The Dusty Circumgalactic Medium of NGC 891: Outflows and Accretion

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 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/yr/kpc2, indicating the threshold for wind formation is lower than previous work has suggested. 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 collisionof a minihalo with the disk of NGC 891. These observations show that multiplemechanisms transport dust into the circumgalactic medium.