Puzzling Star Formation Rates in Galaxies

The rate at which stars form in Galaxies is expected to decrease with time. The spectral lines in more distant galaxies are shifted to the red end of the spectrum. The light from very distant galaxies left in the distant past, so we would expect that the star formation rate in galaxies with high redshift should increase. That does not appear to be the case: On the puzzling plateau in the specific star formation rate at z=2-7