A 100-parsec elliptical and twisted ring of cold and dense molecular clouds revealed by Herschel around the Galactic Center

Abstract: Thermal images of cold dust in the Central Molecular Zone of the Milky Way, obtained with the far-infrared cameras on-board the Herschel satellite, reveal a 3x10^7 solar masses ring of dense and cold clouds orbiting the Galactic Center. Using a simple toy-model, an elliptical shape having semi-major axes of 100 and 60 parsecs is deduced. The major axis of this 100-pc ring is inclined by about 40 degrees with respect to the plane-of-the-sky and is oriented perpendicular to the major axes of the Galactic Bar. The 100-pc ring appears to trace the system of stable x_2 orbits predicted for the barred Galactic potential. Sgr A* is displaced with respect to the geometrical center of symmetry of the ring. The ring is twisted and its morphology suggests a flattening-ratio of 2 for the Galactic potential, which is in good agreement with the bulge flattening ratio derived from the 2MASS data.

• The paper reveals a twisted 100-parsec elliptical ring of cold, dense molecular clouds containing tens of millions of solar masses near the Galactic Center.
• It employs high-angular resolution observations from Herschel’s PACS and SPIRE cameras, mapping far-IR emissions from 70 to 500 micrometers to detail the CMZ structure.
• The study models the ring dynamics consistent with stable x2 orbits in barred galaxies, offering crucial insights into the gravitational influences shaping molecular cloud distributions.

The Herschel Perspective on the Galactic Center: A Detailed Study

The study by Molinari et al., titled "A 100-parsec elliptical and twisted ring of cold and dense molecular clouds revealed by Herschel around the Galactic Center," provides a comprehensive analysis of the Central Molecular Zone (CMZ) of the Milky Way, employing data from the Herschel space observatory. The paper identifies and characterizes a 100-parsec elliptical ring of cold and dense molecular clouds within the CMZ, a region that includes some of the galaxy's most massive molecular clouds and is dominated by the gravitational influence of the central supermassive black hole.

Observational Data and Methodology

The research utilized high-angular resolution infrared images obtained from Herschel, which extended the analysis of dust and molecular clouds to a new domain of sensitivity and resolution. The study focused on the CMZ's far-infrared dust emission, leveraging the multi-wavelength capabilities of the PACS and SPIRE cameras on Herschel. This instrumentation enabled the mapping of far-IR emissions at wavelengths of 70, 160, 250, 350, and 500 micrometers, revealing a more detailed structure of the CMZ compared to previous studies.

The 100-Parsec Ring

Central to the study's findings is the identification of an intricate elliptical structure of molecular clouds within the CMZ. The ring is estimated to be comprised of several tens of millions of solar masses of cold interstellar matter and exhibits a semi-major axis configuration of approximately 100 by 60 parsecs. Furthermore, the elliptical orientation of the ring is inclined by about 40 degrees relative to the plane of the sky and is oriented perpendicular to the Galactic Bar's major axes.

Modeling the Galactic Dynamics

The paper presents a simplified model suggesting that the 100-pc ring aligns with the stable $x_2$ orbits predicted for barred galaxies. This arrangement implies a non-intersecting elliptical trajectory, providing a self-consistent mechanism to explain the observed velocity distribution across the clouds. The study's model matches well with existing CS survey data, presenting a coherent rotational motion around the Galactic Center.

Implications for Galactic and Stellar Dynamics

The identification of the ring and its twist provides insights into the gravitational dynamics of the CMZ. The paper suggests that the pattern motion, possibly indicating a sloshing dynamic, could be a reaction to bar-induced m=1 modes. Additionally, the ring's twist, with a vertical oscillation frequency twice that of its orbital frequency, offers a constraint on the flattening ratio of the Galactic potential, which supports prior 2MASS observations and theoretical models.

Mass and Structural Implications

The estimated mass of the ring is around $3 \times 10^7$ M$_\odot$, aligning with mass estimates from CO data and SCUBA maps. The displacement of the ring concerning the Galactic Center further raises questions about the CMZ's dynamic response to gravitational disturbances. The study discusses the potential impact of these structures in past active galactic nucleus (AGN) phases of the Milky Way, the recent detection of gamma-ray bubbles potentially being a vestige of such activity.

Conclusion and Future Directions

The study by Molinari et al. exemplifies the critical role of infrared observations in unraveling the complex array of structures and dynamics at our Galaxy's central region. The advancements in spatial resolution and sensitivity provided by Herschel offer a better understanding of the interplay between large scale bar dynamics and molecular cloud formation. Future observations and more refined models will undoubtedly provide further clarity on the implications of these structures in star formation processes and galaxy evolution.