Starburst intensity limit of galaxies at z ≃ 5-6

The peak star formation intensity in starburst galaxies does not vary significantly from the local universe to redshift z similar to 6. We arrive at this conclusion through new surface brightness measurements of 47 starburst galaxies at z similar or equal to 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 similar or equal to 5-6 agrees with those at z similar or equal to 3-4 and z similar or equal to 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 similar or equal to 6 ( be they galactic winds, gravitational instability, or something else). We do see two trends with redshift: First, the UV spectral slope (beta) of galaxies at z similar or equal to 5-6 is bluer than that of z similar or equal to 3 galaxies, suggesting an increase in dust content over time. Second, the galaxy sizes from z similar or equal to 3 to 6 scale approximately as the Hubble parameter H-1(z). Thus, galaxies at z similar or equal to 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 similar or equal to 0 and 6 are consistent with standard expanding cosmology and strongly inconsistent with the tired light model.