Halpha Equivalent Widths from the 3D-HST survey: evolution with redshift
  and dependence on stellar mass
  We investigate the evolution of the Halpha equivalent width, EW(Halpha), with
redshift and its dependence on stellar mass, taking advantage of the first data
from the 3D-HST survey, a large spectroscopic Treasury program with the Hubble
Space Telescope WFC3. Combining our Halpha measurements of 854 galaxies at
0.8<z<1.5 with those of ground based surveys at lower and higher redshift, we
can consistently determine the evolution of the EW(Halpha) distribution from
z=0 to z=2.2. We find that at all masses the characteristic EW(Halpha) is
decreasing towards the present epoch, and that at each redshift the EW(Halpha)
is lower for high-mass galaxies. We measure a slope of EW(Halpha) ~ (1+z)^(1.8)
with little mass dependence. Qualitatively, this measurement is a
model-independent confirmation of the evolution of star forming galaxies with
redshift. A quantitative conversion of EW(Halpha) to sSFR is very model
dependent, because of differential reddening corrections between the continuum
SED and the Balmer lines. The observed EW(Halpha) can be reproduced with a
simple model in which the SFR for galaxies rises to the epoch of z~2.5 and then
decreases with time to z = 0. The model implies that the EW(Halpha) rises to
400 A at z=8. The sSFR evolves faster than EW(Halpha), as the mass-to-light
ratio also evolves with redshift. In this context, we find that the sSFR
evolves as (1+z)^(3.2), nearly independent of mass, consistent with previous
reddening insensitive estimates. We confirm previous results that the observed
slope of the sSFR-z relation is steeper than the one predicted by models, but
models and observations agree in finding little mass dependence.
