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-UV (NUV) and far-UV (FUV) images from 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 $h_{\text{h}} = 1 - 2.3$ kpc and $\approx 10\%$ of the total (reddened) flux in the galaxy found beyond the B-band isophotal level of $\mu_{\text{B}} = 25$ 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 $\mu_{\text{B}} = 25$ 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 $\sim L^{*}$ galaxies.