SOME EXPLANATIONS OF THE MODELS


SL2000_P2_fig11_color.eps
-------------------------

The plot shows the time evolution in a simulation run corresponding
to the bound encounter model displayed in Fig. 11 in Salo 
and Laurikainen (2000; MNRAS 319, 393-413, at p. 404).  The
only difference is in the resolution of the spherical polar grid used
for the evaluation of self-gravity: this is now somewhat finer than
that used in Fig. 11, and contains 144 times 108 times 25 bins in
radial, azimuthal and latitudinal directions, respectively.

- The main galaxy consists of an exponential stellar disk composed of
four million self-gravitating particles + a rigid analytic analytical
spherical halo+bulge model, yielding a combined rotation curve which
corresponds closely to the observed one.  More specifically, the
exponential scale-length is 100 arcsecs, and the disk contains 1/3 of
the total mass of the galaxy within 4 scale-lengths. The initial
Toomre parameter Q=1.5.

- The companion is modeled by a rigid bulge+halo model, and has a
mass of 0.55 times that of the primary.

- The interaction is started at 100 million year before the 
Souther disk crossing. Before this the disk was allowed to evolve
in isolation for about 1 Gyr, forming mild multiarm patterns
visible in the first frame.

- The frames show the projection of the system to the sky plane
(assuming Tully's orientation parameters, i=20 and PA=170 degrees)
at various times, together with the projected companion orbit.
The solid curve denotes the portion of the orbit on the side
toward the observer.

- The size of the individual frames is 800 by 800 arcsecs,
corresponding to about 37 kpc (for distance of 9.6 Mpc).

-The time shown in plots is in millions of years, counted from the
time when the companion orbit crosses the disk plane of the primary,
to the direction toward observer (nearly at South)

-The current location of the companion is about 15 kpc behind the main
disk and its projected radial velocity is about 100 km/sec with respect
to the main system. 

- The far tail of particles is tilted by about 40 degrees with respect
to the main disk (in fact to the opposite side of the sky-plane),
providing a plausible explanation for the apparant 'reversed' velocity
field observed by Rots. et al.
Likewise, the particles North of companion have large recession
velocities, due to most recent disk plane crossing at NE).
These two features were our main arguments for prefering
the bound model (see SL2000-Paper 1)

- It is important to note that the distribution of stars is shown:
gas component would have much sharper density contrast.

The density amplitude of spirals in the model agree reasonably well
with the NIR observations, including the amplitude variations along
the arms, which variations have sometimes been used as an argument
against the tidal origin of arms.  (see SL2000-paper 2)



SL2001_P1_fig3_color.eps
------------------------

color version of fig 3 in SL2001 paper 1
Time in frames is as above

SL2000_model.eps
------------------------

Comparison of our simulation model in SL2000,
simultaneously with the optical (Digital
Sky Survey image), HI (contours, from Rots et al 1990), and the
near-IR observations (small insert, depicting the square-shape region
marked on the optical image; Space Telescope archives, image by
Scoville and Evans), with I also attach here. The model in the left
frame shows the last portions of the simulated orbit (solid line
indicates companion orbit on the side toward observer).