The authors discuss new evidence which supports the existence of thick disks in elliptical/SO galaxies. Numerical simulations of weak interactions with thick disk systems produce shell structures very similar in appearance to those observed in many shell galaxies. The authors think this model presents a more plausible explanation for the formation of shell structures in elliptical/SO galaxies than does the merger model and, if correct, supports the existence of thick disks in elliptical/SO galaxies

Thomson, R. C.

Wright, A. E.

English

NASA Technical Reports Server

Do elliptical galaxies have thick disks? 
R c Thomson 
Institute of Astronomy, Madingley Road, 
Cambridge CB3 OHA, UK 
A E Wright 
Australia Telescope National Facility *, 
PO Box 276, Parkes, N.S.W. 2870, Australia 
February 18, 1990 
Abstract. We discuss new evidence which supports the existence of thick disks in 
elliptical/SO galaxies. Numerical simulations of weak interactions with thick disk sys- 
tems produce shell structures very similar in appearance to those observed in many 
shell galaxies. We think this model presents a more plausible explanation for the for- 
mation of shell structures in elliptical/SO galaxies than does the merger model and, if 
correct, supports the existence of thick disks in elliptical/SO galaxies. 
1 Introduction 
The discovery of shell structures in elliptical galaxies (Malin and Carter 1980) was 
unexpected, and showed that these systems were not completely featureless. M a n  
and Carter (1983) estimated that about 17% of all elliptical galaxies had shell systems. 
More recently, Seitzer and Schweizer (1989) found that over hdf of the objects in a 
sample of 74 ~llipticals/SOs exhibited shells. Shell structures are therefore not rare - 
they are a c o ~ m o n  phenomena in 
el for shell formati~n is by merger with a~other galaxy 
ernquist and ~ u i n n  1988). Several problems 
explain a s y ~ ~ e t r i c  
galaxy which has a similar stellar population to the that of the primary galaxy and 
which somehow produces shell structures with a radial brightness distribution similar 
*The Australia Telescope National Facility is operated in association with the Division of Radiophysics 
by CSIRO. 
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https://ntrs.nasa.gov/search.jsp?R=19910007649 2020-03-19T19:48:06+00:00Z
to that of the underlying galaxy. We think the combination of all these factors makes 
this explanation of what is now known to be a relatively common phenomenon, highly 
improbable. 
2 A weak interaction model for shell galaxies 
Numerical simulations of interacting thick disk systems (Thomson and Wright 1990) 
have shown that shell structures can be readily formed by this process. In these simu- 
lations, the thick disk is represented by a quasi-spherical distribution of massless test 
particles, with both the primary and secondary galaxies modelled by spherical Plummer 
potentials, and the interaction characterized by a parabolic orbit of the secondary. The 
test particles are set in motion at a tangent to their radius vector with a speed equal to 
their appropriate circular velocity, and the position of each particle is calculated under 
the influence of the combined time varying potential. 
When such an interacting system is viewed along the effective axis of rotation, the 
induced stellar density wave is seen to have a spiral morphology (Fig.1; top row). 
When such a system is viewed nearly edge on, asymmetric shell structures are clearly 
visible (Fig.1; middle row). The asymmetry is more pronounced when the orbit of the 
secondary galaxy cuts through the simulated thick disk. 
In the classification scheme described by Prieur (1989), type 1 shell systems are inter- 
leaved and well aligned along the major axis of the elliptical. According to our model, 
type 1 shell systems arise in elliptical galaxies which have a thick disk and a conspicuous 
spiral density wave viewed approximately edge on. When such a thick disk system is 
viewed more face-on, the density wave is seen to form fuller, but not complete arcs, and 
the shells also exhibit some spirality (Fig.1; bottom row). We associate this projection 
with type 2 (‘all-round’) shell systems. Furthermore, the observations that elongated 
shell galaxies are preferentially surrounded by type 1 shell systems, and that round-like 
shell galaxies are preferentially surrounded by type 2 systems (Prieur 1989) imply that 
these ellipticals are oblate spheroids when interpreted in terms of our model. 
Shell galaxies that cannot be classified as type 1 or 2, are classified as type 3, which 
includes those galaxies with too few shells or which have complex structure (eg, Fornax 
A). In our model, galaxies with only a small number of shells may occur soon after 
the encounter (before the density wave has wound up by differential rotation). At 
the present stage of our modelling, we are unable to reproduce complex shell struc- 
tures, probably because of the oversimplified assumptions of our model. However, we 
think that these complex shell systems are also caused by (possibly strong) interactions 
since these are observed in galaxies which are otherwise known to be interacting (eg, 
N G C 1549/ 1 553, N G C 13 1 6( Fornax A) /NG C 1 3 1 7). 
756 
3 Thick disks in elliptical/SO galaxies 
Featureless thick disks (ie, with no conspicuous spiral density wave) could exist in all 
elliptical galaxies without being observed. As such, most of the angular momentum 
in these systems could remain hidden. It is interesting to note that most of the ellip- 
tical galaxies in Malin and Carter’s catalog of shell galaxies (Malin and Carter 1983) 
have been classified as SO by Thronson et al. 1989. If this reclassification is correct, 
considerable angular momentum must be present in these (possibly thick disk) systems. 
Detailed surface photometry of elliptical galaxies has revealed significant deviations 
from perfect ellipses (Jedrzedjewsi 1987, Fkanx 1988). In this respect, pointed isophotes 
(positive cos48 term) are thought to indicate the presence of a thin disk. We think 
that ‘boxy’ isophotes (negative cos48 term) may be the signature of thick disks in 
oblate spheroids viewed edge on. The extremely box-shaped elliptical IC3370 has been 
observed by Jarvis (1987). The maximum rotation velocity on the major axis is in good 
agreement with theoretical models of an oblate spheroid supported by rotation alone. 
For this reason, Jarvis proposed classifying this galaxy as SOpec. We think this galaxy 
is not peculiar, but just an extreme case of an elliptical galaxy with a featureless, but 
easily detectable, thick disk. 
4 Discussion 
The existence of thick disks in elliptical galaxies is postulated on the evidence of regular 
shell structures seen in a large number of these systems. By analogy with thin disk 
systems, if the thick disks extends much further than the visible bulge, elliptical galaxies 
may have much more angular momentum than previously thought. The stellar orbits 
in the outer regions of these galaxies could then be dominated by rotation. (NB. The 
shell system in NGC3923 extends more than 80kpc from the galaxy centre!) Due to 
the extremely low surface brightness in these regions, only thick disk systems with 
identifiable features (shells) could have reliable velocities measured, and even then only 
with great difficulty. 
It is interesting to note that our thick disk model can explain the observed connection 
between shell morphology (type 1 vs type 2) and roundness in terms of a simple pro- 
jection effect. The merger model cannot explain this connection in a unified way - the 
differences must be modelled by different initial conditions, and cannot be explained 
by projection effects alone (Dupraz and Combes 1986). 
All galaxies interact to some extent, and our model only requires the existence of a 
dynamically cool population of stars in nearly circular orbits to produce shells similar 
to those seen in many elliptical/SO galaxies. Our simulations show that only weak 
interactions with galaxies l/lOth the mass of the primary and at distances ~40kpc  
are needed to produce visible shell structures. Taking into account the small impact 
parameter and small relative velocity required by the merger model, the cross section 
757 
< . !. .. . . 
.. . , ... * I . .  . . . :. . . . . ... 
Figure 1: Weak interaction model simulation with 10000 particles and a secondary 
galaxy l/lOth mass of primary. Time unit - lo9 years; initial radius of galaxy 80kpc. 
Top row - view from above; middle row - view from front; bottom row - view 
from 45'. For clarity, only the band of particles from the projected equatorial region 
(121 < 0.2) is shown. 
for producing shell structures by weak interactions in thick disks is many orders of 
magnitude greater than the cross section for shell production by merger. 
In conclusion, we think that at least some elliptical galaxies have thick disks and that 
shell systems are formed in these thick disks by weak interaction with another galaxy. 
5 References 
Dupraz,C., & Combes,F., 1986. Astr. Astrophys, 166, 53. 
Franx,M., 1988. Ph.D. Thesis, Leiden. 
Hernquist,L., & Quinn,P.J., 1988. Astrophys. J., 331, 682. 
Jarvis,B.J., 1987. IAU Symp. No. 127, ed. De Zeeuw,T., Reidel, Dordrecht. 
Jedrzejewski,R.I., 1987. IA U Symp. No.  127, ed. De Zeeuw,T., Reidel, Dordrecht. 
Malin,D.F., & Carter,D., 1980. Nature, 285, 643. 
Malin,D.F., & Carter,D., 1983. Astrophys. J., 274, 534. 
Prieur,J.-L., 1989. European Southern Observatory Scientific Preprint. 
Quinn,P.J., 1984. Astrophys. J., 279, 596. 
Seitzer,P., & Schweizer.F., 1989. Preprint. 
Thomson,R.C., & Wri 
Thronson,H.A., Sally, king,P., 1989. Astron. J., 9 
., 1990. Cambridge University Astronomy Preprint. 
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Do elliptical galaxies have thick disks?

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