Ultraviolet Halos Around Spiral Galaxies. I. Morphology

TL;DR

This study investigates ultraviolet halos around spiral galaxies, revealing their morphology and suggesting they are likely reflection nebulae caused by dust scattering, with variations observed in starburst galaxies.

Contribution

It provides the first detailed morphological analysis of UV halos around a diverse sample of spiral galaxies, highlighting differences in starburst galaxies and proposing a reflection nebula origin.

Findings

UV halos are present around all sampled galaxies.

Starburst galaxies exhibit more extensive, filamentary UV halos.

UV halos likely originate from dust scattering, not outflows.

Abstract

We examine ultraviolet halos around a sample of highly inclined galaxies within 25 Mpc to measure their morphology and luminosity. Despite contamination from galactic light scattered into the wings of the point-spread function, we find that UV halos occur around each galaxy in our sample. Around most galaxies the halos form a thick, diffuse disk-like structure, but starburst galaxies with galactic superwinds have qualitatively different halos that are more extensive and have filamentary structure. The spatial coincidence of the UV halos above star-forming regions, the lack of consistent association with outflows or extraplanar ionized gas, and the strong correlation between the halo and galaxy UV luminosity suggest that the UV light is an extragalactic reflection nebula. UV halos may thus represent 1-10 million solar masses of dust within 2-10 kpc of the disk, whose properties may change with height in starburst galaxies.