H I mapping very often reveals a markedly different dynamical picture of systems than suggested by the distribution of the optical light. Particularly striking examples are: the extensive tidal streamers found connecting the members of the M81 group (van der Hulst 1979, Yun et al. 1994); the 200 kpc rotating H I ring in the M96 group (Schneider et al. 1989); a pair of purely gaseous tidal tails emerging from the E4 galaxy NGC 1052 (van Gorkom et al. 1986), from the E2 galaxy NGC 5903 (Appleton, Pedlar & Wilkinson 1990) and from the Sa galaxy NGC 7213 (Hameed, Blank & Young in preparation); the H I bridge/tail morphology of the "Virgo Cloud" H I 1225+01 (Giovanneli et al. 1991, Chengalur et al. 1995); plumes of H I pulled off the Sb galaxy NGC 678 by the Epec galaxy NGC 680 and the associated intergalactic H I cloud (van Morsel 1988). As a result of these and many similar discoveries, we conclude that the true fraction of peculiar objects must be considerably larger than derived from purely optical studies. Based on H I studies, Sancisi (1997) suggest that at least one in four galaxies has suffered a recent merger or experienced an accretion event.
Even in systems already identified as optically peculiar, H I mapping frequently reveals structures that provide critical insights into their dynamical nature by revealing connections not seen at other wavelengths (e.g., Figure 2). Examples include: tidal H I in QSOs (Lim & Ho 1999); the nearly 200 kpc long tidal plumes emerging from the ring galaxy Arp 143 (Appleton et al. 1987; see Figure 4) and the IR luminous starburst Arp 299 (Hibbard & Yun 1999); the 275 kpc diameter H I disk around the mildly interacting system Mrk 348 (see Figure 3); the H I tail and counter-arm in the starburst galaxy NGC 2782 (Smith 1991); the extended tidal streamers in the starburst/blowout system NGC 4631 (Weliachew et al. 1978); the extended disk and streamers in the dIrr NGC 4449 (Hunter et al. 1998); two H I tails emerging from the blue compact dwarf II Zw 40 (van Zee et al. 1998). The fact that these features are easily visible in H I but lack optical counterparts is likely due to the fact that in disk galaxies H I is generally more extended than the stars.
Figure 2. (a) VLA D-array observations of the Cartwheel Ring Galaxy (Higdon 1996). The H I plume reveals a connection between the ring and the northernmost galaxy. Prior to this observation it was not clear which of the three galaxies in the field collided with the ring galaxy. (b) VLA C+D-array observations of the shell elliptical NGC 2865 (Schiminovich et al. 1995). Inset shows the main body of the galaxy. The H I has rotational kinematics. (c) & (d) VLA B+C array observations of the proto-typical shell galaxy Arp 230 (Schiminovich, van Gorkom & van der Hulst, in preparation). Left: integrated H I. Right: H I velocity field, showing that the outer H I is arranged into a rotating disk.
It is not clear if H I mapping is the most efficient means for revealing low-level peculiarities. When a similar amount of observing time (~ few to a dozen hours) is invested in deep optical imaging, some remarkable results have emerged: faint optical loops and streamers have been discovered around what were long thought to be normal unperturbed disk galaxies (see Malin & Hadley 1997, Zheng et al. 1999). While the optical observations do not include the kinematic information provided by H I observations, they may be the only signatures of very evolved interactions, when the H I has faded away or been ionized.