Star-Forming or Starbursting? The Ultraviolet Conundrum

TL;DR

This study investigates the causes of dispersion in the IRX-beta diagram for star-forming regions, finding that dust geometry and extinction laws are key factors, with implications for understanding galaxy dust attenuation.

Contribution

The paper provides new insights into the causes of IRX-beta dispersion by analyzing individual star-forming regions and highlighting the role of dust properties and geometries.

Findings

Dispersion not correlated with stellar age.

Range of dust geometries and extinction curves cause dispersion.

Positive correlation between dust attenuation and metal content.

Abstract

Compared to starburst galaxies, normal star forming galaxies have been shown to display a much larger dispersion of the dust attenuation at fixed reddening through studies of the IRX-beta diagram (the IR/UV ratio "IRX" versus the UV color "beta"). To investigate the causes of this larger dispersion and attempt to isolate second parameters, we have used GALEX UV, ground-based optical, and Spitzer infrared imaging of 8 nearby galaxies, and examined the properties of individual UV and 24 micron selected star forming regions. We concentrated on star-forming regions, in order to isolate simpler star formation histories than those that characterize whole galaxies. We find that 1) the dispersion is not correlated with the mean age of the stellar populations, 2) a range of dust geometries and dust extinction curves are the most likely causes for the observed dispersion in the IRX-beta diagram 3) together with some potential dilution of the most recent star-forming population by older unrelated bursts, at least in the case of star-forming regions within galaxies, 4) we also recover some general characteristics of the regions, including a tight positive correlation between the amount of dust attenuation and the metal content. Although generalizing our results to whole galaxies may not be immediate, the possibility of a range of dust extinction laws and geometries should be accounted for in the latter systems as well.