NGC 2188: A Case Study for Disk-Halo Interaction
A deep Hα image of the nearly edge-on (i ~ 86°; Tully, 1988) irregular galaxy NGC 2188 shows the presence of spectacular filaments of ionized gas extending from a large star-forming complex several hundred pc into the halo of the galaxy (Dettmar, 1994). The origin and ionization mechanism of such extraplanar diffuse ionized features is not yet clear. One promising idea is that the extraplanar DIG is due to large scale mass circulation, i.e. a galactic “fountain” (Shapiro & Field, 1976) or “chimney” (Norman & Ikeuchi, 1989). Hα filaments could then be the photoionized walls of such chimneys or fountains.