Hydrodynamical simulations of the barred spiral galaxy NGC 6782

1
Hydrodynamical simulations of the barred spiral
galaxy NGC 6782
Lien-Hsuan Lin1,2 , Chi Yuan1,2, and R.
Buta3 1Department of Physics, National Taiwan
University, Taiwan, R.O.C. 2Institute of
Astronomy and Astrophysics, Academia Sinica,
Taiwan, R.O.C 3Department of Physics and
Astronomy, University of Alabama, Alabama, U.S.A.
Abstract NGC 6782 is a type (R1R2)SB(r)a galaxy
with multiple ring patterns. It has a nearly
circular bright nuclear ring connected with a
pair of almost straight dust lanes, the other
ends of which attach to a diamond-shaped (or
pointy oval) inner ring. Two faint arms in turn
extend from the two tips of the inner ring to the
outermost parts of the galaxy, forming a faint
double outer ring-pseudoring morphology. In this
study we use numerical simulations to show that
such striking features can be reproduced by
imposing a strong bar to a gaseous disk system.
Since strong bar potentials may, through
instabilities, lead to chaotic sub-structures,
they present challenging problems to the
numerical simulations. Our simulations are
performed with our own Antares code, which
employs Cartesian coordinates and the
higher-order Godunov scheme with unsplit flux
calculated from the exact Riemann solver.
Calculations are carried out with and without
self-gravitation, which produce similar results.
In both cases, the bar is able to drive spiral
density waves simultaneously at both the outer
Lindblad resonance (OLR) and the inner Lindblad
resonance (ILR). When the bar potential is
strong enough, 41 higher harmonic spiral density
waves are excited, which interact with waves
excited at the ILR and give rise to the
diamond-shaped structure. All the essential
features are in excellent agreement with
observations. Our work is in part supported by
National Science Council, Taiwan,
NSC95-2752-M-001-009-PAE, and by NSF grant
AST050-7140.
Observations Fig. 1 is a multicolor optical image
of NGC 6782 released by Space Telescope Science
Institute in 2002. Fig. 2 is a B-band image and
Fig. 3 is the 2D Fabry-Perot velocity field which
is observed by R. Buta using the CTIO 4-m
telescope. The detector scale is 0.349
arcsec/pixel corresponding to a field of view of
about 3.6 x 3.6 arcminutes. These images have
north at top, east to left. The PA of the major
bar of the galaxy is 177. The IA of the galaxy
is 27 and the PA of the line of nodes is 35.
Fig. 4 is the rotation curve derived from the
velocity field of Fig. 3.
Fig. 1
Fig. 2
Fig. 3
Numerical method and simulations We use the
higher-order Godunov method with 2nd-order
accuracy which employs the exact Riemann solver
on Cartesian coordinates, so the problem with the
inner boundary conditions is avoided. Cases
involving self-gravitation are solved by FFT
Poisson solvers. The initial conditions are a
bell-shaped surface density distribution and a
circular motion corresponding to the nearly flat
rotation curve (green line in Fig. 4) which is
fitted from the observed rotation curve. From
this fitted rotation curve, we obtain the angular
velocity curves plotted in Fig. 5. The bar
potential adopted is shown in Fig. 6 and the
angular speed of the bar is chosen to be 25
km/s/kpc.
Fig. 4
Fig. 5
Fig. 6
Simulations vs Observations Fig. 7 is the
projected density distribution of the simulation
result. Its superimpositions onto Figs. 1 and 2
are shown in Figs. 8 and 9 respectively. Fig. 8
shows that for the nuclear ring, the dust lanes,
as well as the pointy oval inner ring,
Fig. 7
Fig. 8
Fig. 9
simulation results match well with observations.
At the northwestern outer arm in Fig. 2, there
are two faint branches near the tip of the inner
ring, which are also reproduced by our
simulation. The only part our simulation cannot
fit well is the outer part of the outer arms. The
arms in our simulation are much broader than
those in observation. Fig. 10 is the velocity
field of the simulation result. Fig. 11 shows
the superposition of its contours and the
observation. It is particular noteworthy that at
the southeastern part of the inner ring, the
contours bend outwards along the spiral arms for
both simulation and observation. It is one of the
distinct features for the density waves excited
at the ILR.
Self-gravitation Figs. 12 and 13 are the results
for our simulations in the absence and presence
of self-gravitation respectively. They are almost
identical except for a slight difference around
the very high-density nuclear rings.
Fig. 10
Fig. 11
Conclusions In this study, we successfully use a
simple model to explain almost all the striking
features of NGC 6782.  We confirm it to be a
moderately strong barred galaxy, in agreement
with Buta and Block 2001. To obtain the good fit
with observations, the pattern speed of the bar
has to be slow, which is consistent with the view
of Byrd et al. 1994 that NGC 6782 is a slow
pattern speed barred galaxy. Moreover, we believe
self-gravitation of the gas disk is unimportant
in this case.
Fig. 12
Fig. 13