z ~ 4 Hα Emitters in the Great Observatories Origins Deep Survey: Tracing the Dominant Mode for Growth of Galaxies

Abstract

We present evidence for strong Hα 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 μm bands, more than 70% of the galaxies show clear excess at 3.6 μm compared to the expected flux density from stellar continuum only. We provide evidence that this 3.6 μm excess is due to Hα emission redshifted into the 3.6 μm band, and classify these 3.6 μm excess galaxies to be Hα emitter (HAE) candidates. The selection of HAE candidates using an excess in broadband filters is sensitive to objects whose rest-frame Hα equivalent width (EW) is larger than 350 Å. The Hα inferred star formation rates (SFRs) of the HAEs range between 20 and 500 M_☉ yr^(–1) and are a factor of ~6 larger than SFRs inferred from the UV continuum. The ratio between the Hα luminosity and UV luminosity of HAEs is also on average larger than that of local starbursts. Possible reasons for such strong Hα 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 β and L_(Hα)/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 Hα emission appears to be star formation history. The Hα 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 ≳ 50% of the stellar mass density that is encompassed in massive (M_* > 10^(11) M_☉) galaxies at z ~ 3. This "strong Hα phase" of star formation plays a dominant role in galaxy growth at z ~ 4, and they are likely progenitors of massive red galaxies at lower redshifts.