Comparison of the extraplanar H$\alpha$ and UV emissions in the halos of nearby edge-on spiral galaxies

TL;DR

This study compares extraplanar Hα and UV emissions in 38 edge-on galaxies, revealing a strong correlation and suggesting a multiphase nature of halo gas and dust, with implications for understanding galactic halos and star formation.

Contribution

It demonstrates a consistent correlation between extraplanar Hα and UV emissions and highlights the role of extraplanar dust in scattering Hα photons, advancing knowledge of galactic halo composition.

Findings

Extraplanar dust presence coincides with Hα emission.

Scale heights of Hα and UV emissions are strongly correlated.

Extraplanar dust properties extend to more active galaxies with galactic winds.

Abstract

We compare vertical profiles of the extraplanar H$\alpha$ emission to those of the UV emission for 38 nearby edge-on late-type galaxies. It is found that detection of the "diffuse" extraplanar dust (eDust), traced by the vertically extended, scattered UV starlight, always coincides with the presence of the extraplanar H$\alpha$ emission. A strong correlation between the scale heights of the extraplanar H$\alpha$ and UV emissions is also found; the scale height at H$\alpha$ is found to be $\sim0.74$ of the scale height at FUV. Our results may indicate the multiphase nature of the diffuse ionized gas and dust in the galactic halos. The existence of eDust in galaxies where the extraplanar H$\alpha$ emission is detected suggests that a larger portion of the extraplanar H$\alpha$ emission than that predicted in previous studies may be caused by H$\alpha$ photons that originate from H II regions in the galactic plane and are subsequently scattered by the eDust. This possibility raise a in studying the eDIG. We also find that the scale heights of the extraplanar emissions normalized to the galaxy size correlate well with the star formation rate surface density of the galaxies. The properties of eDust in our galaxies is on a continuation line of that found through previous observations of the extraplanar polycyclic aromatic hydrocarbons emission in more active galaxies known to have galactic winds.