The peculiar shape of the inner galactic rotation curve
01 Nov 1986-Monthly Notices of the Royal Astronomical Society (Oxford University Press)-Vol. 223, Iss: 2, pp 377-389
TL;DR: In this article, the authors present a morphologie of the bulbe de la galaxie, deduite d'observations HI and CO, with a size r=500 kpc.
Abstract: La courbe de rotation apparente de la galaxie, deduite d'observations HI et CO, presente un pic etroit (ν max ≃250-260 kms −1 ) a r=500 pc, puis un lent declin pour 600 pc≤r≤1,5 kpc. On montre que cette morphologie ne peut etre reconciliee avec les observations que si le bulbe de la galaxie n'est pas a symetrie axiale et le potentiel resultant triaxial
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TL;DR: In this article, a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf is presented, which is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (3 Gyr) tidal debris streams.
Abstract: We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential that can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)Φ = 0.72 and intermediate/major axis ratio (b/a)Φ = 0.99 at radii 20 kpc < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7°, 0°), (0°, 90°), and (97°, 0°) respectively, corresponding to a nearly oblate ellipsoid whose minor axis is contained within the Galactic disk plane. This particular disk/halo orientation is difficult to reconcile within the general context of galactic dynamics (and cold dark matter models in particular), suggesting either that the orientation may have evolved significantly with time or that inclusion of other non-axisymmetric components (such as the gravitational influence of the Magellanic Clouds) in the model may obviate the need for triaxiality in the dark matter halo. The apparent proper motion of Sgr in this model is estimated to be (μ l cos b, μ b ) = (–2.16, 1.73) mas yr–1, corresponding to a Galactocentric space velocity (U, V, W) = (230, –35, 195) km s–1 . Based on the velocity dispersion in the stellar tidal streams, we estimate that Sgr has a current bound mass M Sgr = 2.5+1.3 –1.0 × 108 M ☉. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies. These models predict a strong evolution in the MDF of the model Sgr dwarf with time, indicating that the chemical abundances of stars in Sgr at the present day may be significantly different than the abundances of those already contributed to the Galactic stellar halo. We conclude by using the new N-body model to re-evaluate previous claims of the association of miscellaneous halo substructure with the Sgr dwarf.
488 citations
TL;DR: In this article, the authors review the main problems presented by these complicated dynamical systems and summarize the effort so far made towards their solution, emphasizing results which appear secure, and conclude that the observed motions are consistent with material within the bar streaming along highly elongated orbits aligned with the rotating major axis.
Abstract: Some 30% of disc galaxies have a pronounced central bar features in the disc plane and many more have weaker features of a similar kind. Kinematic data indicate that the bar constitutes a major non-axisymmetric component of the mass distribution and that the bar pattern tumbles rapidly about the axis normal to the disc plane. The observed motions are consistent with material within the bar streaming along highly elongated orbits aligned with the rotating major axis. A barred galaxy may also contain a spheroidal bulge at its centre, spirals in the outer disc and, less commonly, other features such as a ring or lens. Mild asymmetries in both the light and kinematics are quite common. The authors review the main problems presented by these complicated dynamical systems and summarize the effort so far made towards their solution, emphasizing results which appear secure.
481 citations
Cites background from "The peculiar shape of the inner gal..."
...It has frequently been suggested that such features indicate a tri-axial, rather than an axially symmetric, bulge (e.g. de Vaucouleurs 1974, Kormendy 1979, 1982, Gerhard and Vietri 1986, and many others)....
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01 Feb 2000
TL;DR: In this article, detailed simulations of the velocity distribution, f(v), in the outer parts of an exponential stellar disk with nearly flat rotation curve and a rotating central bar have been performed.
Abstract: Hydrodynamic modeling of the inner Galaxy suggests that the radius of the outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the Sun. How does this resonance affect the distribution function in the outer parts of a barred disk, and can we identify any effect of the resonance in the velocity distribution actually observed in the solar neighborhood? To answer these questions, detailed simulations of the velocity distribution, f(v), in the outer parts of an exponential stellar disk with nearly flat rotation curve and a rotating central bar have been performed. For a model resembling the old stellar disk, the OLR causes a distinct feature in f(v) over a significant fraction of the outer disk. For positions up to 2 kpc outside the OLR radius and at bar angles of ~10°–70°, this feature takes the form of a bimodality between the dominant mode of low-velocity stars centered on the local standard of rest (LSR) and a secondary mode of stars predominantly moving outward and rotating more slowly than the LSR. Such a bimodality is indeed present in f(v) inferred from the Hipparcos data for late-type stars in the solar neighborhood. If one interprets this observed bimodality as induced by the OLR—and there are hardly any viable alternatives—then one is forced to deduce that the OLR radius is slightly smaller than R0. Moreover, by a quantitative comparison of the observed with the simulated distributions, one finds that the pattern speed of the bar is 1.85 ± 0.15 times the local circular frequency, where the error is dominated by the uncertainty in bar angle and local circular speed. Also, other, less prominent but still significant, features in the observed f(v) resemble properties of the simulated velocity distributions, in particular a ripple caused by orbits trapped in the outer 1:1 resonance.
462 citations
TL;DR: In this article, a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (< 3 Gyr) tidal debris streams is presented.
Abstract: We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (< 3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential which can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)_Phi = 0.72 and intermediate/major axis ratio (b/a)_Phi = 0.99 at radii 20 < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7, 0), (0, 90), and (97, 0) respectively, corresponding to a nearly-oblate ellipsoid whose minor axis is contained within the Galactic disk plane. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies. (Abridged).
376 citations
Cites background from "The peculiar shape of the inner gal..."
...If a disk were to form in an elliptical halo potential such as that proposed in §4.3, simulations suggest that the disk too should become elongated and non-axisymmetric (see, e.g., Gerhard & Vietri 1986; Debattista et al. 2008)....
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TL;DR: A model of the bar and old stellar disk of the Galaxy has been derived from the survey of the Diffuse Infrared Background Experiment (DIRBE) of the Cosmic Background Explorer at wavelengths of 1.25, 2.2, 3.5, and 4.9 μm as mentioned in this paper.
Abstract: A model of the bar and old stellar disk of the Galaxy has been derived from the survey of the Diffuse Infrared Background Experiment (DIRBE) of the Cosmic Background Explorer at wavelengths of 1.25, 2.2, 3.5, and 4.9 μm. It agrees very well with the data, except in directions in which the near-infrared optical depth is high. Among the conclusions drawn from the model is that the Sun is located approximately 16.5 pc above the midpoint of the Galactic plane. The disk has an outer edge 4 kpc from the Sun and is warped like the H I layer. It has a central hole roughly the diameter of the inner edge of the 3 kpc molecular cloud ring, and within that hole lies a bright, strong, early-type bar, tilted approximately 14° from the Sun-Galactic center line. The model has 47 free parameters. The model is discussed in detail, and contour plots and images of the residuals are presented.
325 citations