Mean Halpha+[NII]+[SII] EW Inferred for Star-Forming Galaxies at
  z=5.1-5.4 Using High-Quality Spitzer/IRAC Photometry
  Recent Spitzer/InfraRed Array Camera (IRAC) photometric observations have
revealed that rest-frame optical emission lines contribute signficantly to the
broadband fluxes of high-redshift galaxies. Specifically, in the narrow
redshift range z~5.1-5.4 the [3.6]-[4.5] color is expected to be very red, due
to contamination of the 4.5-micron band by the dominant Halpha line, while the
3.6-micron filter is free of nebular emission lines. We take advantage of new
reductions of deep Spitzer/IRAC imaging over the GOODS-North+South fields
(Labbe+2015) to obtain a clean measurement of the mean Halpha equivalent width
from the [3.6]-[4.5] color in the redshift range z=5.1-5.4. The selected
sources either have measured spectroscopic redshifts (13 sources) or lie very
confidently in the redshift range z=5.1-5.4 based on the photometric redshift
likelihood intervals (11 sources). Our z_{phot}=5.1-5.4 sample and
z_{spec}=5.10-5.40 spectroscopic sample have a mean [3.6]-[4.5] color of
0.31+/-0.05 mag and 0.35+/-0.07 mag, implying a rest-frame equivalent width
EW(Halpha+[NII]+[SII]) of 665+/-53 Angstroms and 707+/-74 Angstroms,
respectively, for sources in these samples. These values are consistent albeit
slightly higher than derived by Stark+2013 at z~4, suggesting an evolution to
higher values of the Halpha+[NII]+[SII] EW at z>2. Using the 3.6micron band,
which is free of emission line contamination, we perform robust SED fitting and
find a median specific star formation rate of sSFR = 17_{-5}^{+2} Gyr^{-1},
7_{-2}^{+1}x higher than at z~2. We find no strong correlation (<2sigma)
between the Halpha+[NII]+[SII] EW and the stellar mass of sources. Before the
advent of JWST, improvements in these results will come through an expansion of
current spectroscopic samples and deeper Spitzer/IRAC measurements.
