Two modes of gas flow in a single barred galaxy

We investigate stationary gas flows in a fixed, rotating barred potential. The gas is assumed to be isothermal with an effective sound speed c_s, and the equations of motion are solved with smoothed particle hydrodynamics (SPH). Since the thermal energy in cloud random motions is negligible compared to the orbital kinetic energy, no dependence of the flow on c_s is expected. However, this is not the case when shocks are involved. For low values of c_s an open, off-axis shock flow forms that is characteristic for potentials with an inner Lindblad resonance (ILR). Through this shock the gas streams inwards from x_1 to x_2-orbits. At high sound speeds the gas arranges itself in a different, on-axis shock flow pattern. In this case, there is no gas on x_2-orbits, demonstrating that the gas can behave as if there were no ILR. The critical effective sound speed dividing the two regimes is in the range of values observed in the Milky Way. We give a heuristic explanation for this effect. A possible consequence is that star formation may change the structure of the flow by which it was initiated. Low-mass galaxies should predominantly be in the on-axis regime. A brief comparison of our SPH results with those from a grid-based hydrodynamic code is also given.Comment: 12 pages, Latex. 9 figures (gif). Submitted to MNRAS. Also available with figures as ps-file at http://www.astro.unibas.ch/dynamics/papers.htm

Gas Dynamics and Large-Scale Morphology of the Milky Way Galaxy

We present a new model for the gas dynamics in the galactic disk inside the Sun's orbit. Quasi-equilibrium flow solutions are determined in the gravitational potential of the deprojected COBE NIR bar and disk, complemented by a central cusp and, in some models, an outer halo. These models generically lead to four-armed spiral structure between corotation of the bar and the solar circle; their large-scale morphology is not sensitive to the precise value of the bar's pattern speed, to the orientation of the bar with respect to the observer, and to whether or not the spiral arms carry mass. Our best model provides a coherent interpretation of many observed gas dynamical features. Its four-armed spiral structure outside corotation reproduces quantitatively the directions to the five main spiral arm tangents at |l|<=60deg observed in a variety of tracers. The 3-kpc-arm is identified with one of the model arms emanating from the ends of the bar, extending into the corotation region. The model features an inner gas disk with a cusped orbit shock transition to an x_2 orbit disk of radius R~150pc. The bar's corotation radius is fairly well--constrained at R_c=3.5 +/- 0.5 kpc. The best value for the orientation angle of the bar is probably 20-25deg, but the uncertainty is large since no detailed quantitative fit to all features in the observed lv-diagrams is yet possible. The Galactic terminal velocity curve from HI and CO observations out to l=+/-45deg (=5 kpc) is approximately described by a maximal disk model with constant mass-to-light ratio for the NIR bulge and disk.Comment: 24 pages, 16 figures, Latex, to appear in MNRAS. Available with full resolution figures at http://www.astro.unibas.ch/galaxies/papers.htm

Gas Dynamics in the Milky Way: Second Pattern Speed and Large-Scale Morphology

We present new gas flow models for the Milky Way inside the solar circle. To this end we use SPH simulations in gravitational potentials determined from the NIR luminosity distribution (including spiral arms) which are based on the COBE/DIRBE maps. Gas flows in models which include massive spiral arms clearly match the observed 12CO lvplot better than if the potential does not include spiral structure. Besides single pattern speed models we investigate models with separate pattern speeds for the bar and spiral arms. The most important difference is that in the latter case the gas spiral arms go through the bar corotation region, keeping the gas aligned with the arms there. In the (l,v) plot this results in characteristic regions which appear to be nearly void of gas.In single pattern speed models these regions are filled with gas because the spiral arms dissolve in the bar corotation region. Comparing with the 12CO data we find evidence for separate pattern speeds in the Milky Way.From a series of models the preferred range for the bar pattern speed is Om_p=60\pm5 /Gyr, corresponding to corotation at 3.4\pm0.3kpc. The spiral pattern speed is less well constrained, but our preferred value is Om_sp\approx 20 /Gyr. A further series of gas models is computed for different bar angles, using separately determined luminosity models and gravitational potentials in each case. We find acceptable gas models for 20<=\phibar<=25. The model with (\phibar=20, Om_p=60 /Gyr, Om_sp=20 /Gyr) gives an excellent fit to the spiral arm ridges in the observed (l,v) plot.Comment: Paper accepted for publication in MNRAS. The paper contains many figures. These are not included in the version available here to save download time. A full version can be downloaded from http://latour.stochastik.math.uni-goettingen.de/~downloads/sphpaper.ps.g

Ranunculus sect. Batrachium (Ranunculaceae): Contribution to an excursion flora of Austria and the Eastern Alps

A treatment of Ranunculus sect. Batrachium is presented for the Eastern Alpine territory as covered by the forthcoming 4th ed. of the “Excursion flora of Austria and the entire Eastern Alps”. A newly designed key as well as alternative keys for terrestrial modificants and vegetative specimens are presented. A new outline for the section is applied including (1) the Ranunculus fluitans group (R. fluitans and R. pseudofluitans); (2) the R. trichophyllus group (R. trichophyllus, R. confervoides, and R. rionii); (3) the R. aquatilis group, including the R. aquatilis subgroup with R. aquatilis (the only member present in Europe), the R. peltatus subgroup (R. peltatus and R. baudotii) as well as related hybridogenous taxa (R. penicillatus s. str.) and (4) R. circinatus. The distribution of all species is given for the Austrian federal states, the Bavarian Alps, Liechtenstein and the South Alpine regions of Switzerland, Italy and Slovenia

Gas dynamics and large-scale morphology of the Milky Way galaxy

We present a new model for the gas dynamics in the galactic disc inside the orbit of the Sun. Quasi-equilibrium flow solutions are determined in the gravitational potential of the deprojected COBE near-infrared bar and disc, complemented by a central cusp and, in some models, an outer halo. These models generically lead to four-armed spiral structure between corotation of the bar and the solar circle; their large-scale morphology is not sensitive to the precise value of the pattern speed of the bar, to the orientation of the bar with respect to the observer, or to whether or not the spiral arms carry mass. Our best model provides a coherent interpretation of many observed gas dynamical features. Its four-armed spiral structure outside corotation reproduces quantitatively the directions to the five main spiral arm tangents at |l|≤60 observed in a variety of tracers. The 3-kpc arm is identified with one of the model arms emanating from the ends of the bar, extending into the corotation region. The model features an inner gas disc with a cusped orbit shock transition to an x2 orbit disc of radius R∼150 pc. The corotation radius of the bar is fairly well constrained at Rc≃3.5±0.5 kpc. The best value for the orientation angle of the bar is probably 20-25, but the uncertainty is large since no detailed quantitative fit to all features in the observed (l,υ) diagrams is yet possible. The Galactic terminal velocity curve from Hi and CO observations out to l≃±45 (∼5 kpc) is approximately described by a maximal disc model with constant mass-to-light ratio for the near-infrared bulge and dis

Milky Way Gas Dynamics

The Milky Way is made up of a central bar, a disk with embedded spiral arms, and a dark matter halo. Observational and theoretical constraints for the characteristic parameters of these components will be presented, with emphasis on the constraints from the dynamics of the Milky Way gas. In particular, the fraction of dark matter inside the solar radius, the location of the main resonances, and the evidence for multiple pattern speeds will be discusse

Gas dynamics in the Milky Way: second pattern speed and large-scale morphology

We present new gas flow models for the Milky Way inside the solar circle.We use smoothed particles hydrodynamics (SPH) simulations in gravitational potentials determined from the near-infrared (NIR) luminosity distribution of the bulge and disc, assuming a constant NIR mass-to-light ratio, with an outer halo added in some cases.The luminosity models are based on the COBE/DIRBE maps and on clump giant star counts in several bulge fields and include a spiral arm model for the disc. Gas flows in models that include massive spiral arms clearly match the observed 12CO (l, v) diagram better than if the potential does not include spiral structure.Furthermore, models in which the luminous mass distribution and the gravitational potential of the Milky Way have four spiral arms are better fits to the observed (l, v) diagram than two-armed models. Besides single-pattern speed models we investigate models with separate pattern speeds for the bar and spiral arms.The most important difference is that in the latter case the gas spiral arms go through the bar corotation region, keeping the gas aligned with the arms there.In the (l, v) plot this results in characteristic regions that appear to be nearly devoid of gas.In single-pattern speed models these regions are filled with gas because the spiral arms dissolve in the bar corotation region. Comparing with the 12CO data we find evidence for separate pattern speeds in the Milky Way.From a series of models the preferred range for the bar pattern speed is Ωp= 60 ± 5 Gyr−1, corresponding to corotation at 3.4 ± 0.3 kpc.The spiral pattern speed is less well constrained, but our preferred value is Ωsp≈ 20 Gyr−1.A further series of gas models is computed for different bar angles, using separately determined luminosity models and gravitational potentials in each case.We find acceptable gas models for 20°≲ϕbar≲ 25°.The model with (ϕbar= 20°, Ωp= 60 Gyr−1, Ωsp= 20 Gyr−1) gives an excellent fit to the spiral arm ridges in the observed (l, v) plo