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 ~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_*), 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~4.4-5.8. The individual detections and stacks
show that the IRX-$\beta$ relation at z~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 (SMC). 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 1dex on average, at a fixed mass compared to previously
studied IRX-M_* relations at z<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>4 are characterised by (i) a steeper attenuation curve than at z<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_*/$M_\odot$>10) at z~5-6 already exhibit an obscured fraction of star
formation of ~45%, indicating a rapid build-up of dust during the epoch of
reionization.
