z ~ 4 Hα Emitters in the Great Observatories Origins Deep Survey: Tracing the Dominant Mode for Growth of Galaxies
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.
Additional Information© 2011 The American Astronomical Society. Received 2011 March 18; accepted 2011 June 14; published 2011 August 12. We thank the entire GOODS team for their effort in compiling and reducing different components of the data. We thank Rychard Bouwens and Avishai Dekel for helpful comments and discussions and the anonymous referee for a thorough referee report. This work is based, in part, on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Part of the NIR data are based on observations obtained with WIRCam, a joint project of CFHT, Taiwan, Korea, Canada, France, at the Canada–France–Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.
Published - Shim2011p15685Astrophys_J.pdf