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| ≤ 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 disk with a cusped orbit shock transition to an x 2 orbit disk of radius R ∼ 150 pc.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 − 25 • , but the uncertainty is large since no detailed quantitative fit to all features in the observed (l, v) 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 disk model with constant mass-to-light ratio for the NIR bulge and disk.
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) plot.
We analyze formation of grand-design two-arm spiral structure in the nuclear regions of disk galaxies. Such morphology has been recently detected in a number of objects using high-resolution near-infrared observations. Motivated by the observed (1) continuity between the nuclear and kpc-scale spiral structures, and by (2) low arm-interarm contrast, we apply the density wave theory to explain the basic properties of the spiral nuclear morphology. In particular, we address the mechanism for the formation, maintenance and the detailed shape of nuclear spirals. We find, that the latter depends mostly on the shape of the underlying gravitational potential and the sound speed in the gas. Detection of nuclear spiral arms provides diagnostics of mass distribution within the central kpc of disk galaxies. Our results are supported by 2D numerical simulations of gas response to the background gravitational potential of a barred stellar disk. We investigate the parameter space allowed for the formation of nuclear spirals using a new method for constructing a gravitational potential in a barred galaxy, where positions of resonances are prescribed.
We present a new model of the three-dimensional distribution of molecular gas in the Milky Way Galaxy, based on CO line data.Our analysis is based on a gas-flow simulation of the inner Galaxy using smoothed-particle hydrodynamics (SPH) using a realistic barred gravitional potential derived from the observed COBE/DIRBE near-IR light distribution. The gas model prescribes the gas orbits much better than a simple circular rotation model and is highly constrained by observations, but it cannot predict local details. In this study, we provide a 3D map of the observed molecular gas distribution using the velocity field from the SPH model. A comparison with studies of the Galactic Center region suggests that the main structures are reproduced but somewhat stretched along the line-of-sight, probably on account of limited resolution of the underlying SPH simulation. The gas model will be publicly available and may prove useful in a number of applications, among them the analysis of diffuse gamma-ray emission as measured with GLAST.
Abstract. We present new high-resolution observations of the nucleus of the counter-rotating LINER NGC 4826, made in the J = 1-0 and J = 2-1 lines of 12 CO with the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve 0.8 (16 pc) resolution in the 2-1 line, fully resolve an inner molecular gas disk which is truncated at an outer radius of 700 pc. The total molecular gas mass (3.1 × 10 8 M ) is distributed in a lopsided nuclear disk of 40 pc radius, containing 15% of the total gas mass, and two one-arm spirals, which develop at different radii in the disk. The distribution and kinematics of molecular gas in the inner 1 kpc of NGC 4826 show the prevalence of different types of m = 1 perturbations in the gas. Although dominated by rotation, the gas kinematics are perturbed by streaming motions related to the m = 1 instabilities. The non-circular motions associated with the inner m = 1 perturbations (lopsided instability and inner one-arm spiral) agree qualitatively with the pattern expected for a trailing wave developed outside corotation ("fast" wave). In contrast, the streaming motions in the outer m = 1 spiral are better explained by a "slow" wave. A paradoxical consequence is that the inner m = 1 perturbations would not favour AGN feeding. An independent confirmation that the AGN is not being generously fueled at present is found in the low values of the gravitational torques exerted by the stellar potential for R < 530 pc. The distribution of star formation in the disk of NGC 4826 is also strongly asymmetrical. The observed asymmetries, revealed by HST images of the inner disk, follow the scales of the various m = 1 perturbations identified in the molecular gas disk. Massive star formation is still vigorous, fed by the significant molecular gas reservoir at R < 700 pc. There is supporting evidence for a recent large mass inflow episode in NGC 4826. The onset of m = 1 instabilities of the type observed in NGC 4826 may be a consequence of secular evolution of disks with high gas mass contents. These observations have been made in the context of the NUclei of GAlaxies (NUGA) project, aimed at the study of the different mechanisms for gas fueling of Active Galactic Nuclei (AGN).
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