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Abstract The construction of an accurate 3D model of the Milky Way center is necessary to understand inflow processes that drive its overall evolution and to compare our Galactic nucleus to other galaxies’ nuclei. A main point of contention is the line-of-sight location of sources observed toward the central 10 pc of the Galaxy, including recent star formation (the Sgr A East supernova remnant and Sgr A HIIregions) and copious gas (the 50 and 20 km s⁻¹molecular clouds, the circumnuclear disk, and the Sgr A West ionized “minispiral” that encircles the central supermassive black hole, Sgr A*). Some models place all of these structures within a radius of 5 pc from Sgr A*, while others place the 20 and 50 km s⁻¹clouds at a distance of at least 30−50 pc away from Sgr A* along the line of sight. We present new radio and millimeter observations of the molecular gas toward the central ∼10 pc, from which we have constructed an alternative 3D model that is consistent with both prior radio observations and orbital gas kinematics. Our model places the 20 km s⁻¹cloud, 50 km s⁻¹cloud, and Sgr A East more than 10 pc in front of Sgr A*. While this model does not conclusively rule out a connection between the 50 and 20 km s⁻¹clouds and the circumnuclear disk, we argue that prior evidence for these connections is tenuous, especially given the complex spatial and kinematic overlap of structures along the line of sight. 

Abstract The Milky Way’s Central Molecular Zone (CMZ) is the largest concentration of dense molecular gas in the Galaxy, the structure of which is shaped by the complex interplay between Galactic-scale dynamics and extreme physical conditions. Understanding the 3D geometry of this gas is crucial, as it determines the locations of star formation and subsequent feedback. We present a catalog of clouds in the CMZ using Herschel data. Using archival data from the APEX and MOPRA CMZ surveys, we measure averaged kinematic properties of the clouds at 1 and 3 mm. We use archival ATCA data of the H₂CO (1_1,0–1_1,1) 4.8 GHz line to search for absorption towards the clouds, and 4.85 GHz Green Bank Telescope (GBT)C-band data to measure the radio continuum emission. We measure the absorption against the continuum to provide new constraints for the line-of-sight positions of the clouds relative to the Galactic Center, and find a highly asymmetric distribution, with most clouds residing in front of the Galactic Center. The results are compared with different orbital models, and we introduce a revised toy model of a vertically oscillating closed elliptical orbit. We find that most models describe the position–position–velocity structure of the gas reasonably well, but find significant inconsistencies in all cases regarding the near versus far placement of individual clouds. Our results highlight that the CMZ is likely more complex than can be captured by these simple geometric models, along with the need for new data to provide further constraints on the true 3D structure of the CMZ. 

Abstract A comprehensive 3D model of the central 300 pc of the Milky Way, the Central Molecular Zone (CMZ) is of fundamental importance in understanding energy cycles in galactic nuclei, since the 3D structure influences the location and intensity of star formation, feedback, and black hole accretion. Current observational constraints are insufficient to distinguish between existing 3D models. Dust extinction is one diagnostic tool that can help determine the location of dark molecular clouds relative to the bright Galactic Center emission. By combining Herschel and Spitzer observations, we developed three new dust extinction techniques to estimate the likely near/far locations for each cloud in the CMZ. We compare our results to four geometric CMZ orbital models. Our extinction methods show good agreement with each other, and with results from spectral line absorption analysis from Walker et al. Our near/far results for CMZ clouds are inconsistent with a projected version of the Y. Sofue two-spiral-arms model, and show disagreement in position–velocity space with the S. Molinari et al. closed elliptical orbit. Our results are in reasonable agreement with the J. M. D. Kruijssen et al. open streams. We find that a simplified toy-model elliptical orbit that conserves angular momentum shows promising fits in both position–position and position–velocity space. We conclude that all current CMZ orbital models lack the complexity needed to describe the motion of gas in the CMZ, and further work is needed to construct a complex orbital model to accurately describe gas flows in the CMZ. 

Abstract Polycyclic aromatic hydrocarbon (PAH) emission is widely used to trace the distribution of molecular gas in the interstellar medium, exhibiting a tight correlation with CO(2–1) emission across nearby galaxies. Using PHANGS-JWST and PHANGS-Atacama Large Millimeter/submillimeter Array (ALMA) data, we identify localized regions where this correlation fails, with CO flux exceeding that predicted from 7.7μm PAH emission by more than an order of magnitude. These outlier regions are found in 20 out of 70 galaxies and are located in galaxy centers and bars, without signs of massive star formation. We explore two scenarios to explain the elevated CO-to-PAH ratios, which can either be due to suppressed PAH emission or enhanced CO emissivity. We examine PAH emission in other bands (3.3 and 11.3μm) and the dust-continuum-dominated bands (10 and 21μm), finding consistently high CO-to-PAH (or CO-to-dust continuum) emission ratios, suggesting that 7.7μm PAH emission is not particularly suppressed. In some outlier regions, PAH sizes and spectral energy distribution of the radiation differ slightly from nearby control regions with normal CO-to-PAH ratios, though without a consistent trend. We find that the outlier regions show higher CO velocity dispersions (Δv_CO). This increase in Δv_COlowers CO optical depth and raises its emissivity for a given gas mass. Our results favor a scenario where shear along the bar lanes and shocks at the bar ends elevate CO emissivity, leading to the breakdown of the CO–PAH correlation. Future JWST spectroscopy and deep ALMA observations of CO isotopologues will provide critical tests of this scenario. 

Abstract The Central Molecular Zone (CMZ) is the way station at the heart of our Milky Way Galaxy, connecting gas flowing in from Galactic scales with the central nucleus. Key open questions remain about its 3D structure, star formation properties, and role in regulating this gas inflow. In this work, we identify a hierarchy of discrete structures in the CMZ using column density maps from Paper I (C. Battersby et al.) We calculate the physical (N(H₂),T_dust, mass, radius) and kinematic (HNCO, HCN, and HC₃N moments) properties of each structure as well as their bolometric luminosities and star formation rates. We compare these properties with regions in the Milky Way disk and external galaxies. Despite the fact that the CMZ overall is well below the Gao-Solomon dense gas star formation relation (and in modest agreement with the Schmidt–Kennicutt relation), individual structures on the scale of molecular clouds generally follow these star formation relations and agree well with other Milky Way and extragalactic regions. We find that individual CMZ structures require a large external pressure (P_e/k_B> 10⁷⁻⁹K cm⁻³) to be considered bound; however, simple estimates suggest that most CMZ molecular-cloud-sized structures are consistent with being in pressure-bounded virial equilibrium. We perform power-law fits to the column density probability distribution functions of the inner 100 pc, SgrB2, and the outer 100 pc of the CMZ as well as several individual molecular cloud structures and find generally steeper power-law slopes (−9 <α< −2) compared with the literature (−6 <α< −1). 

Abstract The Central Molecular Zone (CMZ) is the largest reservoir of dense molecular gas in the Galaxy and is heavily obscured in the optical and near-IR. We present an overview of the far-IR dust continuum, where the molecular clouds are revealed, provided by Herschel in the inner 40° (∣l∣ < 20°) of the Milky Way with a particular focus on the CMZ. We report a total dense gas (N(H₂) > 10²³cm⁻²) CMZ mass of $\sim 2_{-1}^{+2}\times 10^7$M_⊙and confirm that there is a highly asymmetric distribution of dense gas, with about 70%–75% at positive longitudes. We create and publicly release complete fore/background-subtracted column density and dust temperature maps in the inner 40° (∣l∣ < 20°) of the Galaxy. We find that the CMZ clearly stands out as a distinct structure, with an average mass per longitude that is at least 3× higher than the rest of the inner Galaxy contiguously from 1 $\stackrel{\text{°}}{.}$8 >ℓ> −1 $\stackrel{\text{°}}{.}$3. This CMZ extent is larger than previously assumed, but is consistent with constraints from velocity information. The inner Galaxy’s column density peaks towards the SgrB2 complex with a value of about 2 × 10²⁴cm⁻², and typical CMZ molecular clouds are aboutN(H₂) ∼ 10²³cm⁻². Typical CMZ dust temperatures range from ∼12–35 K with relatively little variation. We identify a ridge of warm dust in the inner CMZ that potentially traces the base of the northern Galactic outflow seen with MEERKAT. 

Context. The central molecular zone (CMZ), surrounding the Galactic centre, is the largest reservoir of dense molecular gas in the Galaxy. Despite its relative proximity, the 3D structure of the CMZ remains poorly constrained, primarily due to projection effects. Aims. We aim to constrain the line-of-sight location of two molecular clouds in the CMZ - the 50 and 20 km/s clouds - and to investigate their possible physical connection using stellar kinematics and photometry. This study serves as a pilot for future applications across the full CMZ. Methods. We estimated the line-of-sight position of the clouds by analysing stellar kinematics, stellar densities, and stellar populations towards the cloud regions and a control field. Results. We find an absence of westward moving stars in the cloud regions, which indicates that they lie on the near side of the CMZ. This interpretation is supported by the stellar density distributions. The similar behaviour observed in the two clouds, as well as in the region between them (the ridge), suggests that they are located at comparable distances and are physically linked. We also identified an intermediate-age stellar population (2-7 Gyr) in both regions, consistent with that observed on the near side of the CMZ. We estimated the line-of-sight distances at which the clouds and the ridge become kinematically detectable (i.e. where the proper motion component parallel to the Galactic plane differs from that of the control field at the 3σ level) by converting their measured proper motions parallel to the Galactic plane using a theoretical model of the stellar distribution. We find that the 50 and 20 km/s clouds are located at 43 ± 8 pc and 56 ± 11 pc from Sgr A^*, respectively, and that the ridge lies at 56 ± 11 pc; this supports the idea that the clouds are physically connected through the ridge. 

Abstract The Galactic center (GC) is an extreme region of the Milky Way that is host to a complex set of thermal and nonthermal structures. In particular, the GC contains high-density gas and dust that is collectively referred to as the Central Molecular Zone (CMZ). In this work, we study a subset of HNCO filaments identified in Band 3 Atacama Large Millimeter/submillimeter Array (ALMA) observations of the GC obtained by the ALMA CMZ Exploration Survey that are comparable to high-density filaments identified in the Galactic disk. We compare the orientation of the magnetic field derived from 214μm Stratospheric Observatory for Infrared Astronomy and 850μm James Clerk Maxwell Telescope observations with the filament orientation to determine which mechanisms dominate the formation of these filaments. We observe a large range of magnetic orientations in our observed filaments indicating the complex environments the filaments are located in. We also compare the observational results to synthetic datasets created using an MHD model of the GC. Our analysis reveals that the dominant mechanisms local to the HNCO filaments vary throughout the GC with some filaments being dominated by supersonic turbulence and others by subsonic turbulence. The comparison to synthetic observations indicates that the observed filaments are in magnetically dominated environments that could be supporting these filaments against collapse. Our results on the CMZ filaments are also compared to results obtained on similar filaments located in the Galactic disk, and we find that the filaments studied here are possible CMZ analogs to the dense filamentary “bones” observed previously in the Galactic disk. 

The central regions of the Milky Way constitute a unique laboratory for a wide swath of astrophysical studies; consequently, the inner ∼400 pc have been the target of numerous large surveys at all accessible wavelengths. In this paper, we present a catalog of sources at 25 and 37μm located within all of the regions observed with the SOFIA/FORCAST instrument in the inner ∼200 pc of the Galaxy. The majority of the observations were obtained as part of the SOFIA Cycle 7 Galactic Center Legacy program survey, which was designed to complement the Spitzer/MIPS 24μm catalog in regions saturated in the MIPS observations. Due to the wide variety of source types captured by our observations at 25 and 37μm, we do not limit the FORCAST source catalog to unresolved point sources, or treat all sources as if they are pointlike sources. The catalog includes all detectable sources in the regions, resulting in a catalog of 950 sources, including point sources, compact sources, and extended sources. We also provide the user with metrics to discriminate between the source types. 

Abstract Magnetic fields may play a crucial role in setting the initial conditions of massive star and star cluster formation. To investigate this, we report SOFIA-HAWC+ 214μm observations of polarized thermal dust emission and high-resolution GBT-Argus C¹⁸O(1-0) observations toward the massive Infrared Dark Cloud (IRDC) G28.37+0.07. Considering the local dispersion ofB-field orientations, we produce a map of the B-field strength of the IRDC, which exhibits values between ∼0.03 and 1 mG based on a refined Davis–Chandrasekhar–Fermi method proposed by Skalidis & Tassis. Comparing to a map of inferred density, the IRDC exhibits aB–nrelation with a power-law index of 0.51 ± 0.02, which is consistent with a scenario of magnetically regulated anisotropic collapse. Consideration of the mass-to-flux ratio map indicates that magnetic fields are dynamically important in most regions of the IRDC. A virial analysis of a sample of massive, dense cores in the IRDC, including evaluation of magnetic and kinetic internal and surface terms, indicates consistency with virial equilibrium, sub-Alfvénic conditions, and a dominant role forB-fields in regulating collapse. A clear alignment of magnetic field morphology with the direction of the steepest column density gradient is also detected. However, there is no preferred orientation of protostellar outflow directions with theB-field. Overall, these results indicate that magnetic fields play a crucial role in regulating massive star and star cluster formation, and therefore they need to be accounted for in theoretical models of these processes.