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We investigate the rest-frame Ultraviolet (UV, $\lambda\sim2000$\AA) surfacebrightness (SB) evolution of galaxies up to $z\sim6$ using a variety of deepHubble Space Telescope imaging. UV SB is a measure of the density of emissionfrom mostly young stars and correlates with an unknown combination of starformation rate, initial mass function, cold gas mass density, dust attenuation,and the size evolution of galaxies. In addition to physical effects, the SB is,unlike magnitude, a more direct way in which a galaxy's detectability isdetermined. We find a very strong evolution in the intrinsic SB distributionwhich declines as $(1+z)^{3}$, decreasing by 4-5 mag arcsec$^{-2}$ between$z=6$ to $z=1$. This change is much larger than expected in terms of theevolution in UV luminosity, sizes or dust extinction and we demonstrate thatthis evolution is 'unnatural' and due to selection biases. We also find nostrong correlation between mass and UV SB. Thus, deep HST imaging is unable todiscover all of the most massive galaxies in the distant universe. Throughsimulations we show that only $\sim15$\% of galaxies that we can detect at$z=2$ would be detected at high-$z$. We furthermore explore possible origins ofhigh SB galaxies at high-$z$ by investigating the relationship betweenintrinsic SB and star formation rates. We conclude that ultra-high SB galaxiesare produced by very gas rich dense galaxies which are in a unique phase ofevolution, possibly produced by mergers. Analogues of such galaxies do notexist in the relatively nearby universe.

Surface Brightness Evolution of Galaxies in the CANDELS GOODS Fields up to $z \sim 6$: High-z Galaxies are Unique or Remain Undetected