Exploring the Dust Content of Galactic Winds with Herschel. I. NGC 4631

TL;DR

This study uses Herschel far-infrared observations to analyze dust structures and their relation to star formation and tidal interactions in the edge-on galaxy NGC 4631, revealing complex extraplanar features and dust properties.

Contribution

It provides a detailed multi-wavelength analysis of dust morphology and properties in NGC 4631, highlighting the disk-halo connection and potential tidal origins of extraplanar dust.

Findings

Filamentary and chimney-like dust features extend up to 6 kpc above the galaxy plane.

Elevated dust temperatures and spectral indices are found in the central superbubble region.

Extraplanar dust may originate from tidal interactions or past star formation activity.

Abstract

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 PACS images at 70 and 160 um 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 Halpha, 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 larger scale in the SPIRE 250, 350, and 500 um data matches the extraplanar H~I streams previously reported in NGC 4631 and suggests a tidal origin. The PACS 70/160 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 (beta > 2) 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.