  In previous studies, it has been shown that the large line widths observed in
high surface brightness H-alpha emitters at low and high redshifts are likely
due to the mechanical energy injected by intense star formation. Here we
discuss the possibility that the high surface brightnesses observed are not due
to star formation, but due to cosmological gas accretion. We assume that all of
the accretion energy is dissipated as shocks from the accreting gas. We show
that in order to explain the high surface brightnesses both the mass accretion
rate and energy would have to be much higher than expected from simulations or
from equating the star formation with the accretion rate. We also investigate
scaling relations between the surface brightness expected from accretion and
for star formation through mechanical heating and photo-ionization, trying to
identify a regime where such accretion may become evident in galaxies.
Unfortunately, the surface brightness necessary to detect the gas in optical
line emission is about an order of magnitude lower than what has currently been
achieved with near-infrared observations of distant galaxies.
