Sizing up Lyman-alpha and Lyman Break Galaxies

2011 
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 H(z) −1 . In contrast, Lyman-� 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-�emission. We base these conclusions on new size measurements for a sample of about 170 Lyman-� 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-�selected galaxies with half-light radii from the literature, and derive surface brightnesses for the entire combined sample.
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