Sizing up Lyman-alpha and Lyman Break Galaxies
  We show that populations of high redshift galaxies grow more luminous as they
grow in linear size. This is because the brightness per unit area has a
distinct upper limit due to the self-regulating nature of star-formation. As a
corollary, we show that the observed increase in characteristic luminosity of
Lyman Break Galaxies (L*) with time can be explained by their increase in size,
which scales as 1/H(z). In contrast, Lyman-alpha selected galaxies have a
characteristic, constant, small size between redshift z=2.25 and 6.5. Coupled
with a characteristic surface brightness, this can explain their non-evolving
ultraviolet continuum luminosity function. This compact physical size seems to
be a critical determining factor in whether a galaxy will show Lyman-alpha
emission. We base these conclusions on new size measurements for a sample of
about 170 Lyman-alpha selected galaxies with Hubble Space Telescope broad-band
imaging, over the redshift range 2.25 < z < 6. We combine these with a similar
number of Lyman-alpha selected galaxies with half-light radii from the
literature, and derive surface brightnesses for the entire combined sample.
