||The Unusual and Ubiquitous Population of Hα Emitters at z ∼ 4: Where are all the Mergers?
||477, Galaxy Mergers in an Evolving Universe
||Shim, H.; Chary, R.-R.; Dickinson, M.; Lin, L.; Spinrad, H.; Stern, D.; Yan, C.-H.
||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.