## 3. N tilted corotating ringsThe generalization of Eq. (40) gives the Lagrangian for an The Hamiltonian, is Because the are non-negative, all of the terms in are non-negative if which is the case for oblate halo. Thus the ring system with is stable in the presence of dissipative forces. The total canonical angular momentum is In the absence of dissipation and are constants of the motion. As in the case of one or two rings, it is useful to let The equations of motion can then be written as with , where the generalize of Eq. (34). As discussed in Sect. 2.1, a Newtonian drag force on the ring motion due to dynamical friction with the halo matter can be taken into account by including the term on the right hand side of Eq. (55). The influence of the relative friction discussed in Sect. 2.3 can be accounted for by including the terms on the right hand side of Eq. (55). Here, and with and . With the dissipation terms (56) and (57) included one can readily show that (see also Sect. 4). We have not found an analogous result for in the presence of dissipation. We have developed and tested codes to solve Eqs. (55) (including
the terms (56) and (57)) for and 49 rings. For
the results presented here, the rings are taken to be uniformly spaced
in is also solved to give , which is the line-of-nodes angle relative to the axis. Comparisons of the temporal responses of 25 and 49 ring sytems for
different initial conditions show generally good agreement of the
warps for the time intervals studied. This
indicates that rings gives a valid
representation of a continuous disk. However, in contrast with a
continuous disk (HT), an ring disk has
discrete modes with separate frequencies -
The lowest frequency mode of the power spectrum, in Fig. 15 (see also Sect. 2.3 and Sect. 2.5), remains discrete (that is, isolated) as because our disk model has a sharp outer edge so that is integrable (HT). Power spectra obtained for cases with show that increases with much more slowly than the higher frequency modes which is consistent with the behavior observed in Figs. 6 and 12. For the conditions of Fig. 15, the magnitude is considerably smaller than that predicted for the limit of strong self-gravity between the rings where [equation(47)]. In any case, the frequency is so small (the corresponding period is Gyr) that it is irrelevant to the initial value problems considered below. In the following four subsections we consider different possible origins of warps in an otherwise flat galaxy. Sect. 3.1 discusses warp excitation by a passing compact satellite; Sect. 3.2 treats warp excitation by a compact sinking satellite; Sect. 3.3 treats the case of a tilted halo potential. Sect. 3.4 considers the tilt evolution for the case where the initial plane of the disk material is tilted. ## 3.1. Warp excitation by a passing satelliteConsider the excitation of a warp in a galaxy due to the passage of a satellite of mass much less than the galaxy's mass. The satellite's orbit can easily be calculated exactly for , neglecting the back reaction of the perturbed galaxy on the satellite. However, we assume that even at closest approach the satellite is far from the center of the galaxy and therefore calculate the orbit in the halo potential, Eq. (6a), with the ellipticity of the halo neglected for simplicity (). At the closest approach at , the satellite's speed is , and the angle between the satellite and the plane is . Also, at closest approach, the satellite is taken to be at and . We consider both prograde and retrograde satellite passages. In the presence of a satellite of finite mass , the description of the galaxy disk in terms of tilted circular rings breaks down. The centers of the rings are shifted from the origin, and the rings become non-circular. For this reason we consider the response of the galaxy to a `symmetrized' perturbation of a satellite obtained by replacing the actual satellite by two satellites, each of equal mass , with one satellite in the orbit described above and the other in the orbit . With this prescription we then calculate the torque of the satellites on each ring as a function of time, where , with given
by Eq. (8a), is the position vector to a point on the Fig. 16a shows the Briggs plot of the warp response
of the galaxy resulting from the retrograde
passage of a satellite of mass . Note that the
dependence of at the times shown is of the
form of a Fig. 16b shows the warp response for the retrograde case with and without gravitational interactions () between the rings. Clearly these interactions give a strong phase-locking of the inner rings of the disk (roughly, to or 10 to 18 kpc) which have the same as a function of time. The phase-locking of two and three rings was disscussed in Sect. 2.2 and Sect. 2.4.
The results shown in Fig. 16 are obtained from a code which solves Eqs. (55) including the torques of Eq. (59) with rings equally spaced between kpc and kpc. Thus the separation between rings is kpc. The torques are evaluated numerically at each time step. The Newtonian drag is neglected, in Eq. (56). A small relative friction is included, in Eq. (57). [For a disk half-thickness pc and sound speed km/s, this corresponds to a value of Shakura's (1973) viscosity parameter . This friction acts to smooth out sharp `corners' which exist in the curve . Owing to this , decreases by 16% between and 3 Gyr.] The mass of the satellite is , its core radius is kpc, and at closest approach it is at a distance kpc where it has a speed of 400 km/s and is located at an angle above the plane. The mass of the inner disk in Eq. (4) is , and the disk radial scale is kpc. The halo potential is given by Eq. (6a) with ellipticity , core radius kpc, and circular velocity km/s so that for . The mass of halo matter inside of kpc is . The neutral hydrogen has a total mass of and is distributed according to Eq. (5) with kpc. Fig. 17a shows the Briggs plot for the warp resulting from the
prograde passage of the satellite. For early times
( Gyr) the form of is a
The different behavior of the prograde and retrograde cases for
early times ( Gyr) can be understood by
considering the response of a single ring with or without
gravitational interactions between rings. Fig. 17b shows the orbits
of ring No. 25 (radius
kpc) including the gravitational interactions
for prograde and retrograde encounters. Consider the torque on the
ring which enters on the
right hand side of Eq. (55). The tilting of the ring during the
encounter is relatively small and can be neglected for this
discussion. Consequently, is an even function
of ## 3.2. Warp excitation by a sinking satelliteHere, we discuss the behavior of warps excited by a slowly sinking compact minor satellite. The satellite is assumed to be minor in the respect that its mass is much smaller than that of the galaxy plus halo matter within say 30 kpc. The sinking of a more massive satellite is likely to cause substantial thickening of the disk (Walker, Mihos, and Hernquist 1996) which is not observed. Initially, the satellite is assumed to be in an approximately circular bound orbit in the halo gravitational potential at a large radius in the plane at an ange above the plane. The satellite slowly sinks owing to dynamical friction with the halo matter [Eq. (6a) with ] which in the simplest description (BT, p. 428) causes the specific angular momentum of the satellite to decrease as where is the Coulomb logarithm. The time for the satellite to sink from to the galaxy's center is where we have taken . The disk response is found by integrating Eqs. (55) including the numerically evaluated torques (59) for at each time step for a `symmetrized' sinking analogous to the approach discussed in Sect. 3.1. Fig. 18 shows the Briggs plots of the warp response of the galaxy at different times after the sinking () of a satellite of mass in retrograde and prograde orbits. A number of points are observed:
(1) The warp amplitudes are larger than for the passing satellite (of mass ) discussed in Sect. 3.1, but the amplitudes are still smaller than some observed warps (Briggs 1990). (2) Coiling in the curves at early times ( Gyr) tends to disappear at later times ( Gyr). We believe but have not proven that this is due to the phase-mixing of the fast precession modes which propagate over the radial extent of the disk in times Gyr (see also Sect. 3.1). (3) For both the retrograde and prograde cases exhibits a leading spiral wave for Gyr qualitatively of the form observed for M 83 (see Fig. 2c). However, the tilt amplitude is significantly smaller than that for M 83. The dissipative torques due to dynamical friction [Eq. (56)] and that due to relative friction [Eq. (57)] have different consequences for the warp evolution. For a dynamical friction coefficient , the warp amplitudes for the cases of Figs. 18a and 18b are reduced by factors while the line-of-nodes angles are roughly the same. On the other hand, for relative friction coefficients , short radial wavelengths are damped out without appreciably affecting the overall shape of the curves. ## 3.3. Tilted halo potentialDekel and Shlosman (1983) and Toomre (1983) proposed that observed
warps of galaxies may result from the fact that the dark matter halo
is oblate and is rigidly tilted with respect to the inner disk. To
consider this possibility we generalize Eq. (55) to the case of
where and . The damping terms are still given by Eqs. (56) and (57). For a special disk tilt, , the right-hand-side of Eq. (61) is zero so that is time-independent. This corresponds to the Laplacian surface of the disk (BT, p. 413). It is given by where is the Kronecker delta and the prime on the summation means that the diagonal terms are omitted. The damping terms (56) and (57) are of course zero for the Laplacian tilt. If the self-gravity of the rings () is negligible, . Eq. (62) can always be inverted to give if . This is because the ring system is stable for (see discussion following Eq. (53b)) which implies . A general solution of Eq. (61) can evidently be written as , where obeys Eq. (55) which has no reference to the halo tilt. Over a sufficiently long period of time, damping given by Eq. (56) and/or (57) will give as discussed in the paragraph after next. Fig. 19a shows the Laplacian tilt for a representative case with and without the self-gravity of the rings. Fig. 19b shows the Briggs plots at times and 8 Gyr for a disk started from a Laplacian tilt and from a deviation from a Laplacian tilt. The initial deviation is taken to be . This deviation vanishes at and it has a maximum at the outer edge of the disk. Dissipative torques are neglected. This deviation from gives rise to a leading spiral feature at the outer edge of the disk at Gyr. For a deviation with the factor replaced by -0.2 the line-of-nodes angle at the outer edge of the disk () at Gyr has a similar magnitude to that in Fig. 19b, but it is negative. The Briggs plot for the Laplacian tilt is in contrast with the leading spiral wave observed, for example, in M 83 (Figs. 1 and 2) (Briggs 1990). However, the Laplacian tilt may be relevant to cases such as NGC 3718 which shows a relatively straight line-of-nodes. There is no evident reason for a disk to be initially `set up' in a Laplacian tilt. Fig. 19b shows that deviations from the Laplacian tilt evolve to give a line-of-nodes which is not straight. As mentioned, dissipative torques due to dynamical friction [Eq. (56)] and/or relative friction [equation(57)] act to damp out the deviation () from the Laplacian tilt over a period of time. For example, for in Eq. (56) and no relative friction, the maximum line-of-nodes deviation () at 8 Gyr is reduced by a factor from the case with no dissipation shown in Fig. 19b. On the other hand for and no dynamical friction, the maximum line-of-nodes deviation is reduced by a factor .
## 3.4. Initially tilted disk planeHere we consider the possibility that at some intial time an outer
gaseous disk is formed which is tilted with respect to the plane of
the inner disk. This situation could arise by the capture,
tidal-disruption, radial-spreading, and cooling of a low mass gas
cloud by a disk galaxy. For a spherical cloud of mass
and radius in an
approximately circular orbit of radius where is the initial tilt angle of the outer disk. Similar behavior is found for other smooth variations of . The smooth variation of avoids the excitation of short radial wavelength modes. Fig. 20 shows the nature of the warp evolution resulting from the initial conditions (63). The spiral for Gyr is qualitatively similar to that observed for M 83 (see Fig. 2c). From results not shown here the similarity continues up to Gyr after which time and become large at the outer edge of the disk.
Fig. 21 shows a surface plot of the disk at
Gyr. A leading spiral wave appears in the
Briggs plot (panel
The dissipative torques due to dynamical friction [Eq. (56)] and that due to relative friction [Eq. (57)] affect the results of Fig. 20 in different ways. For a dynamical friction coefficient , and no relative friction, the maximum warp amplitude at Gyr is reduced by about . On the other hand, for no dynamical friction, but a relative friction coefficient , the warp at Gyr is essentially unchanged. © European Southern Observatory (ESO) 1998 Online publication: September 17, 1998 |