Starburst Intensity Limit of Galaxies at z simeq 5-6

The peak star formation intensity in starburst galaxies does not vary significantly from the local universe to redshift z ~ 6. We arrive at this conclusion through new surface brightness measurements of 47 starburst galaxies at z simeq 5-6, doubling the redshift range for such observations. These galaxies are spectroscopically confirmed in the Hubble Ultra Deep Field (HUDF) through the GRism ACS program for Extragalactic Science (GRAPES) project. The starburst intensity limit for galaxies at z simeq 5-6 agrees with those at z simeq 3-4 and z simeq 0 to within a factor of a few, after correcting for cosmological surface brightness dimming and for dust. The most natural interpretation of this constancy over cosmic time is that the same physical mechanisms limit starburst intensity at all redshifts up to z simeq 6 (be they galactic winds, gravitational instability, or something else). We do see two trends with redshift: First, the UV spectral slope (β) of galaxies at z simeq 5-6 is bluer than that of z simeq 3 galaxies, suggesting an increase in dust content over time. Second, the galaxy sizes from z simeq 3 to 6 scale approximately as the Hubble parameter H^-1(z) . Thus, galaxies at z simeq 6 are high-redshift starbursts, much like their local analogs except for slightly bluer colors, smaller physical sizes, and correspondingly lower overall luminosities. If we now assume a constant maximum star formation intensity, the differences in observed surface brightness between z simeq 0 and 6 are consistent with standard expanding cosmology and strongly inconsistent with the tired light model.