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 H \alpha 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 ( H \alpha ) using a sample of galaxies at z \simeq 1.3 with EW ( H \alpha ) 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 \geq 1 in the CANDELS UDS and GOODS-S fields .
Confining our analysis to a constant stellar mass range ( 9.5 < \log ( M _ { \star } / { M } _ { \odot } ) < 10.5 ) , we find that the median EW ( H \alpha ) evolves only modestly with redshift , reaching a rest-frame value of EW ( H \alpha ) = 301 \pm { 30 } Å by redshift z \simeq 4.5 .
Furthermore , using estimates of star-formation rate ( SFR ) based on both UV luminosity and H \alpha line flux , we use our galaxy samples to compare the evolution of EW ( H \alpha ) and specific star-formation rate ( sSFR ) .
Our results indicate that over the redshift range 1 < z < 5 , the evolution displayed by EW ( H \alpha ) and sSFR is consistent , and can be adequately parameterized as : \propto ( 1 + z ) ^ { 1.0 \pm 0.2 } .
As a consequence , over this redshift range we find that the sSFR and rest-frame EW ( H \alpha ) of star-forming galaxies with stellar masses M _ { \star } \simeq 10 ^ { 10 } \mbox { $ M _ { \odot } $ } are related by : EW ( H \alpha ) /Å = ( 63 \pm 7 ) \times sSFR/Gyr ^ { -1 } .
Given the current uncertainties in measuring the SFRs of high-redshift galaxies , we conclude that EW ( H \alpha ) provides a useful independent tracer of sSFR for star-forming galaxies out to redshifts of z = 5 .