  We present the results of a study which uses spectral energy distribution
(SED) fitting to investigate the evolution of the equivalent width (EW) of the
Halpha emission line in star-forming galaxies over the redshift interval 1<z<5.
After first demonstrating the ability of our SED-fitting technique to recover
EW(Ha) using a sample of galaxies at z~1.3 with EW(Ha) measurements from 3D-HST
grism spectroscopy, we proceed to apply our technique to samples of
spectroscopically confirmed and photometric-redshift selected star-forming
galaxies at z>=1 in the CANDELS UDS and GOODS-S fields. Confining our analysis
to a constant stellar mass range (9.5<log(M/Msun)<10.5), we find that the
median EW(Ha) evolves only modestly with redshift, reaching a rest-frame value
of EW(Ha)=301+/-30 Angs by redshift z~4.5. Furthermore, using estimates of
star-formation rate (SFR) based on both UV luminosity and Ha line flux, we use
our galaxy samples to compare the evolution of EW(Ha) and specific
star-formation rate (sSFR). Our results indicate that over the redshift range
1<z<5, the evolution displayed by EW(Ha) and sSFR is consistent, and can be
adequately parameterized as: propto (1+z)^(1.0+/-0.2). As a consequence, over
this redshift range we find that the sSFR and rest-frame EW(Ha) of star-forming
galaxies with stellar masses M~10^(10) Msun are related by:
EW(Ha)/Ang=(63+/-7)sSFR/Gyr^(-1). Given the current uncertainties in measuring
the SFRs of high-redshift galaxies, we conclude that EW(Ha) provides a useful
independent tracer of sSFR for star-forming galaxies out to redshifts of z=5.
