AHubble Space TelescopeWFPC2 Investigation of the Disk-Halo Interface in NGC 891

Abstract

We present deep narrowband observations with high spatial resolution of extraplanar diffuse ionized gas in the halo of NGC 891, obtained with the WFPC2 on board the HST. Our Hα observations, centered on the northern part of NGC 891, reveal an extended gaseous halo, which fills almost the entire field of view of our WFPC2 observation. Whereas NGC 891 has been studied extensively with ground-based telescopes, here the small-scale structure of the extended emission-line gas is presented at a high spatial resolution of 01, corresponding to 4.6 pc at the distance to NGC 891. The majority of the Hα 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. Despite the prominent dust lane, many bubbles, shells, and supershells can be discerned in the midplane. We also investigate extraplanar dust features, which are best visible in unsharp-masked images of our broadband F675W image, and we compare them with the spatial distribution of DIG filaments. The high-|z| dust is detected out to distances of 2.2 kpc above/below the galactic midplane. 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 Hα emission line features in the midplane of NGC 891 with similar structures that are known in our Milky Way and in the LMC.