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We investigate the evolution of the Halpha equivalent width, EW(Halpha), withredshift and its dependence on stellar mass, taking advantage of the first datafrom the 3D-HST survey, a large spectroscopic Treasury program with the HubbleSpace Telescope WFC3. Combining our Halpha measurements of 854 galaxies at0.8<z<1.5 with those of ground based surveys at lower and higher redshift, wecan consistently determine the evolution of the EW(Halpha) distribution fromz=0 to z=2.2. We find that at all masses the characteristic EW(Halpha) isdecreasing 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 amodel-independent confirmation of the evolution of star forming galaxies withredshift. A quantitative conversion of EW(Halpha) to sSFR is very modeldependent, because of differential reddening corrections between the continuumSED and the Balmer lines. The observed EW(Halpha) can be reproduced with asimple model in which the SFR for galaxies rises to the epoch of z~2.5 and thendecreases with time to z = 0. The model implies that the EW(Halpha) rises to400 A at z=8. The sSFR evolves faster than EW(Halpha), as the mass-to-lightratio also evolves with redshift. In this context, we find that the sSFRevolves as (1+z)^(3.2), nearly independent of mass, consistent with previousreddening insensitive estimates. We confirm previous results that the observedslope of the sSFR-z relation is steeper than the one predicted by models, butmodels and observations agree in finding little mass dependence.

Halpha Equivalent Widths from the 3D-HST survey: evolution with redshift and dependence on stellar mass