Mapping dust through emission and absorption in nearby galaxies

TL;DR

This study combines Herschel infrared imaging and optical spectroscopy to map dust in nearby galaxies, revealing complex dust distribution effects on attenuation measurements and providing empirical relations between dust and reddening.

Contribution

It introduces high-resolution dust maps from Herschel data alongside optical reddening maps, highlighting the limitations of stellar reddening as a dust tracer and establishing empirical relations for dust attenuation.

Findings

Stellar continuum reddening poorly traces overall dust content.

Bright HII regions are often in dusty areas.

Empirical relation between Balmer reddening and dust surface density.

Abstract

Dust has long been identified as a barrier to measuring inherent galaxy properties. However, the link between dust and attenuation is not straightforward and depends on both the amount of dust and its distribution. Herschel imaging of nearby galaxies undertaken as part of the KINGFISH project allows us to map the dust as seen in emission with unprecedented sensitivity and ~1 kpc resolution. We present here new optical integral field unit spectroscopy for eight of these galaxies that provides complementary 100-200 pc scale maps of the dust attenuation through observation of the reddening in both the Balmer decrement and the stellar continuum. The stellar continuum reddening, which is systematically less than that observed in the Balmer decrement, shows no clear correlation with the dust, suggesting that the distribution of stellar reddening acts as a poor tracer of the overall dust content. The brightest HII regions are observed to be preferentially located in dusty regions, and we do find a correlation between the Balmer line reddening and the dust mass surface density for which we provide an empirical relation. Some of the high-inclination systems in our sample exhibit high extinction, but we also find evidence that unresolved variations in the dust distribution on scales smaller than 500 pc may contribute to the scatter in this relation. We caution against the use of integrated A_V measures to infer global dust properties.