The Nuclear Bulge of the Galaxy. III. Large-Scale Physical Characteristics of Stars and Interstellar Matter

TL;DR

This study analyzes infrared data to characterize the large-scale structure and composition of the Nuclear Bulge in the Galaxy, revealing its morphology, mass, and interstellar matter distribution, and distinguishing it from the Galactic Bulge.

Contribution

It provides a detailed model of the Nuclear Bulge's structure, mass, and interstellar matter distribution, highlighting its unique disk-like morphology and high density compared to the Galactic Bulge.

Findings

Nuclear Bulge is a massive, disk-like structure with high star and molecular gas density.

The total stellar mass of the Nuclear Bulge is approximately 1.4 billion solar masses.

Interstellar matter is highly clumpy, with most mass in dense molecular clouds.

Abstract

We analyse IRAS and COBE DIRBE data at wavelengths between 2.2 and 240 mu of the central 500pc 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 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 a Nuclear Molecular Disk of same size. Its total stellar mass and luminosity are 1.4+-0.6 10^9 M_sun and 2.5+-1 10^9 L_sun, respectively. The total mass of interstellar hydrogen in the Nuclear Bulge is 2+-0.3 10^7 M_sun. 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 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 4 10^7 M_sun of cold and dense material located outside the Nuclear Bulge, which gives rise to the observed asymmetry in the distribution of interstellar matter in the Central Molecular Zone.