If a low mass galaxy passes head-on through the center of a disk galaxy, a ring galaxy is created. In an off-center collision, a partial ring may be created. An example of such a partial ring is seen in the interacting pair NGC 7714/5 (Arp 284). At optical (red) wavelengths, NGC 7714/5 looks like this:

NGC 7714 is to the right (west) and NGC 7715 to the east. The galaxies are separated by 2 arcminutes (25 kiloparsecs). Notice the stellar bridge between the galaxies and the tidal tails extending from both galaxies. Note that NGC 7714 has a partial ring. This image was obtained with the 2.1m telescope at McDonald Observatory in collaboration with Rick Pogge (Ohio State University).

To see a better optical image from the Arp Atlas, click here.

To see a mpeg movie of a simulation of the NGC 7714 and NGC 7715 encounter, click here (from Smith and Wallin 1992, Astrophysical Journal, vol. 393, p. 544). In this model, the smaller galaxy has a mass of 1/3 that of the larger galaxy and the impact parameter is 0.85 times the radius of the larger disk. Compare the final frame with the McDonald optical image shown above. Notice that the ring, the bridge, and the tails are successfully reproduced. This is a restricted 3-body model (only includes gravitational forces between the particles and the two centers of mass; it does not calculate all the individual gravitational forces between the various particles). This model only contains stars.

More recently, Curt Struck (Iowa State) and I have constructed a full N-body model (including all the individual forces), which includes both stars and gas. This model is described in Struck and Smith 2003, Astrophysical Journal, vol. 589, p. 157. To see an animation of such a model, click here.

The distribution of interstellar atomic hydrogen in the NGC 7714/5 system looks like this:

These data were obtained with the Very Large Array, part of the National Radio Astronomy Observatory. This map has 12 arcsecond resolution. The gas distribution is color-coded, with the highest column density red and lowest blue. The yellow contours are optical data from the Digitalized Palomar Observatory Sky Survey, smoothed to 12 arcseconds resolution. Notice the complete loop of gas to the northwest of NGC 7714. Also notice that the gas in the bridge and eastern tail is shifted to the north relative to the stars. Such offsets may be caused by the different behavior of gas and stars during a galaxy collision: the stars do not collide, but the gas clouds do. (From Smith, Struck, and Pogge 1997, Astrophysical Journal, 483, 754).

For an H-alpha+[N II] optical image of this system showing the location of the star forming regions, click here . Note that young stars are forming in the bridge (From Smith, Struck, and Pogge 1997, Astrophysical Journal, 483, 754).

To see Chandra X-ray images of NGC 7714/5, click here (from Smith, Struck, and Nowak 2005, Astronomical Journal, in press).

Spitzer infrared images of Arp 284 were published as part of the `Spirals, Bridges, and Tails' project in Smith et al. (2007), Astronomical Journal, 133, 791. The Spitzer images of Arp 284 are visible here.

Also see our Hubble/Spitzer/GALEX paper on NGC 7714/5:
B. W. Peterson, C. Struck, B. J. Smith, and M. Hancock (2009), `Star Clusters in the Interacting Galaxy System Arp 284', M.N.R.A.S., submitted

To see another galaxy which has probably undergone an off-center collision, click here

To return to Beverly Smith's web page, click here

Last updated: 6/15/09