z ∼ 4 Hα EMITTERS IN THE GREAT OBSERVATORIES ORIGINS DEEP SURVEY: TRACING THE DOMINANT MODE FOR GROWTH OF GALAXIES

We present evidence for strong H alpha emission in galaxies with spectroscopic redshifts in the range of 3.8 < z < 5.0 over the Great Observatories Origins Deep Survey fields. Among 74 galaxies detected in the Spitzer IRAC 3.6 and 4.5 mu m bands, more than 70% of the galaxies show clear excess at 3.6 mu m compared to the expected flux density from stellar continuum only. We provide evidence that this 3.6 mu m excess is due to H alpha emission redshifted into the 3.6 mu m band, and classify these 3.6 mu m excess galaxies to be H alpha emitter (HAE) candidates. The selection of HAE candidates using an excess in broadband filters is sensitive to objects whose rest-frame H alpha equivalent width (EW) is larger than 350 angstrom. The H alpha inferred star formation rates (SFRs) of the HAEs range between 20 and 500 M-circle dot yr (1) and are a factor of similar to 6 larger than SFRs inferred from the UV continuum. The ratio between the H alpha luminosity and UV luminosity of HAEs is also on average larger than that of local starbursts. Possible reasons for such strong H alpha emission in these galaxies include different dust extinction properties, young stellar population ages, extended star formation histories, low metallicity, and a top-heavy stellar initial mass function. Although the correlation between UV slope beta and L-H alpha/L-UV raises the possibility that HAEs prefer a dust extinction curve which is steeper in the UV, the most dominant factor that results in strong H alpha emission appears to be star formation history. The H alpha EWs of HAEs are large despite their relatively old stellar population ages constrained by spectral energy distribution fitting, suggesting that at least 60% of HAEs produce stars at a constant rate. Under the assumption that the gas supply is sustained, HAEs are able to produce greater than or similar to 50% of the stellar mass density that is encompassed in massive (M-* > 10(11) M-circle dot) galaxies at z similar to 3. This strong H alpha phase of star formation plays a dominant role in galaxy growth at z similar to 4, and they are likely progenitors of massive red galaxies at lower redshifts.