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The Central Molecular Zone (CMZ; the central ∼500 pc of the Galaxy) is a kinematically unusual environment relative to the Galactic disc, with high-velocity dispersions and a steep size–linewidth relation of the molecular clouds. In addition, the CMZ region has a significantly lower star formation rate (SFR) than expected by its large amount of dense gas. An important factor in explaining the low SFR is the turbulent state of the star-forming gas, which seems to be dominated by rotational modes. However, the turbulence driving mechanism remains unclear. In this work, we investigate how the Galactic gravitational potential affects the turbulence in CMZ clouds. We focus on the CMZ cloud G0.253+0.016 (‘the Brick’), which is very quiescent and unlikely to be kinematically dominated by stellar feedback. We demonstrate that several kinematic properties of the Brick arise naturally in a cloud-scale hydrodynamics simulation, that takes into account the Galactic gravitational potential. These properties include the line-of-sight velocity distribution, the steepened size–linewidth relation, and the predominantly solenoidal nature of the turbulence. Within the simulation, these properties result from the Galactic shear in combination with the cloud’s gravitational collapse. This is a strong indication that the Galactic gravitational potential plays a crucial role in shaping the CMZ gas kinematics, and is a major contributor to suppressing the SFR, by inducing predominantly solenoidal turbulent modes.

 

Galactic bars can drive cold gas inflows towards the centres of galaxies. The gas transport happens primarily through the so-called bar dust lanes, which connect the galactic disc at kpc scales to the nuclear rings at hundreds of pc scales much like two gigantic galactic rivers. Once in the ring, the gas can fuel star formation activity, galactic outflows, and central supermassive black holes. Measuring the mass inflow rates is therefore important to understanding the mass/energy budget and evolution of galactic nuclei. In this work, we use CO datacubes from the PHANGS-ALMA survey and a simple geometrical method to measure the bar-driven mass inflow rate on to the nuclear ring of the barred galaxy NGC 1097. The method assumes that the gas velocity in the bar lanes is parallel to the lanes in the frame co-rotating with the bar, and allows one to derive the inflow rates from sufficiently sensitive and resolved position–position–velocity diagrams if the bar pattern speed and galaxy orientations are known. We find an inflow rate of $\dot{M}=(3.0 \pm 2.1)\, \rm M_\odot \, yr^{-1}$ averaged over a time span of 40 Myr, which varies by a factor of a few over time-scales of ∼10 Myr. Most of the inflow appears to be consumed by star formation in the ring, which is currently occurring at a star formation rate (SFR) of $\simeq\!1.8\!-\!2 \, \rm M_\odot \, yr^{-1}$, suggesting that the inflow is causally controlling the SFR in the ring as a function of time.

 

Abstract The Galactic bar plays a critical role in the evolution of the Milky Way’s Central Molecular Zone (CMZ), driving gas toward the Galactic Center via gas flows known as dust lanes. To explore the interaction between the CMZ and the dust lanes, we run hydrodynamic simulations in arepo , modeling the potential of the Milky Way’s bar in the absence of gas self-gravity and star formation physics, and we study the flows of mass using Monte Carlo tracer particles. We estimate the efficiency of the inflow via the dust lanes, finding that only about a third (30% ± 12%) of the dust lanes’ mass initially accretes onto the CMZ, while the rest overshoots and accretes later. Given observational estimates of the amount of gas within the Milky Way’s dust lanes, this suggests that the true total inflow rate onto the CMZ is 0.8 ± 0.6 M ⊙ yr −1 . Clouds in this simulated CMZ have sudden peaks in their average density near the apocenter, where they undergo violent collisions with inflowing material. While these clouds tend to counter-rotate due to shear, co-rotating clouds occasionally occur due to the injection of momentum from collisions with inflowing material (∼52% are strongly counter-rotating, and ∼7% are strongly co-rotating of the 44 cloud sample). We investigate the formation and evolution of these clouds, finding that they are fed by many discrete inflow events, providing a consistent source of gas to CMZ clouds even as they collapse and form stars. 

Abstract We present an update to the framework called Simulator of Galaxy Millimeter/submillimeter Emission ( sígame ). sígame derives line emission in the far-infrared (FIR) for galaxies in particle-based cosmological hydrodynamics simulations by applying radiative transfer and physics recipes via a postprocessing step after completion of the simulation. In this version, a new technique is developed to model higher gas densities by parameterizing the probability distribution function (PDF) of the gas density in higher-resolution simulations run with the pseudo-Lagrangian, Voronoi mesh code arepo . The parameterized PDFs are used as a look-up table, and reach higher densities than in previous work. sígame v3 is tested on redshift z = 0 galaxies drawn from the simba cosmological simulation for eight FIR emission lines tracing vastly different phases of the interstellar medium. This version of sígame includes dust radiative transfer with S kirt and high-resolution photoionization models with C loudy , the latter sampled according to the density PDF of the arepo simulations to augment the densities in the cosmological simulation. The quartile distributions of the predicted line luminosities overlap with the observed range for nearby galaxies of similar star formation rate (SFR) for all but two emission lines: [O i ]63 and CO(3–2), which are overestimated by median factors of 1.3 and 1.0 dex, respectively, compared to the observed line–SFR relation of mixed-type galaxies. We attribute the remaining disagreement with observations to the lack of precise attenuation of the interstellar light on sub-grid scales (≲200 pc) and differences in sample selection. 

ABSTRACT To investigate how molecular clouds react to different environmental conditions at a galactic scale, we present a catalogue of giant molecular clouds (GMCs) resolved down to masses of ∼10 M⊙ from a simulation of the entire disc of an interacting M51-like galaxy and a comparable isolated galaxy. Our model includes time-dependent gas chemistry, sink particles for star formation, and supernova feedback, meaning we are not reliant on star formation recipes based on threshold densities and can follow the physics of the cold molecular phase. We extract GMCs from the simulations and analyse their properties. In the disc of our simulated galaxies, spiral arms seem to act merely as snowplows, gathering gas, and clouds without dramatically affecting their properties. In the centre of the galaxy, on the other hand, environmental conditions lead to larger, more massive clouds. While the galaxy interaction has little effect on cloud masses and sizes, it does promote the formation of counter-rotating clouds. We find that the identified clouds seem to be largely gravitationally unbound at first glance, but a closer analysis of the hierarchical structure of the molecular interstellar medium shows that there is a large range of virial parameters with a smooth transition from unbound to mostly bound for the densest structures. The common observation that clouds appear to be virialized entities may therefore be due to CO bright emission highlighting a specific level in this hierarchical binding sequence. The small fraction of gravitationally bound structures found suggests that low galactic star formation efficiencies may be set by the process of cloud formation and initial collapse. 

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. 

Large-scale bars can fuel galaxy centers with molecular gas, often leading to the development of dense ringlike structures where intense star formation occurs, forming a very different environment compared to galactic disks. We pair ∼0.″3 (30 pc) resolution new JWST/MIRI imaging with archival ALMA CO(2–1) mapping of the central ∼5 kpc of the nearby barred spiral galaxy NGC 1365 to investigate the physical mechanisms responsible for this extreme star formation. The molecular gas morphology is resolved into two well-known bright bar lanes that surround a smooth dynamically cold gas disk (R_gal∼ 475 pc) reminiscent of non-star-forming disks in early-type galaxies and likely fed by gas inflow triggered by stellar feedback in the lanes. The lanes host a large number of JWST-identified massive young star clusters. We find some evidence for temporal star formation evolution along the ring. The complex kinematics in the gas lanes reveal strong streaming motions and may be consistent with convergence of gas streamlines expected there. Indeed, the extreme line widths are found to be the result of inter-“cloud” motion between gas peaks;ScousePydecomposition reveals multiple components with line widths of 〈σ_CO,scouse〉 ≈ 19 km s⁻¹and surface densities of$〈{\mathrm{\Sigma }}_{{\mathrm{H}}_2,\mathrm{scouse}}〉\approx 800M_{\odot }{\mathrm{pc}}^{-2}$, similar to the properties observed throughout the rest of the central molecular gas structure. Tailored hydrodynamical simulations exhibit many of the observed properties and imply that the observed structures are transient and highly time-variable. From our study of NGC 1365, we conclude that it is predominantly the high gas inflow triggered by the bar that is setting the star formation in its CMZ.

 

Abstract The Milky Way’s central molecular zone (CMZ) has emerged in recent years as a unique laboratory for the study of star formation. Here we use the simulations presented in Tress et al. 2020 to investigate star formation in the CMZ. These simulations resolve the structure of the interstellar medium at sub-parsec resolution while also including the large-scale flow in which the CMZ is embedded. Our main findings are as follows. (1) While most of the star formation happens in the CMZ ring at R ≳ 100 pc, a significant amount also occurs closer to SgrA* at R ≲ 10 pc. (2) Most of the star formation in the CMZ happens downstream of the apocentres, consistent with the “pearls-on-a-string” scenario, and in contrast to the notion that an absolute evolutionary timeline of star formation is triggered by pericentre passage. (3) Within the timescale of our simulations (∼100 Myr), the depletion time of the CMZ is constant within a factor of ∼2. This suggests that variations in the star formation rate are primarily driven by variations in the mass of the CMZ, caused for example by AGN feedback or externally-induced changes in the bar-driven inflow rate, and not by variations in the depletion time. (4) We study the trajectories of newly born stars in our simulations. We find several examples that have age and 3D velocity compatible with those of the Arches and Quintuplet clusters. Our simulations suggest that these prominent clusters originated near the collision sites where the bar-driven inflow accretes onto the CMZ, at symmetrical locations with respect to the Galactic centre, and that they have already decoupled from the gas in which they were born. 

We present a high-resolution view of bubbles within the Phantom Galaxy (NGC 628), a nearby (∼10 Mpc), star-forming (∼2M_⊙yr⁻¹), face-on (i∼ 9°) grand-design spiral galaxy. With new data obtained as part of the Physics at High Angular resolution in Nearby GalaxieS (PHANGS)-JWST treasury program, we perform a detailed case study of two regions of interest, one of which contains the largest and most prominent bubble in the galaxy (the Phantom Void, over 1 kpc in diameter), and the other being a smaller region that may be the precursor to such a large bubble (the Precursor Phantom Void). When comparing to matched-resolution Hαobservations from the Hubble Space Telescope, we see that the ionized gas is brightest in the shells of both bubbles, and is coincident with the youngest (∼1 Myr) and most massive (∼10⁵M_⊙) stellar associations. We also find an older generation (∼20 Myr) of stellar associations is present within the bubble of the Phantom Void. From our kinematic analysis of the HI, H₂(CO), and Hiigas across the Phantom Void, we infer a high expansion speed of around 15 to 50 km s⁻¹. The large size and high expansion speed of the Phantom Void suggest that the driving mechanism is sustained stellar feedback due to multiple mechanisms, where early feedback first cleared a bubble (as we observe now in the Precursor Phantom Void), and since then supernovae have been exploding within the cavity and have accelerated the shell. Finally, comparison to simulations shows a striking resemblance to our JWST observations, and suggests that such large-scale, stellar-feedback-driven bubbles should be common within other galaxies.

 

Wind flow and sediment transport across a northern California beach‐foredune system with two adjacent vegetation types are examined for the same incident wind conditions. The invasiveAmmophila arenariawas taller (c. 1 m) with denser coverage than the neighbouringElymus mollisalliance canopy (c. 0.65 m), which consisted of a variety of interspersed native plants. Wind flow was measured with rotating cup and sonic anemometry, while sediment transport was measured using laser particle counters. Wind speed profiles over the two canopies were significantly different because of differing vegetation height, coverage density, and stem stiffness. In both cases, there was a lower zone of semi‐stagnant air (below about 0.3 m) that transitioned upward to a shear zone comprising the upper part of the canopy and immediately above. The shear zone above theElymuscanopy was relatively thin (confined to 0.3–0.5 m above‐ground) whereas the shear zone in theAmmophilacanopy was thicker extending from a height of about 0.5h(his average plant height) to about 1.5h. Vertical profiles of Reynolds shear stress (RSS) and turbulence kinetic energy (TKE) are consistent with the shear layer structure over these two contrasting vegetation canopies. The degree of topographically‐forced and vegetation‐enhanced flow steering was significant, withAmmophilastrongly shifting the highly oblique (55°) incident wind to essentially shore‐perpendicular trajectories. In comparison, the shore‐perpendicular steering effect was not as pronounced for theElymuscanopy. Sediment transport intensity on the beach was continuous, but decreased progressively to the dune toe, and then dropped to essentially zero once the vegetation canopy was encountered (on the stoss slope). Overall, the study illustrates the significant differences in wind flow and turbulence conditions that may occur in contrasting plant canopies on foredunes, suggesting that greater attention needs to be placed on vegetation roughness characteristics in models of foredune morphodynamics and sediment transport potential.