Illuminating the Incidence of Extraplanar Dust Using Ultraviolet Reflection Nebulae with GALEX

Abstract

Circumgalactic dust grains trace the circulation of mass and metals between star-forming regions and gaseous galactic halos, giving insight into feedback and tidal stripping processes. We perform a search for ultraviolet (UV) reflection nebulae produced by extraplanar dust around 551 nearby ( D < 100 Mpc), edge-on disk galaxies using archival near-ultraviolet and far-ultraviolet images from the Galaxy Evolution Explorer (GALEX), accounting for the point-spread function (FWHM = 4″–5″). We detect extraplanar emission ubiquitously in stacks of galaxies binned by morphology and star formation rate, with scale heights of h _h = 1–2.3 kpc and ≈10% of the total (reddened) flux in the galaxy found beyond the B -band isophotal level of μ _B = 25 mag arcsec ^−2 . This emission is detected in 7% of the individual galaxies, and an additional one-third have at least 5% of their total flux found beyond μ _B = 25 mag arcsec ^−2 in a disk component. The extraplanar luminosities and colors are consistent with reflection nebulae rather than stellar halos and indicate that, on average, disk galaxies have an extraplanar dust mass of 5%–15% of that in their interstellar medium. This suggests that recycled material composes at least a third of the inner circumgalactic medium ( R < 10 kpc) in ∼ L * galaxies.

Keywords

• Circumgalactic medium
• Disk galaxies
• Galaxy fountains
• Interstellar dust
• Stellar feedback