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1.
We study the persistence of warps in galactic discs in the presence of massive haloes. A disc is approximated by a set of massive rings, while a halo is represented by a conventional n -body simulation. We confirm the conclusion of Nelson &38; Tremaine that a halo responds strongly to an embedded precessing disc. This response invalidates the approximations made in the derivation of classical 'modified tilt' modes. We show that the response of the halo causes the line of nodes of a disc that starts from a modified tilt mode to wind up within a few dynamical times. We explain this finding in terms of the probable spectrum of true normal modes of a combined disc–halo system. The key physical point is that in each radial range the halo rapidly aligns with the disc, so calculations based on the assumption that, in the presence of a warped disc, the halo retains a regular spheroidal structure are based on a fatally flawed assumption.  相似文献   

2.
We present an analysis of the behaviour of the ‘coarse-grained’ (‘mesoscopic’) rank partitioning of the mean energy of collections of particles composing virialized dark matter halos in a Λ-CDM cosmological simulation. We find evidence that rank preservation depends on halo mass, in the sense that more massive halos show more rank preservation than less massive ones. We find that the most massive halos obey Arnold’s theorem (on the ordering of the characteristic frequencies of the system) more frequently than less massive halos. This method may be useful to evaluate the coarse-graining level (minimum number of particles per energy cell) necessary to reasonably measure signatures of ‘mesoscopic’ rank orderings in a gravitational system.  相似文献   

3.
We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies. A thin baryonic disc having the Kuzmin density profile embedded in a spherical dark matter halo having a density profile proposed by Navarro, Frenk & White is considered. For the purpose of comparison, we also consider the homogeneous and   r −1  profiles for dark matter in a truncated spherical halo. Assuming for simplicity that the dark matter distribution is not affected by mass-loss from discs and the change of baryonic disc matter distribution is homologous, we evaluate the effects of dynamical response in the resulting discs. We found that the dynamical response only for an adiabatic approximation of mass-loss can simultaneously account for the rotation velocity and disc size as observed particularly in dwarf spiral galaxies, thus reproducing the Tully–Fisher relation and the size versus magnitude relation over the full range of magnitude. Furthermore, we found that the mean specific angular momentum in discs after the mass-loss becomes larger than that before the mass-loss, suggesting that the mass-loss would most likely occur from the central disc region where the specific angular momentum is low.  相似文献   

4.
The non‐linear dynamics of bending instability and vertical structure of a galactic stellar disc embedded into a spherical halo are studied with N‐body numerical modelling. Development of the bending instability in stellar galactic disc is considered as the main factor that increases the disc thickness. Correlation between the disc vertical scale height and the halo‐to‐disc mass ratio is predicted from the simulations. The method of assessment of the spherical‐to‐disc mass ratio for edge‐on spiral galaxies with a small bulge is considered. Modelling of eight edge‐on galaxies: NGC 891, NGC 4738, NGC 5170, UGC 6080, UGC 7321, UGC 8286, UGC 9422 and UGC 9556 is performed. Parameters of stellar discs, dark haloes and bulges are estimated. The lower limit of the dark‐to‐luminous mass ratio in our galaxies is of the order of one within the limits of their stellar discs. The dark haloes dominate by mass in the galaxies with very thin stellar discs (NGC 5170, UGC 7321 and UGC 8286) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
We study the formation of galaxies in a Λ cold dark matter (ΛCDM) universe using high-resolution hydrodynamical simulations with a multiphase treatment of gas, cooling and feedback, focusing on the formation of discs. Our simulations follow eight isolated haloes similar in mass to the Milky Way and extracted from a large cosmological simulation without restriction on spin parameter or merger history. This allows us to investigate how the final properties of the simulated galaxies correlate with the formation histories of their haloes. We find that, at   z = 0  , none of our galaxies contains a disc with more than 20 per cent of its total stellar mass. Four of the eight galaxies nevertheless have well-formed disc components, three have dominant spheroids and very small discs, and one is a spheroidal galaxy with no disc at all. The   z = 0  spheroids are made of old stars, while discs are younger and formed from the inside-out. Neither the existence of a disc at   z = 0  nor the final disc-to-total mass ratio seems to depend on the spin parameter of the halo. Discs are formed in haloes with spin parameters as low as 0.01 and as high as 0.05; galaxies with little or no disc component span the same range in spin parameter. Except for one of the simulated galaxies, all have significant discs at   z ≳ 2  , regardless of their   z = 0  morphologies. Major mergers and instabilities which arise when accreting cold gas is misaligned with the stellar disc trigger a transfer of mass from the discs to the spheroids. In some cases, discs are destroyed, while in others, they survive or reform. This suggests that the survival probability of discs depends on the particular formation history of each galaxy. A realistic ΛCDM model will clearly require weaker star formation at high redshift and later disc assembly than occurs in our models.  相似文献   

6.
Angular momentum redistribution within barred galaxies drives their dynamical evolution. Angular momentum is emitted mainly by near-resonant material in the bar region and absorbed by resonant material mainly in the outer disc and in the halo. This exchange determines the strength of the bar, the decrease of its pattern speed, as well as its morphology. If the galaxy has also a gaseous component and/or a companion or satellite, then these also take part in the angular momentum exchange. During the evolution a bar structure forms in the inner parts of the halo as well. This bar is shorter and fatter than the disc bar and stays so all through the simulation, although its length grows considerably with time. Viewed edge-on, the bar in the disc component acquires a boxy or peanut shape. I describe the families of periodic orbits that explain such structures and review the observations showing that boxy/peanut ‘bulges’ are in fact just bars seen edge-on.  相似文献   

7.
The formation of a disk galaxy within a slowly growing dark halo is simulated with a new chemo-dynamical model. The model describes the evolution of the stellar populations, the multi-phase ISM and all important interaction. I find, that the galaxy forms radially from inside-out and vertically from top-to-bottom. The derived stellar age distributions show that the inner halo is the oldest component, followed by the outer halo, the triaxial bulge, the halo-disk transition region and the disk. Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. In particular, the stellar metallicity distribution in the halo of the model resembles the one of M31. The bulge in the model shows, at least two stellar subpopulations, an early collapse population and a population that formed later out of accreted disk mass. In the stellar metallicity distribution of the disk, I find a pronounced ‘G-dwarf problem’ which is the result of a pre-enrichment of the disk ISM with metal-rich gas from the bulge. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

8.
According to the two-infall model for the chemical evolution of the Galaxy the halo and bulge formed on a relatively short timescale (0.8–1.0 Gyr) out of the first infall episode, whereas the disk accumulated much more slowly and ‘inside-out’ during a second independent infall episode. We explored the effects of a threshold in the star formation process, during both the halo and disk phases. In the comparison between model predictions and available data, we have focused our attention on abundance gradients as well as gas, stellar and star formation rate distributions along the disk. We suggest that the mechanism for the formation of the halo leaves detectable imprints on the chemical properties of the outer regions of the disk, whereas the evolution of the halo and the inner disk are almost completely disentangled. This is due to the fact that the halo and disk densities are comparable at large Galactocentric distances and therefore the gas lost from the halo can substantially contribute to building up the outer disk. We predict that the abundance gradients along the Galactic disk have increased in time during the first billion years of the disk evolution and remained almost constant in the last ~5Gyrs. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

9.
We investigate the early phase of the 13 February 2009 coronal mass ejection (CME). Observations with the twin STEREO spacecraft in quadrature allow us to compare for the first time in one and the same event the temporal evolution of coronal EUV dimmings, observed simultaneously on-disk and above-the-limb. We find that these dimmings are synchronized and appear during the impulsive acceleration phase of the CME, with the highest EUV intensity drop occurring a few minutes after the maximum CME acceleration. During the propagation phase two confined, bipolar dimming regions, appearing near the footpoints of a pre-flare sigmoid structure, show an apparent migration away from the site of the CME-associated flare. Additionally, they rotate around the ‘center’ of the flare site, i.e., the configuration of the dimmings exhibits the same ‘sheared-to-potential’ evolution as the postflare loops. We conclude that the motion pattern of the twin dimmings reflects not only the eruption of the flux rope, but also the ensuing stretching of the overlying arcade. Finally, we find that: i) the global-scale dimmings, expanding from the source region of the eruption, propagate with a speed similar to that of the leaving CME front; ii) the mass loss occurs mainly during the period of strongest CME acceleration. Two hours after the eruption Hinode/EIS observations show no substantial plasma outflow, originating from the ‘open’ field twin dimming regions.  相似文献   

10.
A rotating disc galaxy is modelled as a composite system consisting of thin stellar and gaseous discs, which are described by a two-fluid modal formalism. The composite disc system is assumed to retain axisymmetry in the background equilibrium. General density-wave perturbations in the two discs are coupled through the mutual gravitational interaction. We study the basic properties of open and tight spiral density-wave modes in such a composite disc system. Within the Lindblad resonances, perturbation enhancements of surface mass density in stellar and gaseous discs are in phase; this is also true during the initial growth phase of density-wave perturbations. Outside the Lindblad resonances, there exists a possible spiral density-wave branch for which perturbation enhancements of surface mass density in stellar and gaseous discs are out of phase. We discuss implications of these results on the critical parameters for global star formation in barred and normal spiral galaxies and on magnetohydrodynamic density waves within the Lindblad resonances.  相似文献   

11.
We study the stability of poloidal magnetic fields anchored in a thin accretion disc. The two-dimensional hydrodynamics in the disc plane is followed by a grid-based numerical simulation including the vertically integrated magnetic forces. The three-dimensional magnetic field outside the disc is calculated in a potential field approximation from the magnetic flux density distribution in the disc. For uniformly rotating discs we confirm numerically the existence of the interchange instability as predicted by Spruit, Stehle & Papaloizou . In agreement with predictions from the shearing sheet model, discs with Keplerian rotation are found to be stabilized by the shear, as long as the contribution of magnetic forces to support against gravity is small. When this support becomes significant, we find a global instability which transports angular momentum outwardly and allows mass to accrete inwardly. The instability takes the form of a m =1 rotating 'crescent', reminiscent of the purely hydrodynamic non-linear instability previously found in pressure-supported discs. A model where the initial surface mass density Σ( r ) and B z ( r ) decrease with radius as power laws shows transient mass accretion during about six orbital periods, and settles into a state with surface density and field strength decreasing approximately exponentially with radius. We argue that this instability is likely to be the main angular momentum transport mechanism in discs with a poloidal magnetic field sufficiently strong to suppress magnetic turbulence. It may be especially relevant in jet-producing discs.  相似文献   

12.
We study, theoretically and withN-body simulations, the formation of spiral structures in retrograde galaxy encounters. A one-armed leading spiral dominates in a disc if (i) the tidal perturbation from the retrograde companion is large enough, and (ii) the disc is surrounded by a massive halo. From the literature we find that very few spirals in a sample of galaxies with a large companion have leading spiral arms. A possible reason for this is that very few spiral galaxies have a halo with a larger mass than the disc mass.  相似文献   

13.
In the generic CDM cosmogony, dark-matter haloes emerge too lumpy and centrally concentrated to host observed galactic discs. Moreover, discs are predicted to be smaller than those observed. We argue that the resolution of these problems may lie with a combination of the effects of protogalactic discs, which would have had a mass comparable to that of the inner dark halo and be plausibly non-axisymmetric, and of massive galactic winds, which at early times may have carried off as many baryons as a galaxy now contains. A host of observational phenomena, from quasar absorption lines and intracluster gas through the G-dwarf problem, point to the existence of such winds. Dynamical interactions will homogenize and smooth the inner halo, and the observed disc will be the relic of a massive outflow. The inner halo expanded after absorbing energy and angular momentum from the ejected material. Observed discs formed at the very end of the galaxy formation process, after the halo had been reduced to a minor contributor to the central mass budget and strong radial streaming of the gas had died down.  相似文献   

14.
We consider the problem of 4 bodies of equal masses in R 3 for the Newtonian r−1 potential. We address the question of the absolute minima of the action integral among (anti)symmetric loops of class H 1 whose period is fixed. It is the simplest case for which the results of [4] (corrected in [5]) do not apply: the minima cannot be the relative equilibria whose configuration is an absolute minimum of the potential among the configurations having a given moment of inertia with respect to their center of mass. This is because the regular tetrahedron cannot have a relative equilibrium motion in R 3 (see [2]). We show that the absolute minima of the action are not homographic motions. We also show that if we force the configuration to admit a certain type of symmetry of order 4, the absolute minimum is a collisionless orbit whose configuration ‘hesitates’ between the central configuration of the square and the one of the tetrahedron. We call these orbits ‘hip-hop’. A similar result holds in case of a symmetry of order 3 where the central configuration of the equilateral triangle with a body at the center of mass replaces the square. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

15.
We fit the spectra of Cyg X-1 using two component advective flows with Keplerian accretion disks on the equatorial plane surrounded by sub-Keplerian disks when standing shocks are present. The soft photons generated by the bremsstrahlung and synchrotron processes in the sub-Keplerian flow, as well as the multi-colour black body emission from the Keplerian disk are Comptonized by the thermal and non-thermal electrons. By varying Keplerian and sub-Keplerian rates we are able to reproduce the observed soft and hard states as far as X-ray region is concerned and ‘low γ-ray intensity’ and ‘high γ-ray intensity’ states as far as the soft γ-ray region is concerned. We also find two pivotal points where the spectra intersect as is observed in Cyg X-1.   相似文献   

16.
We study the infrared (IR) emission from flared discs with and without additional optically thin haloes. Flux calculations of a flared disc in vacuum can be considered a special case of the more general family of models in which the disc is imbedded in an optically thin halo. In the absence of such a halo, flux measurements can never rule out its existence because the disc flaring surface defines a mathematically equivalent halo that produces the exact same flux at all IR wavelengths. When a flared disc with height H at its outer radius R is imbedded in a halo whose optical depth at visual wavelengths is  τhalo  , the system IR flux is dominated by the halo whenever  τhalo > (1/4) H / R   . Even when its optical depth is much smaller, the halo can still have a significant effect on the disc temperature profile. Imaging is the only way to rule out the existence of a potential halo, and we identify a decisive test that extracts a signature unique to flared discs from imaging observations.  相似文献   

17.
It is argued that the iron nucleosynthesis rate in the universe due to SNI outbursts is dependent on the mass function of star formation. Since the mass function depends on the chemical composition and since the masses of SNI precursors have upper limits, the iron nucleosynthesis rate was low at an earlier evolutionary epoch of the universe when mainly massive stars were formed. The iron nucleosynthesis rate should reach a maximum near z ∼ 0.5. At such or similar value of z the well-known ‘step’ in the cosmic γ-ray background spectrum may be explained by the presence of γ-gray quanta accompanying the radioactive56Co →56Fe decay. An argument is presented against the identification of the hidden mass of the universe with black-hole remnants of ‘type III’ stars.  相似文献   

18.
The results of a three-dimensional model for disc–halo interaction are presented here. The model considers explicitly the input of energy and mass by isolated and correlated supernovae in the disc. Once disrupted by the explosions, the disc never returns to its initial state. Instead it approaches a state where a thin H  i disc is formed in the Galactic plane, overlaid by thick H  i and H  ii gas discs with scaleheights of 500 pc and 1–1.5 kpc, respectively. The upper parts of the thick H  ii disc (the diffuse ionized medium) act as a disc–halo interface, and its formation and stability are directly correlated to the supernova rate per unit area in the simulated disc.  相似文献   

19.
The evidence for unseen mass (which is briefly reviewed) suggests that the cosmological density parameter Ω is at least 0.1–0.2. An Einstein-de-Sitter ‘flat’ universe with Ω = 1 — which is appealing for theoretical reasons — can only be reconciled with the data if the galaxies are more ‘clumped’ than the overall mass distribution, and are poor tracers of the unseen mass even on scales of several Mpc. Possible forms for the unseen mass are discussed; and feedback processes are outlined whereby galaxy formation can be suppressed in underdense regions.  相似文献   

20.
A recent observation with the Hipparcos satellite and some numerical simulations imply that the interaction between an oblate halo and a disc is inappropriate for the persistence of galactic warps. Following on from this , we have compared the time evolution of galactic warps in a prolate halo with that in an oblate halo. The haloes were approximated as fixed potentials, while the discs were represented by N -body particles. We have found that the warping in the oblate halo continues to wind up, and finally disappears. On the other hand, for the prolate halo model, the precession rate of the outer disc increases when the precession of the outer disc recedes from that of the inner disc, and vice versa. Consequently, the warping in the prolate halo persisted to the end of the simulation by retaining the alignment of the line of nodes of the warped disc. Therefore, our results suggest that prolate haloes could sustain galactic warps. The physical mechanism of the persistence of warp is discussed on the basis of the torque between a halo and a disc and that between the inner and outer regions of the disc.  相似文献   

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