Sizing up Lyman-alpha and Lyman Break Galaxies

TL;DR

This paper explains the evolution of high-redshift galaxy luminosities through size growth and surface brightness limits, highlighting differences between Lyman Break and Lyman-alpha galaxies based on size and emission properties.

Contribution

It introduces new size measurements for Lyman-alpha galaxies across redshifts and links galaxy size evolution to their luminosity functions and emission characteristics.

Findings

Lyman Break galaxies grow in size as redshift decreases, explaining their increasing luminosity.

Lyman-alpha galaxies maintain a constant small size over a wide redshift range, accounting for their non-evolving luminosity.

Galaxy size and surface brightness are key factors determining Lyman-alpha emission presence.

Abstract

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.