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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.

We present evidence for unusually strong Halpha emission in galaxies with spectroscopic redshifts in the range of 3.810^11 Msun) galaxies. This 'strong Halpha phase' of star formation plays a dominant role in early galaxy growth, being a feasible progenitors of massive red galaxies at lower redshifts., Comment: Accepted for publication in ApJ

We report narrowband imaging of the 1–0S(1) ro-vibrational transition of molecular hydrogen (H2) from NGC 1275, the central cD galaxy of the Perseus Cluster. We find that the H2 gas has a spatial morphology identical to the optical emission-line nebula associated with this galaxy, a total luminosity in H2 1–0 S(1) only an order of magnitude less than in Hα, and if the line-emitting gas is thermalized a mass (at ∼2000 K) that is over two orders of magnitude smaller than that of the optical emission-line nebula (at ∼10,000 K). The ratio in H2 1–0 S(1) to Hα +[ Nii] line intensities spans a characteristic range of ∼0.02–0.08 throughout the nebula; the brighter inner nebula exhibits patches with (nearly) constant line ratios unrelated to individual filaments. Recent models proposed to explain the peculiar nebular spectrum from the optical to infrared invoke thermalized along with non-thermalized injection of energy from ionizing particles. The energy density of highly relativistic electrons inferred to cause inverse-Compton scattering of hard X-ray emission from the core of the Perseus Cluster decreases steeply beyond a central radius of ∼20 kpc, yet we do not find any changes in the average or range spanned by the H2 1–0 S(1) to Hα +[ Nii] line ratio between the inner (20 kpc) and outer (∼20–50 kpc) nebulae. On the other hand, saturated conduction from the surrounding X-ray gas produces, in the absence of magnetic fields, a heat flux that is approximately constant throughout the nebula: the change in the line ratio with position would then reflect the ability of the X-ray gas to penetrate presumably magnetically threaded filaments at different locations.