A Hubble Space Telescope WFPC2 Investigation of the Disk-Halo Interface   in NGC891
  We present deep narrowband observations with high spatial resolution of extraplanar diffuse ionized gas in the halo of NGC891, obtained with the WFPC2 on-board the HST. Our H-alpha observations, centered on the northern part of NGC891, reveal an extended gaseous halo, which fills almost the entire field of view of our WFPC2 observation. The majority of the H-alpha emission is diffuse. Several discrete features (e.g., filaments) are visible as well. Some of the filaments reach distances of up to 2.2 kpc above the galactic plane, and are extremely collimated, even at high galactic latitudes. We compare the morphology of these filaments with theoretical models, which describe possible transport mechanisms in a general way. We also investigate extraplanar dust features, which are best visible in unsharp-masked images of our broadband F675W image, and we compare them to the spatial distribution of DIG filaments. Individual dust features, however, are not spatially correlated with diffuse ionized gas counterparts, such as individual filaments. Quite interestingly, the orientation of the dust features changes from being mostly aligned perpendicular to the disk at low galactic latitudes, to a parallel alignment at high |z|. We compare the diffuse ionized gas distribution to the hot ionized medium, traced by X-ray observations performed by Chandra. There exists a good correlation of the presence of the warm and hot ionized gas, in particular, an X-ray bright region at |z| ~ 1-1.5 kpc fills the entire northern halo region, whereas the intensity in the midplane is considerably depressed. We also compare the sizes of individual H-alpha emission line features in the midplane of NGC891 with similar structures that are known in our Milky Way and in the LMC. 
Joern Rossa (STScI), Ralf-Juergen Dettmar (AIRUB), Rene A. M.   Walterbos (NMSU), Colin A. Norman (JHU/STScI)