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1. The JWST-NIRCam View of Sagittarius C. II. Evidence for Magnetically Dominated H ii Regions in the Central Molecular Zone

   https://doi.org/10.3847/1538-4357/ad9d0b  

   Bally, John; Crowe, Samuel; Fedriani, Rubén; Ginsburg, Adam; Schödel, Rainer; Andersen, Morten; Tan, Jonathan C; Li, Zhi-Yun; Nogueras-Lara, Francisco; Cheng, Yu; et al (April 2025, The Astrophysical Journal)

   Abstract We present JWST-NIRCam narrowband, 4.05μm Brαimages of the Sgr C Hiiregion, located in the central molecular zone (CMZ) of the Galaxy. Unlike any Hiiregion in the solar vicinity, the Sgr C plasma is dominated by filamentary structure in both Brαand the radio continuum. Some bright filaments, which form a fractured arc with a radius of about 1.85 pc centered on the Sgr C star-forming molecular clump, likely trace ionization fronts. The brightest filaments form a “π-shaped” structure in the center of the Hiiregion. Fainter filaments radiate away from the surface of the Sgr C molecular cloud. The filaments are emitting optically thin free–free emission, as revealed by spectral index measurements from 1.28 GHz (MeerKAT) to 97 GHz (Atacama Large Millimeter/submillimeter Array). But, the negative in-band 1 to 2 GHz spectral index in the MeerKAT data alone reveals the presence of a nonthermal component across the entire Sgr C Hiiregion. We argue that the plasma flow in Sgr C is controlled by magnetic fields, which confine the plasma to ropelike filaments or sheets. This results in the measured nonthermal component of low-frequency radio emission plasma, as well as a plasmaβ(thermal pressure divided by magnetic pressure) below 1, even in the densest regions. We speculate that all mature Hiiregions in the CMZ, and galactic nuclei in general, evolve in a magnetically dominated, low plasmaβregime. 

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2. Simulations of the Milky Way’s central molecular zone – I. Gas dynamics

   https://doi.org/10.1093/mnras/staa3120  

   Tress, Robin G; Sormani, Mattia C; Glover, Simon C; Klessen, Ralf S; Battersby, Cara D; Clark, Paul C; Hatchfield, H Perry; Smith, Rowan J (October 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We use hydrodynamical simulations to study the Milky Way’s central molecular zone (CMZ). The simulations include a non-equilibrium chemical network, the gas self-gravity, star formation, and supernova feedback. We resolve the structure of the interstellar medium at sub-parsec resolution while also capturing the interaction between the CMZ and the bar-driven large-scale flow out to $$R\sim 5\, {\rm kpc}$$. Our main findings are as follows: (1) The distinction between inner (R ≲ 120 pc) and outer (120 ≲ R ≲ 450 pc) CMZ that is sometimes proposed in the literature is unnecessary. Instead, the CMZ is best described as single structure, namely a star-forming ring with outer radius R ≃ 200 pc which includes the 1.3° complex and which is directly interacting with the dust lanes that mediate the bar-driven inflow. (2) This accretion can induce a significant tilt of the CMZ out of the plane. A tilted CMZ might provide an alternative explanation to the ∞-shaped structure identified in Herschel data by Molinari et al. (3) The bar in our simulation efficiently drives an inflow from the Galactic disc (R ≃ 3 kpc) down to the CMZ (R ≃ 200 pc) of the order of $$1\rm \, M_\odot \, yr^{-1}$$, consistent with observational determinations. (4) Supernova feedback can drive an inflow from the CMZ inwards towards the circumnuclear disc of the order of $${\sim}0.03\, \rm M_\odot \, yr^{-1}$$. (5) We give a new interpretation for the 3D placement of the 20 and 50 km s−1 clouds, according to which they are close (R ≲ 30 pc) to the Galactic Centre, but are also connected to the larger scale streams at R ≳ 100 pc. 

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3. The Galactic Center arms inferred from the ALMA CMZ Exploration Survey (ACES)

   https://doi.org/10.1093/pasj/psaf034  

   Sofue, Yoshiaki; Oka, Tomoharu; Longmore, Steven N; Walker, Daniel; Ginsburg, Adam; Henshaw, Jonathan D; Bally, John; Barnes, Ashley T; Battersby, Cara; Colzi, Laura; et al (June 2025, Publications of the Astronomical Society of Japan)

   Abstract Analyzing longitude–velocity diagrams (LVDs) in the CS (J = 2–1) and H$$^{13}$$CN (J = 1–0) molecular lines from the internal release data of the ALMA Central Molecular Zone (CMZ) Exploration Survey and in the $$^{13}$$CO (J = 1–0) line from the Nobeyama Galactic Center (GC) survey, we identify six GC arms as prominent straight LV ridges. In addition to the currently known Arms I to IV, we identify a new inner arm, Arm V, and further highlight the circumnuclear disk (CND) as Arm VI. Integrated intensity maps of the arms on the sky suggest that most of the arms compose ring-like structures inclined from the Galactic plane. We determine the radii (curvatures) of the arms using the velocity-gradient ($dv/dl$) method, assuming that the arms are rotating in circular orbits at a constant velocity of $$\sim$$150 km s$$^{-1}$$. We show that Arms I and II compose the main ring structure of the CMZ with radii $$\sim$$100–120 pc; Arm III is a dense arm 42 pc from the GC; Arm IV is a clear and narrow arm 20 pc from the GC; and Arm V is a faint, long arm of 8.2 pc radius. We show that the CND comprises the sixth arm, Arm VI, of radius $$\sim$$2.3 pc associated with bifurcated spiral fins. We also discuss the association of the 20 and 50 km s$$^{-1}$$ clouds with these arms. The radii of the arms fall on an empirical relation $$R\sim 630 (2/5)^N$$ for $N=1$ (Arm I) to 6 (VI), suggesting discrete rings or a logarithmic spiral with pitch angle $${\sim }22^{\circ }$$. The vertical full extent of the arm increases with radius and is represented by $$z\sim 0.7 (R/1 {\rm \ pc} )^{0.7}$$ pc. The tilt angle of the arms from the Galactic plane, or the warping, increases rapidly toward the GC. 

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4. Evidence of a Cloud–Cloud Collision from Overshooting Gas in the Galactic Center

   https://doi.org/10.3847/1538-4357/ad01be  

   Gramze, Savannah R.; Ginsburg, Adam; Meier, David S.; Ott, Juergen; Shirley, Yancy; Sormani, Mattia C.; Svoboda, Brian E. (December 2023, The Astrophysical Journal)

   Abstract The Milky Way is a barred spiral galaxy withbar lanesthat bring gas toward the Galactic center. Gas flowing along these bar lanes often overshoots, and instead of accreting onto the Central Molecular Zone (CMZ), it collides with the bar lane on the opposite side of the Galaxy. We observed G5, a cloud that we believe is the site of one such collision, near the Galactic center at (ℓ,b) = ( +5.4, −0.4) with the Atacama Large Millimeter/submillimeter Array/Atacama Compact Array. We took measurements of the spectral lines¹²COJ= 2 → 1,¹³COJ= 2 → 1, C¹⁸OJ= 2 → 1, H₂COJ= 3₀₃→ 2₀₂, H₂COJ= 3₂₂→ 2₂₁, CH₃OHJ= 4₂₂→ 3₁₂, OCSJ= 18 → 17, and SiOJ= 5 → 4. We observed a velocity bridge between two clouds at ∼50 and ∼150 km s⁻¹in our position–velocity diagram, which is direct evidence of a cloud–cloud collision. We measured an average gas temperature of ∼60 K in G5 using H₂CO integrated-intensity line ratios. We observed that the¹²C/¹³C ratio in G5 is consistent with optically thin, or at most marginally optically thick¹²CO. We measured $1.5\times {10}^{19}{\mathrm{cm}}^{-2}{\left(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\right)}^{-1}$ for the local X_CO, 10–20× less than the average Galactic value. G5 is strong direct observational evidence of gas overshooting the CMZ and colliding with a bar lane on the opposite side of the Galactic center. 

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5. Magnetic field morphology and evolution in the Central Molecular Zone and its effect on gas dynamics

   https://doi.org/10.1051/0004-6361/202450035  

   Tress, R G; Sormani, M C; Girichidis, P; Glover, S_C O; Klessen, R S; Smith, R J; Sobacchi, E; Armillotta, L; Barnes, A T; Battersby, C; et al (November 2024, Astronomy & Astrophysics)

   The interstellar medium in the Milky Way’s Central Molecular Zone (CMZ) is known to be strongly magnetised, but its large-scale morphology and impact on the gas dynamics are not well understood. We explore the impact and properties of magnetic fields in the CMZ using three-dimensional non-self gravitating magnetohydrodynamical simulations of gas flow in an external Milky Way barred potential. We find that: (1) The magnetic field is conveniently decomposed into a regular time-averaged component and an irregular turbulent component. The regular component aligns well with the velocity vectors of the gas everywhere, including within the bar lanes. (2) The field geometry transitions from parallel to the Galactic plane near ɀ = 0 to poloidal away from the plane. (3) The magneto-rotational instability (MRI) causes an in-plane inflow of matter from the CMZ gas ring towards the central few parsecs of 0.01−0.1 M_⊙yr⁻¹that is absent in the unmagnetised simulations. However, the magnetic fields have no significant effect on the larger-scale bar-driven inflow that brings the gas from the Galactic disc into the CMZ. (4) A combination of bar inflow and MRI-driven turbulence can sustain a turbulent vertical velocity dispersion ofσ_ɀ= 5 km s⁻¹on scales of 20 pc in the CMZ ring. The MRI alone sustains a velocity dispersion ofσ_ɀ≃ 3 km s⁻¹. Both these numbers are lower than the observed velocity dispersion of gas in the CMZ, suggesting that other processes such as stellar feedback are necessary to explain the observations. (5) Dynamo action driven by differential rotation and the MRI amplifies the magnetic fields in the CMZ ring until they saturate at a value that scales with the average local density asB≃ 102 (n/10³cm⁻³)^0.33µG. Finally, we discuss the implications of our results within the observational context in the CMZ. 

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