VII.