| Abstract | We present evidence that ripples ("shells") occur not only in ellipticals, as hitherto believed, but also in disk galaxies of Hubble types S0, S0/Sa, and Sa, and probably even in the Sbc galaxy NGC 3310. This evidence includes the discovery of ripples in the northern disk galaxies NGC 3032, 3619, 4382, 5548 (a Seyfert), and 5739, and in the "diskless S0" NGC 7600. We argue that these ripples cannot usually have resulted from transient spiral waves or other forced vibrations in the existing disks, but instead consist of extraneous sheet-like matter, because they often (1) interleave in radius on opposite sides of the center and (2) occur with pitch angles either of opposite signs within one galaxy or of opposite sign to the pitch angle of exiting spiral arms. The frequency presence of major disk-shaped companions suggests that ripple material may be acquired not only through wholesale mergers, but also through mass transfer from neighbor galaxies. Although statistics concerning the frequency of ripples are not yet available from our ongoing survey of northern E and S0 galaxies, data from the Malin and Carter catalog indicated that the percentage of galaxies with ripples drops from about 10% for E and E/S0's to 6% for S0's and 1% for Sa's. The presence of ripples in early-type galaxies suggests that (1) the disk of at least some galaxies are robust enough to survive major accretion events, (2) such events may lead to episodic growth of bulges, and (3) S0's may form when disk galaxies experience major accretions of mergers involving perhaps about 1/10th to 1/2 their own mass. |
