共查询到20条相似文献,搜索用时 31 毫秒
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Joshua E. Barnes 《Monthly notices of the Royal Astronomical Society》2002,333(3):481-494
Observations indicate that much of the interstellar gas in merging galaxies may settle into extended gaseous discs. Here, I present simulations of disc formation in mergers of gas-rich galaxies. Up to half of the total gas settles into embedded discs; the most massive instances result from encounters in which both galaxies are inclined to the orbital plane. These discs are often warped, many have rather complex kinematics, and roughly a quarter have counter-rotating or otherwise decoupled central components. Discs typically grow from the inside out; infall from tidal tails may continue disc formation over long periods of time. 相似文献
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P. A. Patsis D. E. Kaufmann S. T. Gottesman V. Boonyasait 《Monthly notices of the Royal Astronomical Society》2009,394(1):142-156
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'. 相似文献
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Scott Tremaine 《Monthly notices of the Royal Astronomical Society》1999,307(4):877-883
Partially phase-mixed structures in galaxies occupy a complex surface of dimension D in six-dimensional phase space. The appearance of such structures to observers is determined by their projection into a space the dimensionality K of which is determined by the number of observables (e.g. sky position, distance, radial velocity, etc.). We discuss the expected dimensionality of phase-space structures and suggest that the most prominent features in surveys with K D will be stable singularities (catastrophes). The simplest of these are the shells seen in the outer parts of elliptical galaxies. 相似文献
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James Binney Carlo Nipoti Filippo Fraternali 《Monthly notices of the Royal Astronomical Society》2009,397(4):1804-1815
We examine the proposal that the H i 'high-velocity' clouds (HVCs) surrounding the Milky Way and other disc galaxies form by condensation of the hot galactic corona via thermal instability. Under the assumption that the galactic corona is well represented by a non-rotating, stratified atmosphere, we find that for this formation mechanism to work the corona must have an almost perfectly flat entropy profile. In all other cases, the growth of thermal perturbations is suppressed by a combination of buoyancy and thermal conduction. Even if the entropy profile were nearly flat, cold clouds with sizes smaller than 10 kpc could form in the corona of the Milky Way only at radii larger than 100 kpc , in contradiction with the determined distances of the largest HVC complexes. Clouds with sizes of a few kpc can form in the inner halo only in low-mass systems. We conclude that unless even slow rotation qualitatively changes the dynamics of a corona, thermal instability is unlikely to be a viable mechanism for formation of cold clouds around disc galaxies. 相似文献
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We present a method for recovering the distribution functions of edge-on thin axisymmetric discs directly from their observable kinematic properties. The most generally observable properties of such a stellar system are the line-of-sight velocity distributions of the stars at different projected radii along the galaxy. If the gravitational potential is known, then the general two-integral distribution function can be reconstructed using the shapes of the high-velocity tails of these line-of-sight distributions. If the wrong gravitational potential is adopted, then a distribution function can still be constructed using this technique, but the low-velocity parts of the observed velocity distributions will not be reproduced by the derived dynamical model. Thus, the gravitational potential is also tightly constrained by the observed kinematics. 相似文献
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Stephen J. Weatherley Stephen J. Warren 《Monthly notices of the Royal Astronomical Society》2003,345(3):L29-L34
We reassess the hypothesis that Lyman-break galaxies (LBGs) at redshifts z ∼ 3 mark the centres of the most massive dark matter haloes at that epoch. First we reanalyse the kinematic measurements of Pettini et al. and Erb et al. of the rest-frame optical emission lines of LBGs. We compare the distribution of the ratio of the rotation velocity to the central line width, against the expected distribution for galaxies with random inclination angles, modelled as singular isothermal spheres. The model fits the data well. On this basis we argue that the central line width provides a predictor of the circular velocity at a radius of several kpc. Assembling a larger sample of LBGs with measured line widths, we compare these results against the theoretical ΛCDM rotation curves of Mo, Mao & White, under the hypothesis that LBGs mark the centres of the most massive dark matter haloes. We find that the circular velocities are overpredicted by a substantial factor, which we estimate conservatively as 1.8 ± 0.4 . This indicates that the model is probably incorrect. The model of LBGs as relatively low-mass starburst systems, of Somerville, Primack & Faber, provides a good fit to the data. 相似文献
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Ch. Skokos P. A. Patsis E. Athanassoula 《Monthly notices of the Royal Astronomical Society》2002,333(4):861-870
We investigate the orbital structure in a class of three-dimensional (3D) models of barred galaxies. We consider different values of the pattern speed, of the strength of the bar and of the parameters of the central bulge of the galactic model. The morphology of the stable orbits in the bar region is associated with the degree of folding of the x1 characteristic. This folding is larger for lower values of the pattern speed. The elongation of rectangular-like orbits belonging to x1 and to x1-originated families depends mainly on the pattern speed. A detailed investigation of the trees of bifurcating families in the various models shows that major building blocks of 3D bars can be supplied by families initially introduced as unstable in the system, but becoming stable at another energy interval. In some models without radial and vertical 2:1 resonances we find, except for the x1 and x1-originated families, also families related to the z -axis orbits, which support the bar. Bifurcations of the x2 family can build a secondary 3D bar along the minor axis of the main bar. This is favoured in the slowly rotating bar case. 相似文献
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Henry E. Kandrup 《Monthly notices of the Royal Astronomical Society》2001,323(3):681-687
This paper explores the phenomenon of energy relaxation for stars in a galaxy embedded in a high-density environment that is subjected continually to perturbations reflecting the presence of other nearby galaxies and/or incoherent internal pulsations. The analysis is similar to earlier analyses of energy relaxation induced by binary encounters between nearby stars and between stars and giant molecular clouds in that the perturbations are idealized as a sum of near-random events which can be modelled as diffusion and dynamical friction. However, the analysis differs in one important respect: because the time-scale associated with these perturbations need not be short compared with the characteristic dynamical time t D for stars in the original galaxy, the diffusion process cannot be modelled as resulting from a sequence of instantaneous kicks, i.e. white noise. Instead, the diffusion is modelled as resulting from random kicks of finite duration, i.e. coloured noise, characterized by a non-zero autocorrelation time t c . A detailed analysis of coloured noise generated by sampling an Ornstein–Uhlenbeck process leads to a simple scaling in terms of t c and an effective diffusion constant D . Interpreting D and t c following early work by Chandrasekhar (the 'nearest neighbour approximation') implies that, for realistic choices of parameter values, energy relaxation associated with an external environment and/or internal pulsations could be important on times short compared with the age of the Universe. 相似文献
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We study the self-consistent, linear response of a galactic disc to vertical perturbations, as induced, say, by a tidal interaction. We calculate the self-gravitational potential corresponding to a non-axisymmetric, self-consistent density response of the disc using the Green's function method. The response potential is shown to oppose the perturbation potential because the self-gravity of the disc resists the imposed potential, and this resistance is stronger in the inner parts of a galactic disc. For the m = 1 azimuthal wavenumber, the disc response opposes the imposed perturbation up to a radius that spans a range of 4–6 disc scalelengths, so that the disc shows a net warp only beyond this region. This physically explains the well known but so far unexplained observation that warps typically set in beyond this range of radii. We show that the inclusion of a dark matter halo in the calculation only marginally changes (by ∼10 per cent) the radius for the onset of warps. For perturbations with higher azimuthal wavenumbers, the net signature of the vertical perturbations can only be seen at larger radii – for example, beyond 7 exponential disc scalelengths for m = 10 . Also, for the high- m cases, the magnitude of the negative disc response due to the disc self-gravity is much smaller. This is shown to result in corrugations of the mid-plane density, which explains the puzzling scalloping with m = 10 detected in H i in the outermost regions ∼30 kpc in the Galaxy. 相似文献