Dust outflows from quiescent spiral disks

We have conducted a search for "dust chimneys" in a sample of 10 highly-inc lined spiral galaxies (i = 86 - 90 degrees) which we had previously observe d in the H alpha emission line (Rand 1996). We have procured B-band CCD ima ges for the purpose and employed unsharp-masking techniques to accentuate t he structure of the dust lane. A scattering+absorption radiation transfer m odel enabled us to separate 5 galaxies from the sample which are sufficient ly inclined (i > 87 degrees) for us to reliably identify and quantify dust clouds residing at over 2 scale-heights above the disk. Three of these gala xies possess numerous curvi-linear chimney structures stretching up to 2 kp c from the midplane and the fraction of total galactic dust contained in su ch structures is of order 1%. Optical extinction offers a lower limit to th e amount of dust contained in the extraplanar layer but, by examining the t ransparent submm thermal emission from NGC 891, we fix an upper limit of 5% . Our results are consistent with a similar recent study by Howk & Savage ( 1999) which indicates that about half of quiescent spiral disks possess det ectable dust chimneys. We have compared our optical images with the corresponding H alpha emission -line radiation. We do not find a detailed spatial correspondance between d ust chimneys and either sites of recent star-formation or the extraplanar d iffuse ionized gas. This is somewhat surprising given that FIR-bright galax ies, such as M 82, are known to entrain dust at the working surface of the starburst-driven outflow (traced in H alpha). It is possible a global corre lation exists, with disks experiencing overall higher rates of star-formati on also possessing the greatest number of chimneys. This may indicate a tim escale difference between the two phenomena with the H alpha phase lasting similar to 10(6) yr but chimneys requiring similar to 10(7) yr to form. Additionally, we have investigated the edge-on disk NGC 55 which, being ten times closer than galaxies in our main sample, allows us to examine in gre ater spatial detail the relationship between chimneys and recent star-forma tion. Our discussion touches upon high latitude dust and supershells observ ed in the Milky Way. We rule out quiescent disks as prolific sources of int ergalactic grains and metals but note that the rate at which dust is expell ed from the main dust layer is comparable to the rate at which it is produc ed by disk stars (suggesting that it may be an important regulatory process ).