Are Dusty Galaxies Blue? Insights on UV Attenuation from Dust-Selected Galaxies

TL;DR

This study investigates dust-obscured star-forming galaxies' UV properties, revealing their deviation from standard dust attenuation relations and suggesting they are dominated by short-lived burst events rather than steady star formation.

Contribution

It provides new empirical insights into the UV attenuation behavior of dusty, star-forming galaxies across different redshifts and luminosities, especially highlighting deviations at high star formation rates.

Findings

Galaxies with SFR > 50 M_sun/yr deviate from the IRX-β relation towards bluer colors.

Contamination of high-z dropout searches by dusty galaxies is less than 1%.

DSFGs are likely dominated by short-lived burst events, not steady-state star formation.

Abstract

Galaxies' rest-frame ultraviolet (UV) properties are often used to directly infer the degree to which dust obscuration affects the measurement of star formation rates. While much recent work has focused on calibrating dust attenuation in galaxies selected at rest-frame ultraviolet wavelengths, locally and at high-$z$, here we investigate attenuation in dusty, star-forming galaxies (DSFGs) selected at far-infrared wavelengths. By combining multiwavelength coverage across 0.15--500\,$\mu$m in the COSMOS field, in particular making use of {\it Herschel} imaging, and a rich dataset on local galaxies, we find a empirical variation in the relationship between rest-frame UV slope ($\beta$) and ratio of infrared-to-ultraviolet emission ($L_{\rm IR}/L_{\rm UV}\equiv\,IRX$) as a function of infrared luminosity, or total star formation rate, SFR. Both locally and at high-$z$, galaxies above SFR$\gt$50\,M$_\odot$\,yr$^{-1}$ deviate from the nominal $IRX-\beta$ relation towards bluer colors by a factor proportional to their increasing IR luminosity. We also estimate contamination rates of DSFGs on high-$z$ dropout searches of $\ll1$\%\ at $z\lt4-10$, providing independent verification that contamination from very dusty foreground galaxies is low in LBG searches. Overall, our results are consistent with the physical interpretation that DSFGs, e.g. galaxies with $>50$\,M$_\odot$\,yr$^{-1}$, are dominated at all epochs by short-lived, extreme burst events, producing many young O and B stars that are primarily, yet not entirely, enshrouded in thick dust cocoons. The blue rest-frame UV slopes of DSFGs are inconsistent with the suggestion that most DSFGs at $z\sim2$ exhibit steady-state star formation in secular disks.