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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.