|Annu. Rev. Astron. Astrophys. 1985. 23:
Copyright © 1985 by Annual Reviews. All rights reserved
5.1. Shells, and Ripples
The discovery of large concentric shells of very low surface brightness around otherwise normal elliptical galaxies came as a complete surprise (75). Perhaps the best example of such a shell galaxy is NGC 1344 (24, 75), revealed by application of a photographic amplification technique (73) to deep plates from the UK Schmidt Telescope. Application of another photographic technique devised to reveal low-contrast features (72), the unsharp masking procedure, showed the existence of inner ripples or shells more or less as the continuation of the outer ones (cf. examples in 77). At the same time, detailed studies of merger candidates such as NGC 1316 and NGC 7252 (112, 113) showed not only the existence of the familiar tails identified as the remnants of the encounter, but also the presence of ripples in the light distribution of the central body. Such ripples can also be seen in several galaxies depicted in Arp's Atlas, but until recently they had not received much attention.
Malin & Carter (77) examined all the available film copies of the ESO/SRC (IIIaJ) Southern Sky Survey and presented a catalog of 137 galaxies south of -17° exhibiting shells at large distances from the galaxy in the faint outer envelope. With the unaided eye, they were able to detect features as faint as 26.5 mag arcsec-2, comparable to the measured surface brightness of the large shell around NGC 1344 (as obtained in 24). As Mahn & Carter point out, galaxies with very faint diffuse jets or envelopes, e.g. M89 (74), are probably left out of their catalog, as well as galaxies with only inner ripples. High-resolution plate material is needed in order to discover shells or ripples in the inner part or main body of a galaxy.
Quinn (87, 88) has analyzed several examples of shell galaxies for which high-contrast - prints and unsharp masked photographs were made available to him. He arrived at the following summary of shell properties Shells have been detected around both normal (NGC 3923, NGC 1344) and abnormal ellipticals (NGC 5128, NGC 7070A) but not around spirals and very rarely around S0s. They seem to occur preferentially around galaxies in the field. The shells are presumably three-dimensional structures, since there are no examples of thin, pointy shells, as would be expected if they were intrinsically planar. They do not completely encircle the central galaxy, but rather can be described as roughly concentric arcs, which for higher ellipticity galaxies, e.g. NGC 1344 and NGC 3923, are aligned with the major axis of the galaxy. Shells are found over a large range of radii, with shell separation increasing with radius, and they are staggered or interleaved, i.e. the next outermost shell is on the opposite side of the nucleus. The total number of shells can be quite large, and they can extend to very large distances; for example, NGC 3923 has about 20 shells, the outermost about 180 kpc away from the center (H = 50 km s-1 Mpc-1).
The shells are bluer than the nucleus of the parent galaxy and have little luminous mass. Carter et al. (24) showed that the faint shell around NGC 1344 has a B - R color of 1.2 ± 0.3 and a B - H color of 3.07 ± 0.27, which is to be compared with a B - R color of 1.60 ± 0.15 for the main body and a B - H color of 3.84 for the nucleus. Thus the shell appears to have the colors of the disk of a disk galaxy. Recent results by Fort et al. (42) on the basis of CCD photometry by and large agree with these conclusions. In particular, the colors of several inner ripples are bluer than the underlying galaxy and have equivalent spectral types ranging from F9-K0, earlier than the K-type spectra of the ellipticals themselves but typical for the disk of disk galaxies (111).