Evidence for a wide range of UV obscuration in z ~ 2 dusty galaxies from the GOODS-Herschel survey

TL;DR

This study investigates the causes of wide UV obscuration in z ~ 2 dusty galaxies, revealing that the diversity is mainly due to varying dust content and spatial distribution, with most galaxies showing coincident IR and UV emitting regions.

Contribution

It provides observational evidence that the UV faintness in dusty galaxies at z ~ 2 is primarily caused by differing dust obscuration levels, challenging previous assumptions about mid-IR brightness.

Findings

Most DOGs are not abnormally bright in mid-IR compared to similar galaxies.

A relation exists between IR/UV luminosity ratios and UV continuum indices.

UV obscuration differences are due to dust distribution or total dust content.

Abstract

Dusty galaxies at z ~ 2 span a wide range of relative brightness between rest-frame mid-infrared (8um) and ultraviolet wavelengths. We attempt to determine the physical mechanism responsible for this diversity. Dust-obscured galaxies (DOGs), which have rest-frame mid-IR to UV flux density ratios > 1000, might be abnormally bright in the mid-IR, perhaps due to prominent AGN and/or PAH emission, or abnormally faint in the UV. We use far-infrared data from the GOODS-Herschel survey to show that most DOGs with 10^12 L_Sun < L_IR < 10^13 L_Sun are not abnormally bright in the mid-IR when compared to other dusty galaxies with similar IR (8--1000um) luminosities. We observe a relation between the median IR to UV luminosity ratios and the median UV continuum power-law indices for these galaxies, and we find that only 24% have specific star formation rates which indicate the dominance of compact star-forming regions. This circumstantial evidence supports the idea that the UV- and IR-emitting regions in these galaxies are spatially coincident, which implies a connection between the abnormal UV faintness of DOGs and dust obscuration. We conclude that the range in rest-frame mid-IR to UV flux density ratios spanned by dusty galaxies at z ~ 2 is due to differing amounts of UV obscuration. Of galaxies with these IR luminosities, DOGs are the most obscured. We attribute differences in UV obscuration to either: 1) differences in the degree of alignment between the spatial distributions of dust and massive stars, or 2) differences in the total dust content.