Dust properties and distribution in dwarf galaxies

TL;DR

This study investigates dust extinction and distribution in dwarf galaxies NGC 1569 and NGC 4214, revealing shell-like structures with high local extinction, comparing dust tracers, and analyzing their relation to star-forming regions across different metallicities.

Contribution

It provides detailed mapping of dust extinction and emission in dwarf galaxies, highlighting the spatial variations and correlation with star formation indicators, and suggests dust shell formation around ionizing stars is metallicity-independent.

Findings

Large-scale extinction forms shells around HII regions.

Good agreement between dust emission tracers and extinction maps.

Extinction-corrected Halpha correlates with 24 extmu m emission regardless of metallicity.

Abstract

We present a study of the extinction, traced by the Balmer decrement, in HII regions in the dwarf galaxies NGC 1569 and NGC 4214. We find that the large-scale extinction around the most prominent HII regions in both galaxies forms a shell in which locally the intrinsic extinction can adopt relatively high values ($A_V = 0.8 - 0.9$ mag) despite the low metallicity and thus the low overall dust content. The small-scale extinction (spatial resolution $\sim$0.3'') shows fluctuations that are most likely due to variations in the dust distribution. We compare the distribution of the extinction to that of the dust emission, traced by Spitzer emission at 8 and 24 \mi, and to the emission of cold dust at 850 \mi. We find in general a good agreement between all tracers, expect for the 850 \mi emission in NGC 4214 which is more extended than the extinction and the other emissions. Whereas in NGC 1569 the dust emission at all wavelengths is very similar, NGC 4214 shows spatial variations in the 24-to-850 \mi ratio. We furthermore compared the 24 \mi and the extinction-corrected Halpha emission from HII regions in a sample of galaxies with a wide range of metallicities and found a good correlation between both emissions, independent of metallicity. We suggest that this lack of dependence on metallicity might be due to the formation of dust shells with a relatively constant opacity, like the ones observed here, around ionizing stars.