z {approx} 4 H{alpha} EMITTERS IN THE GREAT OBSERVATORIES ORIGINS DEEP SURVEY: TRACING THE DOMINANT MODE FOR GROWTH OF GALAXIES
- Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125 (United States)
- National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719 (United States)
- Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 106, Taiwan (China)
- Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720 (United States)
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
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 A. The H{alpha} inferred star formation rates (SFRs) of the HAEs range between 20 and 500 M{sub sun} yr{sup -1} and are a factor of {approx}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{sub H{alpha}}/L{sub 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 {approx}> 50% of the stellar mass density that is encompassed in massive (M{sub *} > 10{sup 11} M{sub sun}) galaxies at z {approx} 3. This 'strong H{alpha} phase' of star formation plays a dominant role in galaxy growth at z {approx} 4, and they are likely progenitors of massive red galaxies at lower redshifts.
- OSTI ID:
- 21582910
- Journal Information:
- Astrophysical Journal, Vol. 738, Issue 1; Other Information: DOI: 10.1088/0004-637X/738/1/69; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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