Galaxy Collisions

Curtis Struck

Dept of Physics and Astronomy, Iowa State University, Ames, IA 50010 USA


Figure Captions

Fig. 1. The collisional galaxy system AM1724-622, nick-named the "Sacred Mushroom." The strong ring wave of the primary galaxy was almost certainly induced by an interpenetrating collision. The structure of the companion galaxy was also strongly disturbed. The connecting "bridge" between the two is made up of stars torn off one or both galaxies. (Digital Sky Survey image courtesy of AURA/STScI.)

Fig. 2. The Hubble "tuning fork" galaxy classification scheme (from Hubble 1958).

Fig. 3. Montage of collisional forms, and specifically "bridges and filaments" from Zwicky's (1959) review article on "multiple galaxies".

Fig. 4. Hubble Space Telescope image of the "Cartwheel", a prototypical collisional ring galaxy (courtesy P. N. Appleton and NASA).

Fig. 5. Radius versus time for representative stars in a kinematic model for a collisional ring galaxy as described in the text.

Fig. 6. Phase diagram of radial velocity versus radius (r-vr ) from the kinematic calculation of Figure 5, at dimensionless time t = 20. The loops are the result of orbit crossing in the inner ring, while the positive velocity wave between radii of r = 3.0 - 4.0 shows the orbit-crowding outer ring.

Fig. 7. Contour maps of the gas density for a hydrodynamical simulation of an off-center galaxy collision (Appleton and Struck-Marcell 1987b) Solid contours indicate densities above the initial unperturbed value and dotted contours show lower densities.

Fig. 8. Toomre's ring-to-spiral transition is illustrated by a sequence numerical model evolutions with progressively decreasing companion impact radii. Each row shows a different model. See text and Toomre 1978 for details.

Fig. 9. A self-consistent, N-body plus Smoothed Particle Hydrodynamics simulation from R. A. Gerber's thesis, showing an incipient spiral in a collision like those that produce ring galaxies. In this case the trajectory of the companion galaxy was nearly perpendicular to the primary disk, and the point of closest approach was at the edge of that disk (see Gerber 1993 for details).

Fig. 10. Optical image of the "Whirlpool galaxy" M51, whose beautiful spiral arms are likely a result of the ongoing collision, see text. (Digital Sky Survey image courtesy of AURA/STScI.)

Fig. 11. The development of a "swing-amplified" trailing spiral wave from an initially leading wave from Toomre (1981). Contours represent fixed fractional excess surface density, and the time between snapshots is one half of the rotation period at the corotation point.

Fig. 12. The collisional system NGC 2207/IC 2163 illustrates the ocular waveform. Specifically, the disk of the smaller galaxy (IC 2163) has the characteristic eyelid shape and the double-branched spiral arm. The other spiral arm has been hidden or disrupted by the larger galaxy. (Digital Sky Survey image courtesy of AURA/STScI.)

Fig. 13. The bridge connecting the two galaxies of the Arp 284 system. A VLA multi-array intensity map of the HI gas (grayscale) is superposed on a narrowband red continuum image (contours) smoothed to 12 arcsec resolution (see Smith et al. 1997 for details). The offset in the bridge between the gas (dark ridge) and the old red stars is about 10 arcsec.

Fig. 14. The HI gas bridge in the Cartwheel system from Higdon (1996).

Fig. 15. Three orthogonal views at two times, immediately before and after the collision of two gas-rich, disk galaxies in a numerical hydrodynamical simulation. As in observed systems the collisional splash produces a substantial gas bridge, while leaving the primary disk largely intact. See Struck (1997) for details.

Fig. 16. Sequence of face-on and edge-on views of a numerical N-body simulation of a minor merger from Walker et al. (1996). Note the waves that develop in the face-on view, and the disk thickening in the edge-on view. (Time indicated in units of about 125 million years for typical galaxy scales.)

Fig. 17. The giant tidal plume extending out 40 degrees from the galaxy NGC 3628 in Leo. The angular size of this plume is very large because it is relatively nearby. It is otherwise a representative tidal plume. (Unpublished image courtesy P. N. Appleton.)

Fig. 18. The prototypical shell galaxy NGC 3923. Several sharp shell edges are visible in this negative image, even without special processing. (Digital Sky Survey image courtesy of AURA/STScI.)

Fig. 19. Evolution of the phase and configuration space distribution of a set of test particles falling from rest into a rigid (isochrone) galaxy potential, representing the formation of a shell galaxy (from Quinn 1984). The time in units of the radial period of the most tightly bound particles is shown in the phase plots. Positions are given in units of the potential scale length.

Fig. 20. Prieur's shell galaxy type classification is illustrated by schematics of two systems. NGC 3923 on the left is an aligned system (type 1), and 0422-476 on the right is all-round, type 2 system (from Prieur 1990).

Fig. 21. Collision involving a barred primary from Athanassoula et al. 1996. In this simulation the point of closest approach is near the edge of the barred galaxy. After the collision the bar is almost destroyed, but later an off-centered oval structure develops.

Fig. 22. Multiwavelength observations of four famous merger remnants: a) NGC 4038/39 (Arp 244, "The Antennae"), b) NGC 7252 (Arp 226, "Atoms for Peace"), c) IRAS 19254-7245 ("The Super-Antennae), d) IC 4553/54 (Arp 220). Contours show the surface density of neutral hydrogen gas superposed on optical images (greyscale). The insets show K band (2.2 micron) images of the central regions as greyscale, with white contours representing molecular gas (CO) intensities. The scale-bar represents 20 kpc in each case. See Sanders and Mirabel (1996) for details.

Fig. 23. A well-known polar ring galaxy, NGC 4650A. (Image courtesy European Southern Observatory.)

Fig. 24. Schematic of a polar ring galaxy seen from a variety of different orientations from Whitmore et al. (1990). According to the authors, less than half of these views are readily identifiable as a polar ring galaxy.

Fig. 25. Maps of the distribution of atomic hydrogen in the M81/82 system. The top schematic (from Appleton et al. 1981) summarizes the large scale distribution. The large amount of gas between the bright optical galaxies emphasizes the magnitude of the disturbance in this system. The contour map at bottom shows the high resolution results of Yun et al. (1994) in the northeastern part of the system.

Fig. 26. Hubble Space Telescope image montage of QSO host galaxies, illustrating the frequency of disturbed or interacting hosts (from Bahcall et al. 1997, courtesy AURA/STSci.).

Fig. 27. The five galaxies of Hickson compact group 40 (= Arp 321 = VV116) are shown at the center of this image (Digital Sky Survey image courtesy of AURA/STScI.).

Fig. 28. Broad band image of a distant (z = 0.39) galaxy cluster, CL 0024+1654. Foreground stars are revealed by the white (saturated) dot in the center of the dark image, almost all remaining objects are galaxies, most in this representative cluster. (Unpublished image produced at the University of Hawaii 2.2m telescope by R. J. Lavery and J. P. Henry, provided by R. J. Lavery.)