The Unusual and Ubiquitous Population of Hα Emitters at z ∼ 4: Where are all the Mergers?

Abstract

We present the evidence that strong Hα emitters (HAEs) comprise more than 70 % of galaxies with spectroscopic redshifts of 3.8 < z < 5.0, from the clear flux excess observed in the Spitzer/IRAC 3.6-μm band. HAEs are strongly star-forming galaxies with star formation rate of 20–500 M☉yr^(–1), with unusually large Hα equivalent widths (EW) of > 350 Å. The rest-frame UV to optical morphologies for half of the HAEs do not show any signs of mergers or tidal interactions. The large Hα EW of HAEs despite their relatively old stellar population age suggests that HAEs are producing stars continuously at a constant rate, rather than through multiple bursts invoked by major mergers. The observed number density of HAEs is consistent with the predicted number of halos which have gas infall rate comparable to the observed star formation rate. Therefore, HAEs provide indirect evidence supporting that star formation mechanism other than mergers, such as cold gas accretion, is the dominant driver of star formation at z ∼ 4.

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