Exploring the dust content of galactic winds with Herschel. I. NGC 4631

We present a detailed analysis of deep far-infrared observations of the nearby edge-on star-forming galaxy NGC 4631 obtained with the Herschel Space Observatory. Our Photodetector Array Camera and Spectrometer (PACS) images at 70 and 160 μm show a rich complex of filaments and chimney-like features that extends up to a projected distance of 6 kpc above the plane of the galaxy. The PACS features often match extraplanar H, radio-continuum, and soft X-ray features observed in this galaxy, pointing to a tight disk-halo connection regulated by star formation. On the other hand, the morphology of the colder dust component detected on a larger scale in the Spectral and Photometric Imaging Receiver 250, 350, and 500 μm data matches the extraplanar H I streams previously reported in NGC 4631 and suggests a tidal origin. The PACS 70/160 μm ratios are elevated in the central ∼3.0 kpc region above the nucleus of this galaxy (the "superbubble"). A pixel-by-pixel analysis shows that dust in this region has a higher temperature and/or an emissivity with a steeper spectral index than the dust in the disk, possibly the result of the harsher environment in the superbubble. Star formation in the disk seems energetically insufficient to lift the material out of the disk, unless it was more active in the past or the dust-to-gas ratio in the superbubble region is higher than the Galactic value. Some of the dust in the halo may also have been tidally stripped from nearby companions or lifted from the disk by galaxy interactions