Dust Properties of C ii Detected z ∼ 5.5 Galaxies: New HST/WFC3 Near-IR Observations

We examine the rest-frame ultraviolet (UV) properties of 10 [C II]λ158 μm-detected galaxies at z ~ 5.5 in COSMOS using new Hubble Space Telescope/Wide Field Camera 3 near-infrared imaging. Together with pre-existing 158 μm continuum and [C II] line measurements by the Atacama Large Millimeter/submillimeter Array, we study their dust attenuation properties on the IRX–β diagram, which connects the total dust emission (∝ IRX = log(L_(FIR)/L_(1600)) to the line-of-sight dust column (∝ β). We find systematically bluer UV continuum spectral slopes (β) compared to previous low-resolution ground-based measurements, which relieves some of the tension between models of dust attenuation and observations at high redshifts. While most of the galaxies are consistent with local starburst or Small Magellanic Cloud–like dust properties, we find galaxies with low IRX values and a large range in β that cannot be explained by models of a uniform dust distribution well mixed with stars. A stacking analysis of Keck/DEIMOS optical spectra indicates that these galaxies are metal-poor with young stellar populations that could significantly alter their spatial dust distribution.