Galactic‐Scale Outflow and Supersonic Ram‐Pressure Stripping in the Virgo Cluster Galaxy NGC 4388

The Hawaii Imaging Fabry-Perot Interferometer (HIFI) on the University ofHawaii 2.2m telescope was used to map the Halpha and [O III] 5007 Aemission-line profiles across the entire disk of the edge-on Sb galaxy NGC4388. We confirm a rich complex of highly ionized gas that extends ~4 kpc abovethe disk of this galaxy. Low-ionization gas associated with star formation isalso present in the disk. Evidence for bar streaming is detected in the diskcomponent and is discussed in a companion paper (Veilleux, Bland-Hawthorn, &Cecil 1999; hereafter VBC). Non-rotational blueshifted velocities of 50 - 250km/s are measured in the extraplanar gas north-east of the nucleus. Thebrighter features in this complex tend to have more blueshifted velocities. Aredshifted cloud is also detected 2 kpc south-west of the nucleus. The velocityfield of the extraplanar gas of NGC 4388 appears to be unaffected by theinferred supersonic (Mach number M ~ 3) motion of this galaxy through the ICMof the Virgo cluster. We argue that this is because the galaxy and the high-|z|gas lie behind a Mach cone with opening angle ~ 80 degrees. The shocked ICMthat flows near the galaxy has a velocity of ~ 500 km/s and exerts insufficientram pressure on the extraplanar gas to perturb its kinematics. We considerseveral explanations of the velocity field of the extraplanar gas. Velocities,especially blueshifted velocities on the N side of the galaxy, are bestexplained as a bipolar outflow which is tilted by > 12 degrees from the normalto the disk. The observed offset between the extraplanar gas and the radiostructure may be due to buoyancy or refractive bending by density gradients inthe halo gas. Velocity substructure in the outflowing gas also suggests aninteraction with ambient halo gas.Comment: 29 pages including 5 figures, Latex, requires aaspp4.sty, to appear in ApJ, 520 (July 20, 1999 issue