The Origins of UV-optical Color Gradients in Star-forming Galaxies at z
  ~ 2: Predominant Dust Gradients But Negligible sSFR Gradients
  The rest-frame UV-optical (i.e., $NUV-B$) color is sensitive to both
low-level recent star formation (specific star formation rate - sSFR) and dust.
In this Letter, we extend our previous work on the origins of $NUV-B$ color
gradients in star-forming galaxies (SFGs) at $z\sim1$ to those at $z\sim2$. We
use a sample of 1335 large (semi-major axis radius $R_{\rm SMA}>0.''18$) SFGs
with extended UV emission out to $2R_{\rm SMA}$ in the mass range $M_{\ast} =
10^{9}-10^{11}M_{\odot}$ at $1.5<z<2.8$ in the CANDELS/GOODS-S and UDS fields.
We show that these SFGs generally have negative $NUV-B$ color gradients (redder
centres), and their color gradients strongly increase with galaxy mass. We also
show that the global rest-frame $FUV-NUV$ color is approximately linear with
$A_{\rm V}$, which is derived by modeling the observed integrated FUV to NIR
spectral energy distributions of the galaxies. Applying this integrated
calibration to our spatially-resolved data, we find a negative dust gradient
(more dust extinguished in the centers), which steadily becomes steeper with
galaxy mass. We further find that the $NUV-B$ color gradients become nearly
zero after correcting for dust gradients regardless of galaxy mass. This
indicates that the sSFR gradients are negligible and dust reddening is likely
the principal cause of negative UV-optical color gradients in these SFGs. Our
findings support that the buildup of the stellar mass in SFGs at the Cosmic
Noon is self-similar inside $2R_{\rm SMA}$.
