Gaseous Flows in Galaxies (original) (raw)

Gas accretion on spiral galaxies: Bar formation and renewal

Astronomy & Astrophysics, 2002

The effects of gas accretion on spiral disk dynamics and stability are studied through N-body simulations, including star formation and gas/stars mass exchange. The detailed processes of bar formation, bar destruction and bar reformation are followed, while in the same time the disk to bulge ratio is varying. The accreted gas might be first prevented to flow inwards to the center by the bar gravity torques, which maintains it to the outer Lindblad resonance. While the first bar is weakening, the accreted gas replenishes the disk, increasing the disk-to-bulge ratio, and the disk self-gravity. A second bar is then unstable, with a higher pattern speed, due both to the increased mass, and shorter bar length. Three or four bar episodes have been followed over a Hubble time. Their strength is decreasing with time, while their pattern speed is increasing. Detailed balance of the angular momentum transfer and evolution can account for these processes. The gas recycled through star formation, and rejected through stellar mass loss plays also a role in the disk dynamics. Implications on the spiral galaxy dynamics and evolution along the Hubble sequence, and as a function of redshift are discussed.

Dynamical effect of gas on spiral pattern speed in galaxies

In the density wave theory of spiral structure, the grand-design two-armed spiral pattern is taken to rotate rigidly in a galactic disc with a constant, definite pattern speed. The observational measurement of the pattern speed of the spiral arms, though difficult, has been achieved in a few galaxies such as NGC 6946, NGC 2997, and M 51 which we consider here. We examine whether the theoretical dispersion relation permits a real solution for wavenumber corresponding to a stable wave, for the observed rotation curve and the pattern speed values. We find that the disc when treated to consist of stars alone, as is usually done in literature, does not generally support a stable density wave for the observed pattern speed. Instead the inclusion of the low velocity dispersion component, namely, gas, is essential to obtain a stable density wave. Further, we obtain a theoretical range of allowed pattern speeds that correspond to a stable density wave at a certain radius, and check that for the three galaxies considered, the observed pattern speeds fall in the respective prescribed range. The inclusion of even a small amount (∼ 15%) of gas by mass fraction in the galactic disc is shown to have a significant dynamical effect on the dispersion relation and hence on the pattern speed that is likely to be seen in a real, gas-rich spiral galaxy.

Gas Dynamics in the Galaxy: Total Mass Distribution and the Bar Pattern Speed

The Astrophysical Journal, 2022

Gas morphology and kinematics in the Milky Way contain key information for understanding the formation and evolution of our Galaxy. We present hydrodynamical simulations based on realistic barred Milky Way potentials constrained by recent observations. Our model can reproduce most features in the observed longitude–velocity diagram, including the Central Molecular Zone, the Near and Far 3 kpc arms, the Molecular Ring, and the spiral arm tangents. It can also explain the noncircular motions of masers from the recent BeSSeL2 survey. The central gas kinematics are consistent with a mass of 6.9 × 108 M ⊙ in the Nuclear Stellar Disk. Our model predicts the formation of an elliptical gaseous ring surrounding the bar, which is composed of the 3 kpc arms, the Norma arm, and the bar-spiral interfaces. This ring is similar to those “inner” rings in some Milky Way analogs with a boxy/peanut-shaped bulge (e.g., NGC 4565 and NGC 5746). The kinematics of gas near the solar neighborhood are govern...

Stellar and gas dynamics of late-type barred-spiral galaxies: NGC 3359, a test case

Monthly Notices of The Royal Astronomical Society, 2009

We study the dynamics of a model for the late-type barred-spiral galaxy NGC 3359 by using both observational and numerical techniques. The results of our modelling are compared with photometric and kinematical data. The potential used is estimated directly from observations of the galaxy. It describes with a single potential function, a barred-spiral system with an extended spiral structure. Thus, the study of the dynamics in this potential has an interest by itself. We apply orbital theory and response models for the study of the stellar component, and smoothed particle hydrodynamics for modelling the gas. In particular, we examine the pattern speed of the system and the orbital character (chaotic or ordered) of the spiral arms. We conclude that the spiral pattern rotates slowly, in the sense that its corotation is close to or even beyond the end of the arms. Although a single, slow pattern speed could, under certain assumptions, characterize the whole disc, the comparison with the observational data indicates that probably the bar and the spirals have different angular velocities. In our two pattern speeds model, the best fit is obtained with a bar ending close to its 4:1 resonance and a more slowly rotating spiral. Assuming an 11 Mpc distance to the galaxy, a match of our models with the observed data indicates a pattern speed of about 39 km s−1 kpc−1 for the bar and about 15 km s−1 kpc−1 for the spiral. We do not find any indication for a chaotic character of the arms in this barred-spiral system. The flow in the region of the spirals can best be described as a regular ‘precessing-ellipses flow’.

On the Galactic Spiral Patterns: Stellar and Gaseous

Journal of The Korean Astronomical Society, 2004

The gas response to a proposed spiral stellar pattern for our Galaxy is presented here as calculated via 2D hydrodynamic calculations utilizing the ZEUS code in the disk plane. The locus is that found by Drimmel (2000) from emission profiles in the K band and at 240 µm. The self-consistency of the stellar spiral pattern was studied in previous work (see Martos et al. 2004). It is a sensitive function of the pattern rotation speed, Ω p , among other parameters which include the mass in the spiral and its pitch angle. Here we further discuss the complex gaseous response found there for plausible values of Ω p in our Galaxy, and argue that its value must be close to 20 km s −1 kpc −1 from the strong selfconsistency criterion and other recent, independent studies which depend on such parameter. However, other values of Ω p that have been used in the literature are explored to study the gas response to the stellar (K band) 2-armed pattern. For our best fit values, the gaseous response to the 2-armed pattern displayed in the K band is a four-armed pattern with complex features in the interarm regions. This response resembles the optical arms observed in the Milky Way and other galaxies with the smooth underlying two-armed pattern of the old stellar disk populations in our interpretation. The complex gaseous response appears to be related to resonances in stellar orbits. Among them, the 4:1 resonance is paramount for the axisymmetric Galactic model employed, and the set of parameters explored. In the regime seemingly proper to our Galaxy, the spiral forcing appears to be marginally strong in the sense that the 4:1 resonance terminates the stellar pattern, despite its relatively low amplitude. In current work underway, the response for low values of Ω p tends to remove most of the rich structure found for the optimal self-consistent model and the gaseous pattern is ring-like. For higher values than the optimal, more features and a multi-arm structure appears.

Spiral structure and the dynamics of galaxies

Physics Reports, 1976

Conten ts: 1. General characteristics of spiral galaxies 317 5. Stabilization of density waves by the gas 364 1.1. A brief review of two centuries of observations 317 5.1. Introduction 364 1.2. Theories of spiral structure 321 5.2. Stabilization mechanism 1.3. Outline of the present study 324 5.3. Discussion 2. Mathematical tools 326 6. Quasi-linear theory 3. Dynamical properties of flat stellar systems 328 6.1. Introduction 3.1. Introduction 328 6.2. Derivation of the diffusion equation 3.2. Stellar orbits 328 6.3. Diffusion coefficients 3.3. Distribution functions 341 6.4. The persistence of spiral structure 4. Stability of slightly perturbed disks 346 7. Conclusions and summary 4.1. Introduction 346 Acknowledgements 4.2. Mathematical formulation 347 Appendix 4.3. Instabilities 354 References 4.3.1. The rate of change of angular momentum 354 4.3.2. Growing waves 356 4.3.3. Damped waves 358 4.3.4. Physical significance of the growth rate y 359 4.3.5. Astronomical implications 361

Gas accretion in disk galaxies

2013

Gas accretion is necessary to maintain star formation, spiral and bar structure, and secular evolution in galaxies. This can occur through tidal interaction, or mass accretion from cosmic filaments. Different processes will be reviewed to drive gas towards galaxy centers and trigger starbursts and AGN. The efficiency of these dynamical processes can be estimated through simulations and checked by observations at different redshift, across the Hubble time. Large progress has been made on galaxies at moderate and high redshifts, allowing to interpret the star formation history and star formation efficiency as a function of gas content, dynamical state and galaxy evolution.

Gravitational torques in spiral galaxies: Gas accretion as a driving mechanism of galactic evolution

Astronomy & Astrophysics, 2002

The distribution of gravitational torques and bar strengths in the local Universe is derived from a detailed study of 163 galaxies observed in the near-infrared. The results are compared with numerical models for spiral galaxy evolution. It is found that the observed distribution of torques can be accounted for only with external accretion of gas onto spiral disks. Accretion is responsible for bar renewal-after the dissolution of primordial bars-as well as the maintenance of spiral structures. Models of isolated, non-accreting galaxies are ruled out. Moderate accretion rates do not explain the observational results: it is shown that galactic disks should double their mass in less than the Hubble time. The best fit is obtained if spiral galaxies are open systems, still forming today by continuous gas accretion, doubling their mass every 10 billion years.

Structure and evolution of star-forming gas in late-type spiral galaxies

Proceedings of the International Astronomical Union, 2006

We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628. We investigate the role of the individual H ii regions together with the large-scale gravitational mechanisms which govern star formation and overall evolution in spiral galaxies. Our kinematical analysis (reinforced by literature maps in HI and CO at lower angular resolution) enables us to verify the presence of an inner rapidly rotating inner disk-like component which we attribute to long term secular evolution of the large-scale spiral arms and oval structure. We find that gas is falling in from the outer parts towards the bluer central regions. This could be an early phase in the formation of a pseudo-bulge. We find signatures of radial motions caused by an m = 2 perturbation, which are likely to be responsible for the inflow of material forming the circumnuclear ring and the rapidly rotating inner structure.

Kinematics of Gas and Stars in Gas-Rich Early-Type Galaxies

The Evolution of Galaxies, 2001

We present gaseous and stellar kinematics for 14 gas-rich early-type galaxies. Half of the galaxies show irregular gaseous velocity profiles, with gas/star counter-rotation in 5 galaxies. We also find 5 counter-rotating stellar cores, and 5 more galaxies display stellar nuclei kinematically decoupled from the main stellar body. Our results indicate that the ionized gas is of external origin, and may have been acquired recently; the merging or accretion events that brought the gas into the galaxy have likely affected its stellar kinematics.