Star-Forming or Starbursting? The Ultraviolet Conundrum

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-β diagram (the IR/UV ratio "IRX" versus the UV color "β"). 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 µm 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.

Dissecting the IRX–β dust attenuation relation: exploring the physical origin of observed variations in galaxies

“…Such variations originate from differences in star-formation history (SFH) of the respective regions. Boquien et al (2012) furthermore conclude that the shape of the attenuation curve can play a secondary role in describing deviations from the Meurer, Heckman & Calzetti (1999) relation. The latter is in agreement with our results of a flattening in the IRX -β relation as the FUV/NUV attenuation ratio increases.…”

“…The comparison between the Boquien et al results and our model is interesting, as sub-galactic regions are perhaps more similar to the screen of dust experiment presented in this work than integrated galaxy properties. Boquien et al (2012) find that for all except one galaxy in their sample, the IRX -β relation on sub-galactic scales is located below the Meurer, Heckman & Calzetti (1999) and Overzier et al (2011) relation for local (blue starburst) galaxies. Our results suggest that stellar age and differences in the dust attenuation curves may cause such variations.…”

“…The authors found that deviations from the Meurer, Heckman & Calzetti (1999) IRX -β relation are poorly described by age sensitive parameters such as the 4000Å break index, birthrate parameter, and the mass normalised age. Boquien et al (2012) find that variations can best be described by variations in the intrinsic UV colour in the absence of dust between different subgalactic regions. Such variations originate from differences in star-formation history (SFH) of the respective regions.…”

“…Metal poor systems such as the Magellanic clouds have redder colours than metal-rich systems Bell 2002). There are many other works that have explored variations in the IRX -β relation and their physical cause in local galaxies (e.g., Burgarella, Buat & Iglesias-Páramo 2005;Seibert et al 2005;Cortese et al 2006;Gil de Paz et al 2007;Johnson et al 2007;Panuzzo et al 2007;Cortese et al 2008;Siana et al 2008;Boquien et al 2009;Muñoz-Mateos et al 2009;Wijesinghe et al 2011;Boquien et al 2012;Ye et al 2016). High-redshift observations have demonstrated that dusty star-forming galaxies (DSFGs) have bluer UV continuum slopes for a given IRX (increasingly deviating from the Meurer et al relation as a function of IR luminosity, e.g., Oteo et al 2013;Casey et al 2014;Bourne et al 2017).…”

Encoding of the infrared excess in theNUVrKcolor diagram for star-forming galaxies

“…The dust attenuation curve encodes informations about the nature of the dust grains (sizes, chemical compositions) and the spatial distributions of dust and stars. Boquien et al (2009) have shown the necessity of adopting a range of attenuation laws to reproduce the observed scatter in the IRX vs. β relation. In particular, they show that a gray (i.e., shallow) attenuation curve, such as the starburst curve of Calzetti et al (2000), and a steeper curve, such the SMC extinction curve of Prevot et al (1984), are required to span the observed distribution in the IRX versus β diagram of the starbursting and normal star-forming galaxies.…”