The Star-formation Mass Sequence out to z=2.5
  We study the star formation rate (SFR) - stellar mass (M*) relation in a
self-consistent manner from 0 < z < 2.5 with a sample of galaxies selected from
the NEWFIRM Medium-Band Survey. We find a significant non-linear slope of the
relation, SFR \propto M*^0.6, and a constant observed scatter of 0.34 dex,
independent of redshift and M*. However, if we select only blue galaxies we
find a linear relation SFR \propto M*, similar to previous results at z = 0 by
Peng et al. (2010). This selection excludes red, dusty, star-forming galaxies
with higher masses, which brings down the slope. By selecting on L_IR/L_UV (a
proxy for dust obscuration) and the rest-frame U-V colors, we show that
star-forming galaxies fall in three distinct regions of the log(SFR)-log(M*)
plane: 1) actively star-forming galaxies with "normal" dust obscuration and
associated colors (54% for log(M*) > 10 at 1 < z < 1.5), 2) red star-forming
galaxies with low levels of dust obscuration and low specific SFRs (11%), and
3) dusty, blue star-forming galaxies with high specific SFRs (7%). The
remaining 28% comprises quiescent galaxies. Galaxies on the "normal" star
formation sequence show strong trends of increasing dust attenuation with
stellar mass and a decreasing specific SFR, with an observed scatter of 0.25
dex (0.17 dex intrinsic scatter). The dusty, blue galaxies reside in the upper
envelope of the star formation sequence with remarkably similar spectral shapes
at all masses, suggesting that the same physical process is dominating the
stellar light. The red, low-dust star-forming galaxies may be in the process of
shutting off and migrating to the quiescent population.
