Exploring the Dust Content of Galactic Halos with Herschel III. NGC 891
  We present deep far-infrared observations of the nearby edge-on galaxy NGC
891 obtained with the Herschel Space Observatory and the Spitzer Space
Telescope. The maps confirm the detection of thermal emission from the inner
circumgalactic medium (halo) and spatially resolve a dusty superbubble and a
dust spur (filament). The dust temperature of the halo component is lower than
that of the disk but increases across a region of diameter ~8.0 kpc extending
at least 7.7 kpc vertically from one side of the disk, a region we call a
superbubble because of its association with thermal X-ray emission and a
minimum in the synchrotron scaleheight. This outflow is breaking through the
thick disk and developing into a galactic wind, which is of particular interest
because NGC 891 is not considered a starburst galaxy; the star formation rate
surface density, 0.03 Msun/year per square kiloparsec, and gas fraction, just
10% in the inner disk, indicate the threshold for wind formation is lower than
previous work has suggested. We conclude that the star formation surface
density is sufficient for superbubble blowout into the halo, but the cosmic ray
electrons may play a critical role in determining whether this outflow develops
into a fountain or escapes from the gravitational potential. The high
dust-to-gas ratio in the dust spur suggests the material was pulled out of NGC
891 through the collision of a minihalo with the disk of NGC 891. We conclude
that NGC 891 offers an example of both feedback and satellite interactions
transporting dust into the halo of a typical galaxy.
