Ultraviolet Radiative Transfer Modeling of Nearby Galaxies with Extraplanar Dusts

TL;DR

This study models the ultraviolet halos of six nearby galaxies to investigate the presence and properties of extraplanar dust, revealing that some UV halos may be caused by instrumental effects rather than actual dust layers.

Contribution

It introduces a three-dimensional Monte Carlo radiative transfer model to analyze UV halos, clarifies the presence of extraplanar dust in some galaxies, and discusses potential mechanisms for dust elevation.

Findings

Three galaxies show clear extraplanar dust layers.

Some UV halos may be due to PSF wing effects, not dust.

Extraplanar dust scale-height may be similar to PAH emission height.

Abstract

In order to examine their relation to the host galaxy, the extraplanar dust of six nearby galaxies are modeled, employing a three dimensional Monte Carlo radiative transfer code. The targets are from the highly-inclined galaxies that show dust-scattered ultraviolet halos, and the archival Galaxy Evolution Explorer FUV band images were fitted with the model. The observed images are in general well reproduced by two dust layers and one light-source layer, whose vertical and radial distributions have exponential profiles. We obtained several important physical parameters, such as star formation rate (SFR_UV), face-on optical depth, and scale-heights. Three galaxies (NGC 891, NGC 3628, and UGC 11794) show clear evidence for the existence of extraplanar dust layer. However, it is found that the rest three targets (IC 5249, NGC 24, and NGC 4173) do not necessarily need a thick dust disk to model the ultraviolet (UV) halo, because its contribution is too small and the UV halo may be caused by the wing part of the GALEX point spread function. This indicates that the galaxy samples reported to have UV halos may be contaminated by galaxies with negligible extraplanar (halo) dust. The galaxies showing evidence of the extraplanar dust layer fall within a narrow range on the scatter plots between physical parameters such as SFR_UV and extraplanar dust mass. Several mechanisms possible to produce the extraplanar dust are discussed. We also found a hint that the extraplanar dust scale-height might not be much different from the polycyclic aromatic hydrocarbon emission characteristic height.