Demographics of Star-forming Galaxies since $z\sim2.5$. I. The $UVJ$
  Diagram in CANDELS
  This is the first in a series of papers examining the demographics of
star-forming galaxies at $0.2<z<2.5$ in CANDELS. We study 9,100 galaxies from
GOODS-S and UDS having published values of redshifts, masses, star-formation
rates (SFRs), and dust attenuation ($A_V$) derived from UV-optical SED fitting.
In agreement with previous works, we find that the $UVJ$ colors of a galaxy are
closely correlated with its specific star-formation rate (SSFR) and $A_V$. We
define rotated $UVJ$ coordinate axes, termed $S_\mathrm{SED}$ and
$C_\mathrm{SED}$, that are parallel and perpendicular to the star-forming
sequence and derive a quantitative calibration that predicts SSFR from
$C_\mathrm{SED}$ with an accuracy of ~0.2 dex. SFRs from UV-optical fitting and
from UV+IR values based on Spitzer/MIPS 24 $\mu\mathrm{m}$ agree well overall,
but systematic differences of order 0.2 dex exist at high and low redshifts. A
novel plotting scheme conveys the evolution of multiple galaxy properties
simultaneously, and dust growth, as well as star-formation decline and
quenching, exhibit "mass-accelerated evolution" ("downsizing"). A population of
transition galaxies below the star-forming main sequence is identified. These
objects are located between star-forming and quiescent galaxies in $UVJ$ space
and have lower $A_V$ and smaller radii than galaxies on the main sequence.
Their properties are consistent with their being in transit between the two
regions. The relative numbers of quenched, transition, and star-forming
galaxies are given as a function of mass and redshift.
