Ionization, kinematics, and extent of the diffuse ionized gas halo of NGC 5775

We present key results from deep spectra of the diffuse ionized gas halo of the edge-on galaxy NGC 5775. [N II] lambda 6583 has been detected up to z approximate to 13 kpc above the plane in one of two vertically oriented long slits-making this the spiral galaxy with the greatest spectroscopically detected halo extent in emission. Key diagnostic line ratios have been measured up to z approximate to 8 kpc, allowing the source of ionization and physical stale to be probed. Ionization by a dilute radiation field from massive stars in the disk can explain some of the line ratio behavior, but departures from this picture are clearly indicated, most strongly by the rise of [O III]/H alpha with z. Velocities of the gas in both slits approach the systemic velocity of the galaxy at several kiloparsecs above the plane. We interpret this trend as a decrease in rotation velocity with z, with essentially no rotation at heights of several kiloparsecs. Such a trend was observed in the edge-on galaxy NGC 891, but here much more dramatically. This falloff is presumably due to the gravitational potential changing with z, but will also depend on the hydrodynamic nature of the disk-halo cycling of gas and projection effects. More detailed modeling of the ionization and kinematics of this and other edge-ons will be presented in future papers.