Abstract
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.
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