The ALPINE-ALMA [CII] survey: Dust attenuation properties and obscured star formation at z ∼ 4.4-5.8

We present dust attenuation properties of spectroscopically confirmed star forming galaxies on the main sequence at a redshift of similar to 4.4-5.8. Our analyses are based on the far infrared continuum observations of 118 galaxies at rest-frame 158 mu m obtained with the Atacama Large Millimeter Array (ALMA) Large Program to INvestigate [CII] at Early times (ALPINE). We study the connection between the ultraviolet (UV) spectral slope (beta), stellar mass (M-star), and infrared excess (IRX = L-IR/L-UV). Twenty-three galaxies are individually detected in the continuum at > 3.5 sigma significance. We perform a stacking analysis using both detections and nondetections to study the average dust attenuation properties at z similar to 4.4-5.8. The individual detections and stacks show that the IRX-beta relation at z similar to 5 is consistent with a steeper dust attenuation curve than typically found at lower redshifts (z< 4). The attenuation curve is similar to or even steeper than that of the extinction curve of the Small Magellanic Cloud. This systematic change of the IRX-beta relation as a function of redshift suggests an evolution of dust attenuation properties at z> 4. Similarly, we find that our galaxies have lower IRX values, up to 1 dex on average, at a fixed mass compared to previously studied IRX-M-star relations at z less than or similar to 4, albeit with significant scatter. This implies a lower obscured fraction of star formation than at lower redshifts. Our results suggest that dust properties of UV-selected star forming galaxies at z greater than or similar to 4 are characterised by (i) a steeper attenuation curve than at z less than or similar to 4, and (ii) a rapidly decreasing dust obscured fraction of star formation as a function of redshift. Nevertheless, even among this UV-selected sample, massive galaxies (log M-star/M-circle dot> 10) at z similar to 5-6 already exhibit an obscured fraction of star formation of similar to 45%, indicating a rapid build-up of dust during the epoch of reionization.