The Flattening of Dust Attenuation Curve to z=2.5
  We examine the evolution of dust attenuation curve using a sample of 9504
disk star-forming galaxies (SFGs) selected from the CANDELS and 3D-HST surveys
and a new technique relying on the fact that disk SFGs of similar stellar
masses at the same cosmic epoch are statistically identical in stellar
populations. We attribute the discrepancy in median magnitude between face-on
(b/a>0.7) and edge-on (b/a<=0.4) subsamples solely to dust attenuation, and
obtain the average attenuation in the rest-frame UV and optical as functions of
stellar mass and redshift out to z=2.5. Our results show that the attenuation
curve becomes remarkably flatter at increasing redshift for both massive and
low-mass disk SFGs, and remains likely unchanged with galaxy stellar mass at a
fixed epoch within uncertainties. Compared with the Calzetti law, our dust
attenuation curves appear to be slightly steeper at 0.5<z<1.4 and remarkably
flatter at 1.4<z<2.5. Our findings are consistent with a picture in which the
evolution of dust grain size distribution is mainly responsible for the
evolution of the dust attenuation curve in SFGs; dust shattering becomes a
dominant process at z<~1.4, resulting in an enrichment of small dust grains and
consequently a steeper attenuation curve. We stress that extinction correction
for high-z galaxies should be done using mass- and redshift-dependent
attenuation curves.
