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

R. Launhardt, R. Zylka+1 more|ArXiv.org|Jan 17, 2002

Astrophysics and Star Formation Studies64 references241 citations

TL;DR

This study analyzes IRAS and COBE DIRBE data across 2.2–240 μm to model the large-scale structure of stars and interstellar matter in the Galactic Nuclear Bulge (NB), revealing a massive, disk-like complex with a central R⁻² stellar cluster, a 230±20 pc nuclear stellar disk, and a coextensive nuclear molecular disk. The key contribution is the first photometric mass distribution fully consistent with kinematic data, showing flatter density profiles beyond 30 pc and extreme clumpiness in ISM enabling strong radiation penetration despite high density.

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.

Motivation & Objective

• To disentangle the surface brightness contributions from the Galactic Disk and Nuclear Bulge in the inner 500 pc using multi-wavelength infrared data.
• To model the three-dimensional distribution of interstellar matter and dust to correct for extinction and isolate the true stellar and ISM emission in the Nuclear Bulge.
• To derive a photometric mass distribution for the central 500 pc that is consistent with existing kinematic mass measurements.
• To characterize the morphology, mass, and clumpiness of the interstellar medium in the Nuclear Bulge, particularly the Central Molecular Zone.
• To explain the low average extinction toward the Galactic Center despite high gas and dust densities by analyzing the spatial distribution and filling factor of dense molecular clouds.

Proposed method

• Modeling the Galactic Disk (GD) as an axisymmetric disk with spiral arms, incorporating H I, H₂, and H II components with observed radial and vertical density profiles.
• Using a 3D grid-based model (100 pc³ cells) to simulate dust emission and extinction, calibrated to COBE/DIRBE 240 μm data and FIR color temperatures.
• Applying sech² and double-Gaussian vertical density profiles for H₂ and H I, respectively, with scale heights adjusted to Wouterloot et al. (1990) measurements.
• Incorporating a four-arm logarithmic spiral pattern with Gaussian density contrasts, based on Heyer & Terebey (1998), to reproduce FIR emission variations.
• Calibrating dust temperatures and metallicity (Z/Z⊙ = 2) using Sodroski et al. (1994) and Afflerbach et al. (1997), with adjustments to fit observed FIR emission.
• Separating front- and back-side emission contributions to the NIR surface brightness using the 3D ISM model, enabling accurate extinction correction for the Nuclear Bulge.

Experimental results

Research questions

• RQ1 What is the large-scale spatial distribution of stars and interstellar matter in the Galactic Nuclear Bulge?
• RQ2 How does the photometric mass distribution of the Nuclear Bulge compare with its kinematic mass distribution?
• RQ3 Why is the average extinction toward the Galactic Center lower than expected given the high dust and gas densities?
• RQ4 What is the morphology and mass distribution of the interstellar medium in the Nuclear Bulge, particularly in the Central Molecular Zone?
• RQ5 How does the clumpiness of the interstellar medium affect radiation field penetration and observed emission?

Key findings

• The Nuclear Bulge has a total stellar mass of 1.4 ± 0.6 × 10⁹ M⊙ and luminosity of 2.5 ± 1 × 10⁹ L⊙, with 70% of the luminosity originating from young massive main-sequence stars in the central R⁻² cluster.
• The Nuclear Stellar Disk extends to 230 ± 20 pc with a scale height of 45 ± 5 pc, and its flatter density profile dominates the mass distribution beyond 30 pc, contradicting the simple R⁻² law at larger radii.
• The total interstellar hydrogen mass in the Nuclear Bulge is 2.0 ± 0.3 × 10⁷ M⊙, with 80% in a cold, massive outer torus and 20% in a warm inner disk (R ≈ 110 ± 20 pc).
• Interstellar matter is extremely clumpy, with 90% of the mass in dense, massive molecular clouds and a volume filling factor of only a few percent, enabling strong radiation field penetration.
• An additional 3 × 10⁷ M⊙ of cold, dense material is found at positive longitudes and 1 × 10⁷ M⊙ at negative longitudes, explaining the asymmetry in the Central Molecular Zone's ISM distribution.
• The photometric mass distribution derived here is the first to be fully consistent with the kinematic mass distribution in the central 500 pc of the Galaxy.