[Paper Review] Gas Streams and Spiral Structure in the Milky Way

P. Englmaier|arXiv (Cornell University)|Jan 20, 2000

Astro and Planetary Science3 citations

TL;DR

This paper proposes a non-axisymmetric, frozen mass model for the Milky Way that explains the observed gas dynamics and spiral structure through the combined effects of a central bar and additional mass concentrations on the bar's minor axis. The model reproduces the 4-arm grand-design spiral pattern, including the 3-kpc arm, by simulating gas flows under the gravitational influence of the bar and deprojected stellar structures, successfully matching terminal velocity curves and forbidden velocity features in the inner Galaxy.

ABSTRACT

The observed gas dynamics in the Milky Way can only be explained by a bar in the galactic center. Such a bar is directly visible in the near-IR maps of the bulge, where it causes a distinctive asymmetric light distribution pattern. Another large-scale structure is the grand-design 4-arm spiral pattern, most clearly observed in the spatial distribution of molecular gas and HII-regions. In order to model the observed gas flow structure, we constructed a model for the stellar mass distribution. For the inner 5 $\kpc$ we used the 3D deprojected near-IR light distribution, as observed by the COBE/DIRBE experiment, and added an analytical disk model outside the box as well as a halo model. With this frozen mass distribution, we computed the stationary gas flow for various deprojection parameters and pattern speeds. For all reasonable parameter choices, we obtain a 4-armed spiral pattern, which can be matched to the observed spiral arms. In the bar region, our model can explain the non-circular motion visible in the terminal velocity curve as well as part of the forbidden velocities. Inside the corotation, we also find 4 spiral arms, the nearest arm corresponds to the 3-kpc-arm, although only qualitatively. The missing southern 3-kpc-arm at the far end of the galaxy is explained by running parallel to another arm. Close to the center, we find gas on circular orbits forming a disk. Such a disk has been observed in emission of the CS molecule, however only part of the disk appears to be occupied by dense enough gas to be traced by CS. Further we compare our model to the distribution of OH/IR stars in the inner galaxy.

Motivation & Objective

• To explain the observed non-circular gas motions in the inner Milky Way, particularly the sharp peak in the rotation curve near 0.5 kpc.
• To account for the presence of the 3-kpc arm and other spiral features in molecular gas and HII regions using a non-axisymmetric mass distribution.
• To reconcile photometric observations of the near-IR bulge with gas dynamical data by constructing a consistent stationary gas flow model.
• To investigate whether the 3-kpc arm and other spiral arms can be driven by the bar and additional structural features, rather than independent spiral modes.
• To constrain the role of the bar and spiral arms in shaping the gas distribution and kinematics across the inner Galaxy.

Proposed method

• Constructed a 3D stellar mass distribution using deprojected near-IR light from COBE/DIRBE, including a triaxial bar with axis ratios 1:0.33:0.22 and inclination ~20°.
• Extended the model with an analytical disk component beyond 5 kpc and a halo component to match the total mass budget and rotation curve.
• Computed stationary gas flows under the frozen gravitational potential of the mass model for various deprojection parameters and pattern speeds.
• Incorporated additional mass concentrations at ~4 kpc along the bar’s minor axis to simulate incorrectly deprojected stellar spiral arms.
• Traced gas dynamics using hydrodynamical simulations under the influence of the bar and spiral forcing, focusing on terminal velocity curves and forbidden velocity regions.
• Validated the model against observed distributions of molecular gas, HII regions, and the CS-emitting nuclear disk.

Experimental results

Research questions

• RQ1 Can the observed non-circular motions in the inner Milky Way, particularly the sharp peak in the rotation curve at ~0.5 kpc, be explained by a bar-induced potential rather than an axisymmetric model?
• RQ2 How do the 4-arm grand-design spiral features, especially the 3-kpc arm, arise in the gas distribution despite the lack of current star formation in that feature?
• RQ3 To what extent can the spiral structure in the molecular ring (4–7 kpc) be reproduced by the gravitational forcing of the bar and minor-axis mass concentrations?
• RQ4 Is the 3-kpc arm a stable, long-lived feature, or is it transient, and how does its morphology relate to the orbital dynamics of gas and stars?
• RQ5 Can the observed gas kinematics and spatial distribution be consistently explained by a single, non-axisymmetric mass model including a bar and additional structural features?

Key findings

• The model successfully reproduces the 4-arm grand-design spiral pattern observed in molecular gas and HII regions, particularly in the molecular ring between 4 and 7 kpc.
• The 3-kpc arm is qualitatively explained as a result of bar forcing, with gas on non-circular, elongated orbits that may be dynamically hot and starless, consistent with the lack of HII regions.
• The bar and minor-axis mass concentrations together generate a stable 4-armed spiral pattern in the gas, matching the observed spiral structure in the inner Galaxy.
• The model explains the terminal velocity curve peak at ~0.5 kpc as a result of non-circular motions driven by the bar, resolving discrepancies in axisymmetric models.
• A nuclear gas disk is formed in the central ~200 pc, consistent with CS emission observations, though only partially traced due to density thresholds.
• The model suggests that OH/IR stars are not directly tracing the 3-kpc arm, but may be associated with closed orbits near corotation, possibly linked to star formation triggered at the bar’s end.