The Origin of Warm Ionized Gas Halos in Spiral Galaxies

Richard Rand

I will present morphological and kinematic data on warm, diffuse ionized gas (DIG) halos in nearby edge-on spiral galaxies. The brightness and extent of these DIG halos correlates with the star formation activity in the underlying disks, both globally and locally, and I will argue that DIG halos we see in emission largely arise in star-formation-driven disk-halo flows. I will also talk about recent results on the rotation of DIG halos. We have found that these halos rotate slower than disks.

The vertical gradient in rotation speed seems to relate to the brightness and extent of the halo and therefore also the level of disk star formation, for reasons that are not understood. Simple models of ballistic disk-halo flow predict much shallower vertical gradients of the rotation speed than observed, suggesting hydrodynamical effects may govern the flow.