Influence of initial disk orientation on the final shape of E+S merger remnants
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1
J. Perea , C. García-Gómez , A. del Olmo , E. Athanassoula , S. Leon
1
Instituto de Astrofísica de Andalucía, CSIC, Granada (Spain).
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Dep. Enginyeria Informatica, URV., Tarragona (Spain).
Abstract
We have performed N-body simulations of E+S galaxy mergers using equal mass galaxies in radial
orbits. Our results show that the initial orientation of the disk with respect to the infall direction is the
most important parameter which determines the final appearance of the merger remnant. The
merger retains a memory of the initial disk angular momentum. Thus, when looking at a projection of
the merger were initially the disk was seen edge on, the merger has a tendency of being disky. If the
disk was seen initially face-on, the projection of the merger can show a boxy shape.
Introduction
Elliptical galaxies show fine structure like boxy or disky isophotes. Different dynamical
mechanisms have been invoked to explain these features. Thus, the dichotomy between boxy
and disky ellipticals can be explained simply as an apparent effect due to the selected viewing
angle of the projection of a triaxial structure. (Governato, Reduzzi and Rampazzo, 1993;
Heyl, Hernquist and Spergel, 1994; Lima-Neto and Combes, 1995). The difference in the star
formation history at the epoch of formation of ellipticals has been proposed by Bekki and
Shioya (1997). Extensive numerical simulations of Naab and Burkert (2003) show that
mergers between spiral galaxies with a 3:1 or 4:1 mass ratio tend to produce low-luminosity,
fast-rotating elliptical galaxies. They conclude also that the homogeneous class of massive
boxy ellipticals are most likely formed by a different process. Here we explore the influence of
the inital disk orientation in the properties of remnants resulting from the merger between
giant ellipticals and disk galaxies.
Figure 1
Simulations
We have performed N-body simulations of the interaction between giant ellipticals and
disk galaxies initially in radial orbits. A total of 500.000 particles for each simulation were
integrated using the Gadget (Springel et al, 2001) parallel treecode. The galaxies had
initially equal masses with a dark to visible mass ratio of 10:1. For the elliptical galaxy we
adopted a Hernquist zero ellipticity model while for the spiral we used the Milky Way
galaxy model of Kuijken & Dubinski (1995).
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Observatoire de Marseille, Marseille (France).
Results
In all the simulations the galaxies merge, but the fate of the galaxy disk is strongly
dependent on the initial conditions. In the perpendicular and tilt model, the disk is
completely destroyed. In the parallel model, however, the disk, while suffering
considerable thickening, ends up in the center of the elliptical. This merger remnant has
similar properties as the Centaurus A galaxy. In Figure 3 we show in the left panel, the
particles of the merger that initially were within the elliptical galaxy and in the right panel
the particles that initially formed the disk galaxy. We can see that the disk is still clearly
visible.
Figure 3
In Figure 4 we show the galaxy ellipticity as a function of the radius for three orthogonal
projections of the merger remnants. Almost all the merger remnants are triaxial. In the
case of the perpendicular model, the projection where initially the disk was seen face on,
results in the rounder system, reflecting the fact that the disk is completely destroyed. In
the other simulations the ellipticity of the merger changes with radius. This is due to the
contribution of the dispersed disk particles.
We have also analyzed the shape of the projected isophotes to check the possible influence
of the disk orientation on the final shape of the merger remnant. We have calculated the
b4 coefficients of the Fourier analysis of the isophotes as a function of radius. The results
are shown in Figure 5. The parallel model shows only positive values of the b4 parameter
showing a disky tendency in the isophotes. This is consistent with the fact that the disk is
not completely destroyed in this case. The rest of the simulations may show a boxy
tendency of some of the isophotes. Boxy isophotes are preferably found in the projections
where initially the disk is not seen edge on.
Figure 4
We have selected three different initial conditions. In the first model the galaxy disk lies in
the the same plane as the galaxy orbit, being their angular momentum perpendicular to
this trajectory. In the second model, the disk is perpendicular to the galaxy orbit and its
angular momentum is parallel to the trajectory, and finally in the third model, the angular
momentum is tilted with respect to the galaxy orbit. We named these simulations as
parallel, perpendicular and tilt model respectively. We show these initial conditions in
Figure 1. The evolution of the tilt model is shown in Figure 2. Dark matter is not displayed
in any of these figures. All the simulations ended at a time of 10Gyr.
Figure 2
Figure 5
Figure 5
Conclusions
The merger between giant elliptical and disk galaxies do not always destroy completely the stellar
galaxy disk. The particles in the merger remnant retain a memory of the initial angular momentum of
the disk.
The direction of the initial disk angular momentum with respect to the direction of the galaxy infall is
then the key parameter which determines the final properties of the merger remnant. In the parallel
model, where the angular momentum is initially perpendicular to the galaxy orbit, the disk enters the
elliptical without being destroyed suffering considerable thickening. This results in strong disky
mergers when viewing the remnant in a direction where initially the disk was seen edge-on, which
have a similar shape as the galaxy Centaurus A. Thus, strongly disky giant ellipticals can be formed
also through the merger of a giant elliptical and a disk galaxy.
References
Bekki, K., Shioya, Y., 1997, ApJ, 478, L17.
Governato, F., Reduzzi, L., Rampazzo, R., 1993, MNRAS, 261, 379
Heyl, J.S., Hernquist, L., Spergel, D.N., 1994, ApJ, 427, 165
Kuijken, K. & Dubinski, J. 1995, MNRAS, 277, 1341.
Lima-Neto, G.B., Combes, F., 1995, A&A, 294, 657
Naab, T., Burkert, A., ApJ, 597, 893
Springel, V., Yoshida, N., White, S.D.M, 2001, New. Astron. 6, 51.
When the initial angular momentum is initially parallel to the galaxy orbit, the disk is completely
destroyed due to the phase wrapping mechanism. If this merger remnant is viewed in a projection
where initially the disk was seen face on, then the merger looks strongly boxy. Intermediate angles
between the initial angular momentum and the galaxy orbits result in merger with intermediate
properties depending also on the viewing angle. It is important to stress that all the features of the fine
structure of the merger remnants are mainly due to the particles that were initially in the disk galaxy
which are not completely mixed with the particles of the elliptical and thus can be interpreted as the
signatures of recent mergers.